Reflex model scale question (Burkhard?).

[HankF]

What controls the visual scale of the models in Reflex? The VoyagerE heli model from the Reflex site is visually larger than a Raptor heli when, in reality, it is only 80% the size of a Raptor.

RFG2 has a single parameter which sets the scale.

Hank

[BurkhardE]

Reflex too, it is the wingspan for aircraft and the main rotor diameter for helicopters. Normally, you set these to 1 m (1000 mm) in RMK and Reflex will scale up or down to the value set in the physical parameters. For aircraft, you might build a slightly bigger wing than 1000 mm, if you want to exactly map e.g. an elliptical wing planform to the standard tapered one (equivalent coefficients). But that's already tricky, maybe some model authors are simply in doubt about that point. Some have deliberately set other parameters (weight, inertia and so on too) to have a more visible model in Reflex.

[CustomPC]

Hank,

What is the correct Rotor Diametre for the VoyagerE? I've been training on this heli in reflex and would like to make it true to scale.

[HankF]

My VE has a (stock blades) rotor diameter of 38.75 inches or 984 mm.

Burkhard, you imply that parameters other than wingspan or rotor diameter also affect the visible scale of the machine. Is that so? and how so?

Hank

[BurkhardE]

Yes, but it's only wing span or rotor diameter (or the overall size of the model) in RMK. I f you create a new model project in RMK, you have to define what is called 'Dimension' in the 'View' menu. It's explicitely called 'span' e.g. in the top view. With the mouse, you drag a dimension line from wingtip to wingtip. This 'span' (or rotor diameter for a heli) defines 1 meter in RMK. In Reflex, this meter is scaled by the wing span given in the pysical parameters. If you e.g. set the rotor diameter to 0.984 m, all dimensions of the 3D (RMK) model are multiplied by this factor. (You have to divide the real dimensions by this factor in RMK before if you build a model without pictures, only from measured dimensions, as I do.)

On the other hand, the physical parameter 'wingspan' - together with other parameters - defines the wing area, aspect ratio, and taper ratio, i.e. the main aerodynamic characteristics of the wing. Reflex assumes a single taper planform (cornered, not rounded) and calculates it's aerodynamic coefficients (lift, drag, induced drag). To be completely precise, you have to define such a 'cornered' wing so that it gives exactly the same area and coefficient values as a real rounded (e.g. elliptical) wing.

That is achieved by setting the 'span' in RMK (and in Reflex) not to the real (rounded) wingtips, but to the (straight) wingtips of an anticipated tapered wing. Of course you have to know in advance the (smaller than elliptical) span, root and tip chord of this 'standardized' (or idealized?) wing. The Plane Geometry spreadsheets by Blaine Beron-Rawdon I mentioned in another thread are great to do these tricks. In Reflex, the model is rendered in the correct size since the tapered span in RMK and Reflex is smaller than the elliptical span, but the wingtip points in RMK will go beyond the span (have X coordinates greater than 500).

Helicopters are not completely different, but I think it's not necessary to be so precise to consider elliptical rotor blade planform, particularly since that's completely unnecessary for Reflex. I don't know what the authors of heli models intended when choosing a 'wrong' scaling factor. Maybe you find an e-mail address of the builder of the VoyagerE (the name was very complicated) in the RC-Sim download and could ask.

Burkhard

P.S.: At RC-Sim, look in Reflex Planes downloads, page 4, the last but one, 'Reflex Sports'. That's one year old, but there is the author's e-mail address, and he was the author of VE too. Maybe it still works...

[Wright Flyer]

Hank,

When I started to use RMK and edit the parameters in the menus of XTR at first I couldn't see how to scale the plane (in my case) as I drew it to an arbitrary size in RMK and then edited all the fields in the model parameters in the sim and nothing seemed to affect the size at which the model appeared until, that is, I edited the "wingspan" field, started the sim - no visible change - crashed the model and then, when it was re-drawn only THEN did I see it scaled to a different size. That seems to be the key thing - edit the menus then deliberately crash the model to force it to be re-read from the disk and re-built. At htat moment the new size comes into effect. As Burkhard says, the sim is clever enough to base it's physics modelling on the size you specify so, in the case of planes (and I'm sure helis) if you make the model smaller it will "feel" much more "twitchy" in the air.

Cliff

[BurkhardE]

Cliff,

thank you for this hint, I'm always too far in 'theory' to understand a practical question. I should emphasize your statement and add that it applies to engine/prop failure too. Sometimes, when flying the Cargo, it's even necessary to restart the whole program. If once one engine is stopped (for unknown reason), it will not restart without leaving and restarting Reflex. Maybe the model description is transferred to the graphices adapter and stays unchanged there. In some cases it helps therefore to load another model (F2) and then the damaged/changed again.

Independent of that I would definitely not recommend to play with model size. Flight behaviour is composed of several, interdependent parameters. It's even difficult to scale the physical parameters in a consistent manner since some values change not continously. And in any case - even with correct parameters - different size is different model. You won't recognize your own model in a different size! That's why I wonder what the model authors are aiming at who build helis bigger than in reality. (Uh, am I in theory again? :o )

Burkhard

[HankF]

Cliff, That's an interesting point. But surely, Reflex doesn't use the visual model to calculate flight parameters, does it? In any case, the model of the VoyagerE doesn't fly like the real thing at all. It's much too responsive.

Burkhard do you think it possible to program in Reflex the model shown to fly like the real thing before the real thing is built?

Hank

[max z]

I have TRIED to fly the real thing :eek: :o

[BurkhardE]

Max, you tried, what was the result? :D (I'm really interested to hear that!)

Hank, as I understand your question I would definitely say no, sorry. Of course, if it succeded to calculate the aerodynamic coefficients it would be possible to get a feeling for the thing in Reflex. But Reflex uses parameters of an idealized conventional aircraft configuration, what means a slender wing and cross tail. Any unconventional, unusual configuration must be mapped by equivalent coefficients. Try that for the flying pancake, it's impossible! A stubby wing with small span and big chord resembling the pancake would still not be simulated correctly by Reflex.

Add to this that the big propellers have a big influence on the overall airflow around the aircraft. Even RFG3 is not likely to handle that correctly although it has per-component parameters. Problem is that the pancake configuration is an integrated one, not composed of seperate independent elements, but one very complicated element with many interdependencies in itself.

When the prototypes were built there was no way to calculate the aerodynamic characteristics and the flight behaviour. The physics model of Reflex is at that state of the art. Today there are computer programs that calculate the characteristics of a whole aircraft (and therefore even the pancake) from hundreds and thousands of small finite elements. Afaik there is even a simulator working that way: X-Plane. Considering the reasonable price it should be worth a try, especially since a model of the V-173 would be valuable.

And btw, Stefan Kunde, programmer of Reflex, once wrote for the FAQ of RC-Sim: "Flight behaviour is essentially determined by the .par file. It contains all model and physical parameters. The .mod file actually determines only the appearance. Experience shows that despite an objectively identical flight behaviour on account of a different model appearance there will be subjectively perceived a different flight behaviour. Besides only the positions of rotors, propellers and landing gears (at ground contact) influence flight behaviour." (It was 'technical', therefore bad German, translated literally, but nevertheless charge me with errors.)

Burkhard

[max z]

Max, you tried, what was the result?Burkhard
Burkhard, it was very unstable in pitch, lateral stability seemed OK. I had the middle elevons operating as one large trimflap, and the ailerons had mixed in elevator. Flights were too short to be analysed properly :rolleyes: We did get a chance to play with the CofG position, but to no avail. At the last flight the plane made an uncontrolled loop and headed in the direction of parked cars/spectators, and the pilot (not me, way too slow on the reflexes ) made a wise decision and stuffed it into the ground. It is still awaiting repairs...

Max.

[BurkhardE]

Max,

very nice model, what an achievement! It was courageous to simply build such an unusual plane and try it. The (right) deliberate sticky landing is too sad.

The pancake configuration is difficult enough, but I could imagine it's even more difficult with the low Reynolds numbers in model flight. Maybe it was also a factor that both propellers of the model were turning right. The designers of the V-173 might have intended to counteract the tip vortices by arranging the props in-line with them. Maybe also that the model needs some aid to guide the airflow and control separations.

It's likely that such a model would be difficult to calculate/simulate even with powerful modern programs since low Re number aerodynamics are so difficult. The more it's interesting for you or for Hank to build one and make it fly by trial and error.

My hat is off to you!

Burkhard

[HankF]

Hey Max, that's a beautiful model! It must have been scratch built, I haven't seen any flying models or plans.

Can you give us some airframe and power specifications?

I understand the original had 2 - 80 hp engines. I'm not sure if they were cross linked. If they were under the marked squares, I would think the CG would be quite forward probably less than 1/4 chord. It would probably help the model if you could gear the props so they could be as large as the originals so that more of the model was immersed in the prop wash.

Burkhard, thanks for the analysis of Reflex. It is as I suspected. It's a wonder Wolfgang gets as much out of it as he does. It shows that one should be intimately familiar with the "guts" of Reflex to be able to make a convincing model.

Hank

[Wright Flyer]

Cliff, That's an interesting point. But surely, Reflex doesn't use the visual model to calculate flight parameters, does it?

Ah but to get the visuals scaled to a different size you ARE changing one of the calculation parameters (wingspan for planes, rotor diam for helis) and just changing that one item will have a dramatic effect on the "feel" of the physics. but, as Burkhard says, it's not a good idea to change just one of the physical parameters without changing and recalculating all the others.

In many of the planes I've downloaded from rc-sim.de I've noticed that they will have changed the wingspan to (say) 50cm but have left everything else at the default values when they copied the .PAR from another plane. So you'll have a 50cm span with a 30cm chord and 20cm ailerons, or something really odd like that. It's true these planes fly but they can't really reflect reality.

For a plane I've been modelling I spent a lot of time making sure all the physical dimensions were entered as acurately as possible but the fields I can't enter correctly are calculated values like coefficient of lift but in one of those other threads Burkhard gave a link to a program that WILL calculate all those values given only the physical dimensions of the plane. In my case though, it's simply a matter of trial and error (adjusting values then flying to see if it "feels" anything like the real model)

Cliff

[BurkhardE]

Hank,

that's true, but your post might sound too bad. Actually you have to know what you are doing with any aerodynamic software, be it simulators or airfoil calculators. It's simply not the real thing, and you e.g. have to decide whether airflow will seperate or not in certain conditions. For simulators, that refers to stall behaviour, airfoil moment and stability.

Since 1992 Stefan Kunde, programmer of Reflex, manages to keep the simulator up-to-date without abandoning the original approach. Most other simulators use a very similar approach. They all compute the models performance and flight behaviour from few dimensions and aggregate coefficients. Experience shows that all but very few airplane configurations are well matched by this idealized model if only the parameter values are carefully adjusted.

I just prefer to calculate most of them (former topic) to avoid guessing, but there will still remain some uncertainty. If I e.g. have information that the model flicks easily I can adjust that in the simulator, if it doesn't but is docile I can adjust that too. All I have to know is the real model behaviour and common aerodynamics. Maybe for that reason there is no manual for customizing the parameters in Reflex. ;) (Though some explanations in the help file actually could be clearer.)

The flying pancake is an exception from the rule. (In German, there is a saying that exceptions confirm a rule.) IMHO, the problem is not the differential thrust but the overall airflow substantially defined by both the 'wing' and the propellers. Using G3, you might simulate hover flight correctly. But (for me) what Max described (pitch instability, no certain c/g, uncontrolled loop) indicates airflow separations and will likely not be simulated correctly. I even doubt that knowing what really happens would help since G3 can't simulate that at all (as most other sims, maybe except X-Plane).

I don't intend to talk you out of G3, but simply suspect you are chasing after a wrong goal. If I'm wrong please forget all... ;)

Burkhard

[BurkhardE]

Hank and Max,

did you check the Vought website completely? There is a nice little article (http://www.vought.com/heritage/special/html/sv1731.html) about the advanced design concept of the V-173.

It was typical for th 1930s (imo) as the designer tried to avoid all unnecessary drag, i.e. all parts producing drag only and no lift. It was unusual as the 'flying wing' had an aspect ratio of 1. The article explains that the props had to counteract the tip vortices and reduce induced drag, even make it induced thrust at full power!

So the propellers were vital part of the concept and had to be 'cross-shafted', if not in the first place, but at least later (not for syncing, but for safety in case of engine failure). Due to vibrations, articulated propellers were needed. Maybe a cross-shaft is not needed in a model since electric drives don't fail. ;) Flapping props are common for models anyway. But the whole aim of the concept is based on counter-rotating props, port prop left and starbord prop right! Should be no problem with brushless motors, but hard to get the left-turning deviate of propeller.

A second article (http://www.vought.com/heritage/special/html/sv1732.html) mentions the airfoil used: NACA 0015 is symmetrical and only suited to flying wings with further arrangement like a swept and twisted wing. The pancake seems to be a perfect elliptical planform without sweep and twist (washout) and a straight 1/4-chord line. It seems to me that the elements of aerodynamic theory known in the 1930s (namely lifting line/vortex) have been assembled to the concept. (Could be possible to calculate the thing rather precisely using common formulas.)

But maybe the designer neglected stability aspects. The article mentions as a result of the test flights the need for the (later?) attached 'ailevators'. I wonder why they didn't notice that during the extensive wind tunnel tests. I would have expected that wing to be indifferent, neither stable nor unstable, suitable c/g provided. But maybe they expected and recognized a stabilizing effect by propeller operation and that only proved to be too small in full size. (But how about idle flight?)

Anyhow, the ailevators are essential now. The article says that the trailing edge flaps alone were not sufficient. The sketch Hank attached to his post suggests the ailevators to be 'in-line' with the main wing, i.e. there is no decalage. At least two pictures from the Vought website suggest the same. (Hope they don't mind I borrowed the pics.) But especially one in-flight picture clearly shows a decalage, and I think there has to be some. Max's model has apparently no decalage, and that would explain the noticed pitch instability.

Now it's so simple? I withdraw my guess of airflow separations for the time being. Despite the simplicity of the design, there is no simulator to model it correctly (maybe except X-Plane :p ), since most sims are unchangeably based on slender tapered, not stubby elliptical wings, and won't superimpose wing and prop airflow correctly. Nevertheless it should even be possible to calculate some values. So I would build a raw test model (since I am lazy) to prove the concept (as the Vought people did).

Without any kind of decalage no stable flight is possible at all, so why bother about separations. Tilt the ailevators a bit (should even be possible to guesstimate the right decalage and static margin) and install a left-turning prop left. Balance the model near the 1/4-chord line (more forward c/g is safer, as usual), pull a bit elevator to be on the safe side, and try straight and level flight. Should work! (If not, blame me...)

Burkhard

[max z]

Max's model has apparently no decalage, and that would explain the noticed pitch instability.
My ailevators were of the full flying type, activated by torque rods & internal pushrods from a servo in the "engine bay".
Max.

[BurkhardE]

Beautifully built! Did you ever try to estimate a decalage and c/g position in advance?

Burkhard

[max z]

Can you give us some airframe and power specifications?
wing(body)span: 600mm (excluding ailevons)
motors: 2xspeed 400 6V
gearboxes: Robbe planeta 3.81:1
battery: IIRC 8 cells sub-C, could be 10
Propellors: Graupner CamGear 11x8 folders (ever tried to find lefthanders? :rolleyes: )
weight: ?
Max.

[BurkhardE]

Propellors: Graupner CamGear 11x8 folders (ever tried to find lefthanders? :rolleyes: )

found (http://www.ramoser.de/) :)

[max z]

Beautifully built! Did you ever try to estimate a decalage and c/g position in advance?
Uh..no. I was hoping to get there by trial and error. Error won the game..... :o
I started the CofG at about the 25% position. It would dive into the ground immediately, despite full up elevator. Power seemed adequate enough. Then the rear was ballasted gradually, eventually succeeding in sustained flight, but as said very instable in pitch. Results as mentioned before.

Max

[max z]

I had a conversation with Christian Ramoser at the time, and he explained then that he had special propellerblades for pusher installations ( since you cannot simply mount a Varioprop backwards on the shaft) Are you sure his L versions are true lefthanders, i.e intended for left hand rotation when viewed from the rear?

Max.

[BurkhardE]

No, only checked his website. There he writes that a pusher is built by installing the blades turned by 180 deg in the hub. In the shop are pictures of right-turning blades for the left-turning (lh) ones, but the text says the real left-handers have to be simply imagined as 'mirror-operated'. I understand that as real left-turning blades, particularly since the German text explicitely says 'left-turning (corresponds to blade no. ..., only mirrored compared to picture)' (my translation).

Burkhard

[BurkhardE]

Uh..no. I was hoping to get there by trial and error. Error won the game..... :o
I started the CofG at about the 25% position. It would dive into the ground immediately, despite full up elevator. Power seemed adequate enough. Then the rear was ballasted gradually, eventually succeeding in sustained flight, but as said very instable in pitch. Results as mentioned before.

Max

Again, that's too sad. So my idea to test the c/g position is useless resp. you tried that way already. Remains the 'wrong' prop wash at the port side which intensified the tip vortex and the induced drag and whatever and made the airflow asymmetrical at all.

There is a nice little (simple) tool by
Martin Hepperle (http://www.mh-aerotools.de/airfoils/index.htm), at his website under 'Software', the 'A.C. Calculator'. The help file sounds quite encourageing, should be worth a try and should at least give some learning effect.

Burkhard

[max z]

particularly since the German text explicitely says 'left-turning (corresponds to blade no. ..., only mirrored compared to picture)'Burkhard
Haben wir uns eben nicht richtig verstanden, oder damals gab es die noch nicht......
Very limited choice though, only three small sizes, biggest is 8.2"

Max.

[HankF]

I started the CofG at about the 25% position. It would dive into the ground immediately, despite full up elevator.

Well, that sounds like not enough elevator to me. I think the problem is low Reynolds numbers (ie. a scale effect). I suspect that moving the CG back just made it more unstable. This looks like one model that can't be to true scale.

I would imagine that the elevator (as well as the rudders) size will have to be increased proportionately for this thing to work. (Maybe bring the leading edge of the elevators to the leading edge of the rudders?) I also think the contra-rotating (outward at the top) is essential for this machine. I'm not sure decalage is needed however. It would be true that the elevator would have to be up a little while in flight so that the Cl is sufficient for flight. The problem with full flying ailevators is their tendency to stall at low angles. That's probably why the original had a fixed horizontal members.

Interestingly, the original V-173 was piloted once by none other than Charles Lindbergh.

Hank

[BurkhardE]

Concerning learning effect: Thats interesting, I never before checked such an unusual configuration (picture is Hank's plan view of the pancake, filled with colors by hand, dither is due to neither age nor alcohol, but to mouse).

Burkhard

P.S.: Ramoser soll halt sein CAD/CNC anwerfen und ein passendes Blatt spiegeln.

[max z]

This is getting further and further away from simulators.... :)
I would imagine that the elevator (as well as the rudders) size will have to be increased proportionately for this thing to work. (Maybe bring the leading edge of the elevators to the leading edge of the rudders?)
Did that with the rudders, made an extra large set from corex.
It would be true that the elevator would have to be up a little while in flight so that the Cl is sufficient for flight.
The central flap can be moved, IIRC I had it up a bit. Ailevons were used for elevator function, all same as the original.
The problem with full flying ailevators is their tendency to stall at low angles. That's probably why the original had a fixed horizontal members.
Not helped by the fact that I used flat plate surfaces. (The original's surfaces were full flying also.)

[BurkhardE]

Hank,

why do you think the pancake can't scaled down as is and needs bigger elevators as a model? I can't believe that since there is no airfoil moment and it's a tailless / flying wing configuration anyway. The main 'body' is lacking any reflex in the airfoil what has to be replaced by the ailevators.

I agree that some up elevator is needed for level flight to get sufficient angle of attack. The picture shows that clearly with a nose-up attitude of the wing/body and level ailevators. That's what I meant with decalage. That for me suggests a neutral behaviour without any longitudinal moments. The ailevators would then be needed only for a higher degree of stability and a higher responsiveness.

Do you have more information about the ailevators (the pictures are too small to see any details)? Maybe the full-flying type will stall at low aoa, at least with idle power, but anyway I wonder if Max tried only powered flight or a glide too. Would help to distinguish the prop influence. So far I'm only baffled what caused the instability.

Burkhard

[max z]

I wonder if Max tried only powered flight or a glide too.
Only powered. Should have built a chucky first for glide testing.
I am re-reading my data source "Naval Fighters number twenty one"
Interesting anecdote on the original: Charles Zimmerman, the designer, built a tethered remotely controlled electric(!) model to prove his design, the V-162. The aft 1/4 end was hinged, and the model was flown quite satisfactorily, operated by two pilots. Also the need for the ailevators was found as a result of windtunnel tests, i.e. before taking to the air.
The rear flap was an afterthought, used only for landing (with little effect)
No flap to be seen on this picture, but the ailevator design looks like it has adjustable camber, or a fixed LE part:

[HankF]

I guess I'm just not that well informed. I read that, at least initially, the machine didn't have the ailevators at all! That would imply the the aileron function was to be performed by the split main elevator. Its easy to see why that would be marginal! While it is true that the airfoil has a zero moment coefficient I'm not sure for stability reasons that they would want to fly that way. I would think that they'd want a bit of up elevator (you call decalage) so that the CG could be farther forward to take into account CG variability with fuel burn, armament expenditure, etc.

Max, did you use the main flap also as part of the elevator system? (BTW, this discussion is appropo to simulators in my opinion. I hope to see this model in one). I just read your latest entry and it looks like the ailevon has a normally fixed part which could be adjusted for angle of attack (for trim purposes?). It's pretty confusing.

Hank

[max z]

I just read your latest entry and it looks like the ailevon has a normally fixed part which could be adjusted for angle of attack (for trim purposes
It was clearly redesigned to full flying configuration later, the picture in my earlier post shows that. You can also see the counterweights at the LE.
Max, did you use the main flap also as part of the elevator system?
It was servo adjustable (up only), but I used a separate slider on my Tx. I could have operated it on the elevator stick, but I wanted to avoid the complication initially.

Max.

[BurkhardE]

Confusing indeed, but we seem to be confused differently. Max seems to have excellent information. His last picture is very confusing for me. A common reason for providing full flying elevators is a very large pitch range of the main wing (as typical for low aspect ratio). The elevator will have a small angle of attack anyway if controlled properly and stall only if set not properly (e.g. the Cessna 177 is notorious for that).

As Max writes, the ailevators seem to have adjustable camber and even a small area in front of the hinge line as any common elevator on a fixed horizontal tail. I'm completely confused by the fact that the ailevators (both parts) on the picture are cambered positively, producing lift. That would mean that the c/g is aft of the neutral point, or what?

Burkhard

[HankF]

Maybe a drawing of the XF5U would help decipher the control surfaces:

Hank

BTW: Burkhard, the second article you mentioned on the V-173 also makes the comment: "The first flight was of only 13 minutes duration because of very heavy longitudinal stick forces, and having only 20 gallons of fuel aboard. The stick forces were subsequently lightened by adding the trailing-edge stability flaps and ailevator trim tabs."

[HankF]

Here's additional info:

Rear View on 21 August 1947 with taped-on work areas and engine panels removed. The good size "stabilizing flaps " can be seen between the two vertical fins.

Another unique feature of the XF5U-1 was the stability flap, located symmetrically about the centerline of the airplane at the wing trailing edge. The 15 sq. ft. hinged surface required no pilot control but automatically provided for change in airplane trim with change in attitude. The air loads upon the flap adjusted deflection against a spring loaded strut. The stability flap was linked to the tail wheel to insure locking in the up position when the tail wheel was extended.

Max: also check this out. http://home.planet.nl/~otten100/XF5U-1.html

[max z]

Thanks for that link Hank, I sure wished mine was flying that well! That picture of the XF5U-1 streaking past in a banked attitude.....Whow!
Three interesting observations:
1. no flap
2. small propellors, so the propwash is not essential for good flight behaviour
3. a motor/prop failure is terminal.......
Max.

[BurkhardE]

This is getting further and further away from simulators.... :)
Max, I don't think so at all. I'm still trying to understand the behaviour of the plane/model. I would need that to build it in a simulator, and even though I doubt it would be really possible with Reflex I feel challanged by Hank to try it.

It's no problem to adjust the physical parameters in Reflex to any known flight behaviour. It could even be done for such an unusual configuration like the flying pancake so that flight behaviour in Reflex would at least resemble the real one. But e.g. the Jitterbug and the SlowStick are normally configured and the parameters could be calculated. Fine tuning was done comparing the Reflex model to the videos of the real models.

Now that your model is unstable in pitch I need to know the goal to chase after. I still think the props are vital part of the concept, though of course the plane wouldn't crash without power. Just induced drag will be so great that flaps are not needed for landing, logically for me. For cruise though the props have to reduce the otherwise too big drag, and for that purpose they have to rotate in opposite direction. That certainly can not be simulated in Reflex.

Flight behaviour might be approximated (unknown how near since much guesswork needed) with parameter values for a stubby wing, small tail moment arm, and two props. Rotating propwash isn't simulated in Reflex anyway. Of course I would adjust a stable flight behaviour and maybe the simulated model would reveal the special characteristics of the configuration (e.g. fast roll, heavy pitch, ...). But the instability has to be interpreted and eliminated before since it is obviously no normal (real) behaviour.

The article Hank posted the link to is very interesting since it's a case to compare with. Obviously they got the model flying without worrying too much. I'm still baffled.

Burkhard

[max z]

Now that your model is unstable in pitch I need to know the goal to chase after
Burkhard, maybe the configuration is stable after all, maybe I was just changing the wrong parameter, i.e. the CofG. On hindsight, I should have experimented with the flap and tried more reflex to counteract the diving tendency. OTOH I don't see any flap or apparent reflex in Guiseppe's XF5U-1 :confused: ,but then again I think I see a lot of ailevator deflection in the second launch picture.

Max.

[HankF]

A rough calculation on the XF5U shows that at stall speed (approx 40 mph), the Coefficient of lift for the machine has to be greater than 5 or 6. It's quite obvious that the wind over the airframe is much higher than ambient to produce such lift since that airfoil is limited to about 1.2 maximum. So for low speed performance, the large props are essential. Also, given a 22 degree AOA at stall, the vertical component of the thrust would be appreciable.

Since Reflex handles 3D flight to some extent which depends on the airflow over the control surfaces, it should be able to handle the V-173. Did I see an input for airspeed over the tail (or something like that) in Reflex?

Burkhard, I modified the .par file for a heli (VoyagerE) that looks too big and, sure enough, the dimensions are way off. I fixed it as best I could but I couldn't find an input for tail moment to adjust yaw responsiveness. Does Reflex depend on the visual model for this?

Hank

[BurkhardE]

This seems to be one of the hottest threads... ;)

A rough calculation on the XF5U shows that at stall speed (approx 40 mph), the Coefficient of lift for the machine has to be greater than 5 or 6. It's quite obvious that the wind over the airframe is much higher than ambient to produce such lift since that airfoil is limited to about 1.2 maximum. So for low speed performance, the large props are essential. Also, given a 22 degree AOA at stall, the vertical component of the thrust would be appreciable.That's why the cross-shaft is needed, but maybe for landing you could simply throw the model on its belly at high speed, as Guiseppe does (just omit the landing gear). :D
Since Reflex handles 3D flight to some extent which depends on the airflow over the control surfaces, it should be able to handle the V-173. Did I see an input for airspeed over the tail (or something like that) in Reflex?True, that's propwash, Reflex even puts it on the wings (as far as covered by prop disk). But we never know which value is realistic, it's only guessing (or desire).
Burkhard, I modified the .par file for a heli (VoyagerE) that looks too big and, sure enough, the dimensions are way off. I fixed it as best I could but I couldn't find an input for tail moment to adjust yaw responsiveness. Does Reflex depend on the visual model for this?I know the VE, but unfortunately RC-Sim download is down til next week so we can't find out the author's e-mail address to ask him what he intended (see former post #5). The tail moment arm of helicopters is obviously derived from the 3D model, as Stefan Kunde wrote that "the positions of rotors, propellers and landing gears (at ground contact) influence flight behaviour" (see former post #10).

I'm working on the V-173 for Reflex, the 3D model first. Max's posts reassured me and sparked off some ideas for the pysical model. But that will require some days...

Burkhard

Night shift:

[HankF]

Burkhard, That's just great! You certainly do good work.

I hope to get up to speed on modeling in Reflex soon. I have several I'd like to see.

Hank.

[BurkhardE]

Hank, modeling will be easyer using objects. Look at the propeller above, it consists of objects and is an object itself. Objects can even have their own textures. And publish your objects, so modeling will become easier for all of us Reflex constructors. (Now I dropped a broad hint! :D )

Burkhard

[max z]

I'm working on the V-173 for Reflex.
Now there is an incentive for me to buy the sim...... If only it did not have such a heavy pricetag, compounded by the fact that I have to buy a new graphics card as well. I see $$$ prices around the 180-190 mark, but the Reflex website lists it as 229 Euro's. Any cheap adresses for us poor Eurodwellers? Max's posts reassured me and sparked off some ideas for the pysical model.
That is a pleasant surprise, I would have thought my stories of crashes and hairy flights would have put you off. :confused: :)

Max.

[BurkhardE]

Max,

in the long run it is much less expensive to build and fly (and crash) only virtual models. The simulator should pay soon. :D If the forums don't know cheaper sources than Reflex itself you might have to go #pound or $$$.

Anyway you should buy Reflex not (only) for the V-173 since this can't be simulated correctly at all. I'm not going to prove that, but we will likely have a model looking like the Pancake and also flying like a pancake. It's just an UFO (unmatchable flying object). But we could try to duplicate your crash to understand the reason. ;) That's something challenging for me...

Burkhard

Flaps and rudder are working, but it's tiring:

[HankF]

Burkhard, correct me if I'm wrong but it looks possible to create "phantom" parts for Reflex. If so, they might be helpful in getting to model to fly right while being invisible so the model looks right.

BTW, will you be making the ailevons and main flap so they can be linked or not for control purposes? Maybe the main flap could be run off a separate channel or configured as an extra elevator for testing.

I found a company called "eFlight Manuals" who advertizes one for the XF5U and I ordered one. It may have some information on the controls. All I could see from a small thumbnail was an exploded view diagram.

Hank

[BurkhardE]

Hank,

unfortunately none of the parts of the visual model (even if invisible or 'phantom') except the props and the landing gear have any effect on flight behaviour. It's also impossible to alter the channel assignments or 'mixers' built into Reflex. Let me first complete the 3D model and then write a post listing all design decisions I made and limitations I stumbled on. Likely I will make that tomorrow. After that I intend to tinker at physical parameters for Max's model and post again. There will be even more limitations and I expect a real flying pancake adjustable at will but not realistic. You will get a link for download and should comment. Hope the manual you ordered will help.

Burkhard

Snapshot:

[max z]

I found a company called "eFlight Manuals" who advertizes one for the XF5U and I ordered one. It may have some information on the controls. All I could see from a small thumbnail was an exploded view diagram.Hank
Hank, the XF5U-1 never flew, so the controls were never proven to be effective! The book I have (Naval Fighters number twenty one, ISBN 0-942612-21-3, author Steve Ginter) has 14 pages on the V-173 and another 20 on the XF5U-1, including a very nice see-through drawing showing the construction in detail and lots of other data ( but no flight notes ). Worthwile.

Burkhard, I like the way your pancake is coming along!

Max.

[BurkhardE]

Here she is! It's the 3D model only, physical parameters are absolutely dummy to just let the model work in Reflex.

Max, the pan is hot now, the batter is filled in and tomorrow we will add some bacon. At weekend we three will eat up the German/Dutch/American pancake and see if we upset our stomach. Bon appetit! (Em, good night for the moment!)

Burkhard

P.S.: I forgot the model files (http://www.fh-augsburg.de/~erd/privat/Modellflug/download/V-173Reflex.zip). See V173.txt file for path/directory information. Use at own risk. ;) The announced comments will follow tomorrow at this place.

[HankF]

Burkhard, your the master! What a kick in the pants!

I immediately set about modifying the .par file to what I guessed it would look like for the real bird. The exercise was extremely entertaining! Basically, I ended up with the whole machine square (0.9 m) and I put the "tail" on a moment of 0.8 m (asuming it is from the CG). I gave the flaps (elevators) full half span (100% - except for Reflex's minimum of 10%) and the ailerons 60% (100% was too sensitive, again except for Reflex's minimum of 5%). I also increased the max lift Cd a bit. To simulated airflow over the machine I set the AOA for stall at 90 degrees. Lastly, I gave it a thrust/weight ratio of 1.1 to give myself an edge.

Take-offs are hairy to say the least, requiring just a tad of down elevator until the machine is moving or it just flips straight up! Once up to speed, it does quite well. It sure glides well, it could use an airbrake for spot landings with power.

I think I’m beginning to understand this beast.

The thing sits at a 22 degree angle on the ground. The total lift vector of the machine comes from the fuselage (i.e. wing)and the propellers. At very low speeds most of the lift comes from the props so that the combined lift vector is well forward of the CG. If the machine becomes airborne in this state, it will have a pitch-up tendency which may not be able to be overcome by the ailevators. That’s why they provided a “stability” flap on the aft center of the machine. It works automatically against a spring which holds it down while the machine is at low speeds accelerating. By giving the wing camber, it moves the fuselage lift vector aft so that the combined vector is at the CG. As the machine speeds up, airloads retract the stability flap against the spring, moving the fuselage lift vector forward as the prop lift vector is diminishing thereby keeping the combined lift vector at the CG.

Thanks again, Burkhard. You Reflexers really need to try this one!

Hank

[max z]

Burkhard, what can I say! Not being the happy owner of a Reflex sim I cannot fly it or even see it fly, but just looking at the pictures is very satisfying. This looks like the flapjack I had (and still have with a few bits missing ). Very,very nice. Are you going to put surface detail on it? I am a stickler for details, and one odd item to watch out for is that the early star-in-circle insigna's were pointing forward, while the later star-and-bar's were pointing sideways. I guess they could not decide whether it was a wing or a fuselage :D . Another fine detail is that the engine bay hatches are identical, not mirrored, but I will forgive you for mirroring :rolleyes: .
I guess my flying buddies will never understand why I insist on finishing a prototype completely BEFORE testflying it. (Don't understand it myself...)
Well done, and I am glad to be part of this culinary session :)

Max.

[BurkhardE]

Thank you both, glad you like it, and gradually I'm going to understand why you like this plane, it's very impressive to see it 'live'.

Yet let me again see to our pancake. Swinging the pan, flip (not flap!) over. Nothing burnt, nice golden colour on both sides (how true!). I'm cooking a sour pancake, not a sweet one, so prepare for the bacon.

There is no 'eye candy' at all, at least so far. The plane is an impressive figure by it's unusual shape alone (or strange, if even Navy was confused about alignment of insignia?). I was too lazy (or tired) to add some textures, but that seems really necessary since the model looks affected without. I already noticed the two forms of insignia and prefer the star-in-circle one. The engine hatches would be no problem even in an unforgiving environment ;) since they anyway only would be plastered to the surface as a bitmap.

As usual (for me), I used no pictures of the model in the first place (how wonder, I had none). But as usual too, the three-view sketch was a good starting point and guidance to create the overall shape of the model (as a classical RMK project). Now I could overlay the sketch bitmap with several coloured bitmaps (using Micrografx Designer and Picture Publisher) and map them to the model surfaces in RMK.

So the Reflex model resembles the real aircraft, not a real model, but in Reflex it looks like a model. The shape had to be reproduced in main detail to get the special appearance of the plane, a simple plank form won't have done the trick. Still the model is not perfect, it's even a bit faulty. The flap and rudder axes are not aligned properly, an ugly edge shows up when ailevators are deflected, there could be even more polygons for smoother curves, prop blade twist could be better, canopy could be transparent, landing gear struts need fairing, wheel pants, ...

But that's all only nice to have now. The important things:

The visual model (mod file) has one single main 'physical' effect, namely collision detection in simulation. That's why the landing gear is so important for correct behaviour in take-off, landing and taxying (and why you should 'optimize' a model in RMK). Next are the propellers which not only can collide but may produce moments if not aligned with the longitudinal axis (we discussed that). All control surfaces have no other effect than visual!

Unfortunately, Reflex only has 'hard-coded' channel assignments and mixers, and there's a very limited number. I suppose Reflex will aim at a more component-based approach as AFP and G3 already have (as I understand), but til then we're restricted.

The ailevators are the most noticeable control surfaces, and in turn they will be most noticeable used as elevators (I felt). Therefore I chose to define them as not ailerons, but elevators, since there are no 'elevons' in Reflex. At least there are two elevators in Reflex, so it's no problem that they have no common axis. The same is true of the two rudders.

The trailing edge flaps are set up as two plain flaps in Reflex. They could be flaperons, but as such they are useless as we know and despite deflecting up and down any up deflection would have no effect in Reflex anyway (discussed that too). Sadly, now our pancake has no visible ailerons at all (life isn't perfect at all, why should simulators).

The most important decision in RMK is the definition of the 'meter' at the beginning of the project (of course also discussed that before). But there is no real choice since this meter is scaled to the wingspan in Reflex and all wing characteristics depend on it. So in my opinion it has to be the 'diameter' of the pancake. (BTW the pancake turns out to be not perfectly 'circular', chord is 0.975 of span.) As an aftereffect, the ailevators have X-coordinates greater than 500 (mm), but that's no problem at all. Problem is that they are simulated not really correctly in Reflex.

Of course, all features could be definable independently in Reflex. But Stefan Kunde decided to go simple (for himself and us) and made many parameters relative to others. In addition, he built value range checks into the dialog boxes, as certainly every programmer should do. The spanwise position of ailerons is defined as a percentage of span, and not above 100%. Obviously, Stefan had no ailevators in mind. The pancake's will likely have too small effect due to too small moment arm. But the small-span configuration has good roll rate anyway and only small aileron deflection is needed, so we could compensate. That's the next topic to discuss.

I'm writing a book, am I? Anyway, I wonder if you will notice any errors or inconsistencies (resp. would wonder if not) and expect your response.

Burkhard

P.S.: The propeller topic is exhausted now. I molded the left prop as if it were turning left, but in Reflex all propellers turn right (no deviates :D ). That's no big deal since Reflex also doesn't know any propwash spin (yet prop torque, but that's part of the next topic too). You will notice a change in the color of the prop disk if you alter this in the rotor parameters in RMK. But you will also notice the black and white stripes at the prop blades resulting in slightly shining circles on the rotating prop disk. Obviously, Reflex overlays or mixes the colors. Maybe that's why the disk gets too dark (or opaque) from some rpm up. You can't preview these effects in RMK, it's rendering seems to be different.

P.S. 2: The pancake reminds me of an anecdote from the 1970s. In German parliament, opposition (with a wink) put a 'small question' why the cooks in some remote Navy school had to use so heavy pans and why these could not be replaced by lighter ones. The then secretary of defence, popular and humorous Georg Leber, personally replied (with a wink too): For the purpose of complete deterrence we cannot do without heavy pans also in future. ;) (sorry, couldn't resist to tell)

[BurkhardE]

The thing sits at a 22 degree angle on the ground. The total lift vector of the machine comes from the fuselage (i.e. wing)and the propellers. At very low speeds most of the lift comes from the props so that the combined lift vector is well forward of the CG. If the machine becomes airborne in this state, it will have a pitch-up tendency which may not be able to be overcome by the ailevators. That’s why they provided a “stability” flap on the aft center of the machine. It works automatically against a spring which holds it down while the machine is at low speeds accelerating. By giving the wing camber, it moves the fuselage lift vector aft so that the combined vector is at the CG. As the machine speeds up, airloads retract the stability flap against the spring, moving the fuselage lift vector forward as the prop lift vector is diminishing thereby keeping the combined lift vector at the CG.
Hank, the thing is an extreme taildragger, even our STOL DO 27 stands only 11 deg steep on her wheels. There must be strong bending forces in the aircraft during take-off and high-pitch flight (modern: high alpha?) since right prop turns left, left prop turns right due to the diagonal airflow.

Your idea is very good, now I wonder how it's quantitative. It seems the thing can at all fly at low-speed/high-pitch only due to the propwash. The low speed free airflow would give too little lift anyway, if not even the wing would stall. The props blow the air towards the wing, ideally straight in-line. But due to their spinning (and only half-covering the wing) they 'shovel' the air in a slightly upwards direction to the wing bottom making for an effective angle of attack.

Regarding the extreme nose-high attitude though (and looking at the side view), it's quite possible (or sure?) that mainly the front part of the wing is hit by this blow making for the pitch-up moment. In the rear part, air is already redirected and flows parallel to the wing, producing no lift force. The stability flaps have to deflect the air downwards and, on their long lever arm, produce a counteracting pitch-down moment. If flight speed is higher, noticeable free air flow (not blown by the props) will hit the rear wing part, increase air pressure there, retract the flaps against the spring force and take on their role.

It's a naive argument, but did I understand your reflection and could you agree? Question goes to Max if his model crashed at high or low speed. Is it too daring to assume at low speed?

Burkhard

P.S.: Reflex seems to simulate the spatial position of the prop wash and it's main effect quite well, how else could you notice the pitch-up effect.

P.S. 2: On the real thing, the ailevators should be hit by propwash at their top side, but only at higher speed/lower pitch. At low speed/high pitch they will crouch down under the propwash.

[max z]

Question goes to Max if his model crashed at high or low speed. Is it too daring to assume at low speed?
First crashes were straight from launch, plane went into a dive straight away. Did not look that it was running out of speed though. Final crash was at high speed and intentional to avoid collision with cars and/or spectators.

Max.

[BurkhardE]

Thanks Max, had to be the (for me) unexpected. :confused: That's a hard nut to crack! (So far hadn't realized you launched the model.)

Burkhard

[max z]

This is what my book has to say on the flap, as recollected by one of the test pilots:" I had problems getting the tail down effectively under low or no power landing. I called it ground effect. The trailing flap was to provide automatic relief by deflecting up under the ground effect load when perhaps ten feet above the landing surface. It proved effective, but as I recall, not startling. The airplane always had a tendencty to pitch nose down as the flare for landing was made, and more so when power was reduced."

Max.

[max z]

Another tidbit: The Flapjack was able to fly in full control with full power and full up elevator, assuming a 45 deg. attitude!

Max.

[BurkhardE]

Interesting, gives new ideas:

The latter seems to prove the assumption that airflow is determined mainly by the props. They make for a 'cambered' configuration and prevent separation, similar to leading edge slats or boundary layer control by blow-out.

The former could be a combination. Of course there is a 'cushioning' effect near the ground (there are special ground-effect vehicles), and the higher pressure near trailing edge gives a pitch-down moment. But increasing this effect by reducing power could conversely mean increasing power would pitch-up what could prove the ideas concerning overall prop effects in this configuration.

For the high-speed instability we are back to the old theory of lacking 'decalage' since a plain symmetrical wing will produce pitch-down moment with lift. It has to be countered by twist in a swept wing, reflex in the airfoil or a 'reflexed' tail, e.g. the ailevators.

In a former post you wrote you didn't try different decalages. But the pilot must have used the full-flying ailevators anyway. I suppose you couldn't see the ailevator position due to speed and distance. Not unlikely that I can examine the theory in Reflex instead.

Burkhard

[max z]

In a former post you wrote you didn't try different decalages.
I meant with the tail flap, by deflecting it up more.
Max.

[HankF]

What I’ve been trying to do is to construct a flight model for Reflex of the V-173 as close to a real model as I can within the constraints of Reflex without trying to bias the model toward realistic flight and see what comes out and whether it accords with the known facts of the full sized machine.

I started with a square configuration with an aspect ratio of 1. I then envisioned ailerons and flaps on the rear of the wing as full span as Reflex allows and a horizontal “tail” of 100% elevator with an area equal to those of the flaps, with a moment arm to place it at the position of the wing trailing edge. The ailerons were given half the area of the flaps/elevator.

Here’s the resulting .par file. You'll have to change its extension to .par.

Max, I can't get this model to fly under full control at 45 degrees at full power but it can get it up there quite a ways and pretty slow but it starts to get pretty squirrely. You need to get RefleXTR. It's pretty entertaining.

I also found out that Reflex doesn't allow reflexed (no pun intended) flaps.

Hank

[heli-nek]

Hank, please check your file. it is a txt file rather than a par file. And there are no paramenters in there. :)

[HankF]

Thanks Heli-nek, It's fixed - sorry folks.

Hank

[HankF]

Here are a couple of performance estimators at sea-level:

Slip stream velocity (mph) = 42*(HP/D^2)^1/3, HP is horsepower, D is prop diameter in feet.

Static thrust (pounds) = 7.28*(D*HP)^2/3 @ 70% prop efficiency.

For the V-173 with a D of 16.5 feet and an HP of 80 (per prop), Slip stream velocity is approximately 27 mph and Static thrust for both props is approximately 1756 pounds.

For a GW of 2250, the T/W ratio is approximately 0.78

Hank

[BurkhardE]

Hank, I went the other way trying to match Max's model as closely as possible. Maybe you changed some of my parameters (which were those of the threedee480) to a square wing. I had to change all parameters.

Span is 0.6 m and the wing is a perfect ellipse with a straight 1/4-chord line, as Martin Hepperle's A.C.-Calculator revealed. There is neither sweep nor dihedral. Wing area is 0.276 m2. I estimated the weight as that of my threedee480 plus a second 480 drive plus the bigger battery resulting in 1.1 kg GW. That gives wing loading of 40 g/dm2 what is typical for a trainer model (but that's the only thing the V-173 has in common with a trainer). The moments of inertia are derived (guessed) from the threedee480, too.

As drive I chose two of my 3:1 geared 480 7.2V with 11x7 prop and 8 cells NiCd. Torque is neutralized, thrust doubled giving a 1.0 thrust/weight ratio. Propwash speed is 12 m/s.

All areas and neutral points were determined using A.C.-Calculator. The ailevator's moment arm turned out to be only 0.262 m. The fin's moment arm is only a bit longer. But that's no problem since no big pitch and yaw moments are needed.

The flaps were defined in Reflex resembling the real ones very closely. They are deflectable only downwards (as Hank found out too :p ) and have to camber the inner wing parts. That will prevent tip stall on the elliptical or highly tapered wing.

Unfortunately, the aileron function of the ailevators is a problem in Reflex. I chose to exploit the remaining span (flaps subtracted) from 50% to 100% and to use a big aileron chord to replace the big ailevator span.

I'm still wondering obout the damping coefficients (tried to estimate them with Plane Geometry but don't know if it works). Anyway, pitch and yaw damping is very low making for a nervous plane.

My first trial on the wing planform was a 0.5 taper ratio for smallest induced drag, but tip stall made it an error. The equivalent taper ratio of the elliptical planform is 0.63 and somewhat better, but I still had to apply flaps to prevent tip over. Easiest to fly is a rectangular wing planform with 0.6 m span and 0.46 m chord. Ailerons are more effective, too. The tapered wing is in the first par file attached and the rectangular one in the second (c/g adapted in both). Rename .txt to .par!

Airfoil moment is set to 0 except for stall. There is a 1.5 deg 'decalage' and a c/g slightly in front of the overall neutral point (which is a bit aft of the wings neutral point due to the ailevators, see picture below). The model is very sensitive to c/g position! The downwind parameter for the tail is set to less than 1 (0.5) to reflect the fact that ailevators are mostly outside of wing downwash.

The model is very nervous in Reflex. Especially the tapered wing showed a somewhat indifferent behaviour, not to say instable. Sometimes the plane pitched up very fast after a small elevator input, sometimes the opposite. More forward c/g helps but requires some up elevator (of course?).

I doubt the model is simulated correctly in all respects (self-fulfilling prophecy?), but many aspects of it's behaviour should show in Reflex. I suppose problems are pitch nervousness, soft ailerons and c/g. BTW take-off and landing are difficult, model may tilt and props may hit the ground.

Max, you obviously have (of course) three choices:
1 buy Reflex and explore the potential behaviour of the model as far as possible (some unknown limitations),
2 repair the model and try again being aware of more risks than before (mainly c/g position),
3 do both in that order (after all it's winter now and you would be ready to fly in spring ;) ).

Hank, my impression is that lower wing loading helps a lot. Maybe Max should have built a bigger model, weighing the same, with an even higher gear ratio and bigger props. I'd be very pleased if you would try my par files too, rate the model behaviour and compare it to your parameter set.

Burkhard

P.S.: Ever tried to flip a pancake with an elegant swing of the pan and it only flapped despite your best efforts? So does the flying flapjack.

[max z]

Max, you obviously have (of course) three choices:
1 to buy Reflex and exploit the potential behaviour of the model as far as possible (some unknown limitations),
2 to repair the model and try again being aware of more risks as before (mainly c/g position),
3 to do both in that order (after all it's winter now and you would be ready to fly in spring ;) ).
Don't think so. I just finished today with the drawing and parts arrangement of a nice scale twin, the Cessna Bobcat, and sent it away to be CNC cut. Very conventional aeroplane for a change :) , but I will have my hands full until the summer for sure.

Max.

[BurkhardE]

That's a great relief to me, I think the flapjack isn't worth the effort (though it looks exciting). If you had (or made) good pictures of the Cessna (top and bottom, sides and front, no shadows) I would build a Reflex model. And I promise it would fly perfectly (I'm a Cessna enthusiast). What about Reflex now?

Burkhard

[HankF]

Burkhard, Those were very interesting .par files. I made just one change which removed most of the pitch sensitivity, I set the tail moment arm to 0.5m (which should put the elevator effectively at the trailing edge. This makes your rendition much closer to mine.

We ought to agree on a visual scale so that our efforts aren't confused by perspective. I suggest using the scale of 1 meter wing span for the .par file, the same that RMK uses. This would help us in our negotiations of the parameter values.

Based on my suggestion here is my latest .par file (in .txt format). The only change is to size and I've increased the weight to 2 kg. I think the model flys very convicingly like you'd expect it to.

Hank

[BurkhardE]

Hank, thank you very much, I tried immediately, but let me first tell another story.

In the last hours I did the texture job, it was a walkaway. I not even corrected the flaws and deficiencies of the visual model but only plastered some textures on it. Now I'm tired and declare the visual model completed.

Max, I am a stickler for details too, but for the moment I lost all interest. Hope the hatches are good enough and you don't miss anything essential. ;) The fins and rudders are not as they should be, the landing gear struts are still lacking their fairings. But I would not claim it's your model anyway.

As you will notice I chose the star-only insignia since I feel the thing is a wing is a wing is a wing! :)

The zip file (http://www.fh-augsburg.de/~erd/privat/Modellflug/download/V-173Reflex.zip) is updated for the moment, but it's content isn't complete yet.

There is a nasty effect in Reflex. Seen from certain viewing angles the textures look darker than the untextured areas even though it's exactly the same color. Unfortunately, the effect is noticeable especially with yellow. I didn't find a way to avoid it so far, so you should simply ignore it. :o

Enjoy anyway.

Burkhard

P.S. (impossible to do without): You may notice that Reflex is still not perfect in rendering light and shadow (as the other simulators too). The model casts it's shadow but isn't shaded at least by itself, see the fins not shading the wing. Sometimes the slit between the flaps and the wing is too bright since it is rendered as lighted instead of shaded. Maybe it's a more general problem of rendering performance or lack of functionality in DirectX.

[max z]

Burkhard, it is magnificent! As if I am looking at my model. And no, I don't miss anything essential :) . But if you feel like it, maybe the color scheme can be more authentic. Fins are all silver. Underside is silver up to a cheat line, see attached picture. Underside of ailevons is silver. Proptips are red/yellow/dark blue/white. (sorry, could not resist it :D ) And I did not bother about the fairings too......

Great job!
Max.

[HankF]

I have to say that I’m impressed:

With Max’s rendition of the V-173, the workmanship is flawless.
With Burkhard’s RMK modeling, especially in such a short time.
With RefleXTR’s capability to handle such a project in such a convincing fashion.

The similarity of a square wing to the V-173 configuration may be argued but XTR’s handling of it is truly impressive. It’s too bad that Max didn’t get his machine flying properly but we know that at least one model of the XF5U-1 did fly so I believe Max’s machine would have flown just fine when sorted out. For a model, I don’t believe that the central flap is necessary. Now what would be interesting is to build a V-173 model with a square wing with the CG at ¼ chord and see how it works. I’ll bet it flies just like XTR’s version.

Burkhard, you don't have to respond to Max's nit picking (the mark of a true craftsman), I have a picture of the V-173 flying without the wheel fairings or pants. I personally thought it was fine being all yellow since I'm more of a form follows function type of person. BTW, which .PAR file do you like best?

Now all we need is a "sticky" thread at the top of the Forum for Simulator models to put this one in.

Hank

[max z]

Burkhard, you don't have to respond to Max's nit picking :mad: :D

[BurkhardE]

Hank, guess what I did when I read Max's post: :D

Max, is it possible that we have something in common? ;)

Anyway, with Hank I agree that the Reflex model is a means to an end. And the pancake looks quite right in golden color all around (not as if some metal of the pan stuck on it ;) ), good enough to give a realistic impression in the simulator. Besides I would need an additional star texture with silver around, not only a star in yellow. But starting the RMK project I chose only a small 512x512 bitmat to keep the mod file small. Now it's nearly filled and no more room for a star. There would be even the cheat line (by chance) for the yellow-silver border but also an additional texture needed for the rounding. Why didn't you post the picture earlier? ;)

Now the model will just remain all yellow, but here is my nit picking: The wheels are affectedly black and glossy, they should be dark grey and matt. Oh, and the propellers! How could I only build them in this default position, although I told others so often that a three-blader has to be lined up one blade straight down (as in the picture). Maybe I will correct still just these flaws. :)

Burkhard

[BurkhardE]

Hank, actually I like your second par file best, but that may be since I still feel much of my good old threedee480 stick-type pattern model in it (I used it's par file for the V-173). I'm still afraid that we adjust the Reflex model the way we think it should be.

Nevertheless, I'm impressed too (to my surprise). I started this project expecting a simulation-to-reality resemblance of about 50%. Now my feeling is near 80% even for such a strange configuration and we're coming across the limitations of Reflex. But wasn't just that the goal we were heading for?

I speculated that a low wing loading would make the pancake more controllable, you proved just that. Now I'm tempted to try still another variant of my par file (which is close to the real thing regarding the figures), only scaling the size up (and the dependent parameters). I still feel that my second par file is closest to Max's model and shows much of it's real behaviour. So I try to prove that a lighter model wouldn't be so c/g sensitive and could be controlled easier. ;)

But we seem to have revealed a weak point of Reflex. In the other thread ('preparing' this one) we discussed differential thrust in Reflex. It was evident that it handles multi-engine configurations differently in mod and par file. The mod file not only renders several props correctly (though only right-turning and synchronous) but even gives the moment arm for yaw moments due to breakdown of one engine.

The par file contains (as in most other sims too) only aggregate parameters for the whole drive. It was no real problem for us that there is no propwash spin though it would help a lot just for the V-173 (to counteract the wing tip vortices and reduce induced drag). But we supposed there is a propwash at all hitting all parts of the model behind the prop disk.

Now obviously aileron effectiveness of the V-173 model only depends on flight speed, not on propwash. There must be an effect of propwash on ailerons in Reflex, even depending on prop diameter given in the mod file. Any one of the many 3D fun flyers shows that since it couldn't be torqued otherwise. I suppose Reflex simply forgot to distribute the propwash to multiple props and simulates a single centerline prop.

Now we need a sticky thread for our project, you wrote. Seems to me we could continue endlessly and would stay on top of the threads list automatically. If we would run out of unexplored points of Reflex they might have a new version and we could start over again. :D

Burkhard

[max z]

How about................. :) :)

[HankF]

I suppose Reflex simply forgot to distribute the propwash to multiple props and simulates a single centerline prop

It seems to me that Reflex is in fact taking the dual prop thrust into account since a damaged prop makes the machine yaw and/or spin the way you would expect. As to whether there are dual prop wash effects, that would be hard to test for but since XTR has been upgraded to reflect more accurately 3D flight (high alpha) it may be doing a fair approximation. The ailerons remain effective at very low speeds so the prop wash must be affecting them.

BTW I sure want to thank you guys for such an interesting exercise and resulting model.

Hank

[Daniel11]

BurkhardE, how on earth do you create viritual models for Reflex? Could you make a Slowstick?


-Daniel

[BurkhardE]

BurkhardE, how on earth do you create viritual models for Reflex?I can't explain, it just happens :D (it's a long story, see just this thread).

Could you make a Slowstick?Yes, that even happened! Look here (http://time.fh-augsburg.de/~erd/Modellflug/textDownloads.shtml) ! (looked at my avatar?)

Enjoy it!

Burkhard

[Daniel11]

Is the program to make planes free?


-Daniel

[BurkhardE]

Is the program to make planes free?Yes, it is. And there will probably be an English tutorial soon.

[Daniel11]

Link please....


-Daniel

[BurkhardE]

:D
Max, where do you get all this strange configurations from? That would exceed all limitations! ;)

(I know it's a silly question, but it's suited to let the game go on...)

Burkhard

[Daniel11]

BurkhardE, could you post a link to download RMK?


-Daniel

[BurkhardE]

Link please....
The program directly from Reflex (http://www.reflex-sim.de/) (in the shop, but no pay), the English tutorial is not yet available.

[Daniel11]

When will the english version be available?
I have DSL so I started the download, but what is the file size?



-Daniel

[BurkhardE]

When will the english version be available?
I have DSL so I started the download, but what is the file size?Don't know both, sorry.

[BurkhardE]

Hank, just tried and found out about the ailerons. You defined the ailerons nearly full span so they are hit by a centerline propwash in any case. In my par file ailerons go from 50 to 100% span so they are outside the propwash.

I assume Reflex takes the prop diameter from the mod file. Some time ago, I tried with other models and managed to lift off nearly vertically, without ground roll, only by propwash generating lift at the inner wing part. Since then I no longer set the real propwash speed for tail (and aileron) effectiveness, but a lower value instead (as the Reflex people do).

The 12 m/s for the V-173 were an exception from this (private) rule to test for lift. But Reflex seems to assume an untilted propwash flowing around the wing symmetrically, giving no lift with the 0015 airfoil. Ground effect seems to be missing, too (even with flaps down).

And thank you too! (for the motivation, ...)

Burkhard

P.S.: Found a solution for the 'changing model problem' you described. When I used your model, stopped simulation and selected my smaller model (F2), the smaller size was not rendered. By mistake I hit F11, then simply Enter, that helped!

[HankF]

Burkhard,

Crashing it also forces a rerendering but it's not as elegant. On a slightly different matter, can RMK handle twisted polygons (points not coplanar)? If so, how much twist can they have before rendering becomes a problem?

Hank

[BurkhardE]

Hank,

That's difficile. I think twisted polygons are no real problem, but they often look nasty. It also depends on the order you define several adjacent polygons. And if you managed to let it look good it often looks bad on the other side of the model after mirroring. I use to play with the rounding of edges (Gouroud shading). And try the Shift-Tab key to tilt the polygon vector! But most times I end up using only untwisted polygons, mostly rectangles (maybe a little bit twisted), and triangles. It's all a matter of trial. (One exception: propeller blades. Due to their built-in twist they are no problem at all. :) )

Burkhard

[max z]

:D
Max, where do you get all this strange configurations from? That would exceed all limitations! ;)
Burkhard, this was based on a vague memory, later confirmed (after designing it), of an Australian design, the "Stratos". Flies very well, and was published in the UK mag. QEFI.
http://www.rcgroups.com/forums/showthread.php?t=36230&highlight=diamona
describes it all.

Max.

[BurkhardE]

Max, the model looks even nicer if one knows it flies well, and nice to see you! :)

You would have managed to fly also the pancake, not too slow on the reflexes (though it would help a lot to practise in Reflex ;) ). I tried some parameter changes and found out that most problems should be 'normal' ones. Maybe I'm exaggerating, but I feel now we have about 90% realism in Reflex.

Due to low moment of inertia and short tail moment arm the model is pitch-nervous. When I put the c/g far forward - what means only a few millimeters - it was much easier to handle. Correspondingly the 'decalage' (tail angle of incidence) was set to -3 deg. Now the model is neutrally stable at full power/speed (strong drive provided). At lower power/speed you have to apply elevator, of course. It's still possible to pitch up the model to ground attitude, at least with some power.

As expected, slow flight is difficult due to high wing loading. As every high-load low-aspect-ratio airplane, the pancake flies like a brick when slowed down. It rapidly sags and pitches up even higher. You have to briskly apply down elevator and/or power to regain control, if that is possible at all (the model might also tip-over).

It helped to set a combi mixer (rudder with aileron) in the transmitter. The workload was low enough for me so I managed to fly around, do some aerobatics, fly slowly (though shaky), and touch down in three-point attitude (avoiding nose-over), even though with some power. Max, you could do more than that for sure. :)

Hank, as you already revealed the V-173 is way easier to handle if wing loading is low. To prove that again I changed my latest parameter file (just described) once more (both attached). The areas are doubled, so distances and moments of inertia are multiplied by 1.41, whereas all airfoil and damping coefficients stay unchanged. Weight and drive are unchanged too, wing loading is halved to parkflyer value of 20 g/dm2.

This parameter set flies nearly as nice as yours! I feel confirmed that low aspect ratio must be combined with low weight or you really need a master pilot (at least a younger one ;) ). Now even I manage to land the thing smoothly! :D

I had to try this since I feel the parameter set is as close to the real thing as possible except wing loading. Hence that unexpected feeling of realism!

Burkhard

P.S.: The real V-173 had a very low wing loading of 36 kg/m2. Even dear old Cessna 150 has 45 kg/m2!

P.S. 2: The zip file (http://www.fh-augsburg.de/~erd/privat/Modellflug/download/V-173Reflex.zip) is updated again.

[HankF]

Burkhard, a couple of questions.

Where does XTR get it's propeller information (diameter and location(s) on the airplane).

Where does XTR get it's tail aspect ratio information (elevator and rudder)? I see values for area but no dimension inputs.

Where did you set the span for the V-173 in the .mod file (disk or ailevons)?

I see you want to set the wing chords at less than the span. Also you have a sweepback. Do you have some logic for that?

Where is the nose of the V-173 in relation to the wing leading edge? How does a person set the CG in relation to the wing?

I have others but they can wait.

Hank

[BurkhardE]

Hank, let me reply in list form too: ;)

Diameter (if at all) and location of props (sure) are from the mod file.

Reflex simply assumes a tail aspect ratio as it does with tail airfoil.

Span is of the main wing, not ailevators.

Chord is set so the area of the tapered or rectangular Reflex wing is the same as that of the real model's elliptic wing. The sweepback helps with a more stable flight behaviour, and I would like to have more of it. Alas, the ailevators are only good for less than 2 deg equivalent sweep (estimated with Plane Geometry).

The airplane's nose doesn't matter in Reflex, only c/g position. It's related to the leading edge of the 'virtual' wing in the par file. There are only few words about that in the Reflex help file. I think they use a 'mean' leading edge of a tapered wing, since they explicitly mention that for a swept wing (but I'm not sure about that).

You might locate the par file c/g position in the mod file too (by calculation with the ratio, relative to wing aerodynamic center, and considering wing planform) and would find good resemblance to the c/g position of the real model at least for standard configurations (my experience).

Burkhard

[BurkhardE]

Hank,

I improved the landing gear (at least I think it's an improvement). Now the spring strut is longer, the stiffness is lowered. I adapted all par files, yours too. It's all in the zip file (http://www.fh-augsburg.de/~erd/privat/Modellflug/download/V-173Reflex.zip) updated again. You might also take a look at the V-173.txt file. Shouldn't we give the model to the public at RC-Sim? (I prepare for that, but for now we are not yet finished with the project.)

Burkhard

[HankF]

Shouldn't we give the model to the public at RC-Sim?
Certainly, when we agree we're finished.

I've included a plan sketch of my latest attempt superimposed on your drawing. I hope it is clearer what I've been trying to do. We are getting pretty close since these modifications haven't changed the handling much. Pitch sensitivity is up a bit.

Please note that I've dramatically increased the top and front view drag areas and the stabilizer incidence to -2 degrees. The incidence was added just to trim the machine at 1/2 throttle.

The machine comes pretty close to being able to fly at full power at a high angle of attack.

BTW, improving the landing gear was really needed. I hope it doesn't crash as easily (haven't tried it yet). (ed: I did try it and its much better).

Please delete my two .par files from your .zip and add my latest below.
Your -4 par file and mine fly very close to the same except for the added drag in my version. (I'd sure like it if you'd go to the 1 meter standard wingspread.)

Hank

[BurkhardE]

Hank, download doesn't work, says invalid file. :confused:

[BurkhardE]

Hank, that's amazing! :D The directional stability is so good I can manage it to touch down like a feather. I didn't dare to set more sweep since the real V-173 hadn't any, but it certainly had gone well with.

Now I understand why you asked the questions. You could be even more unconcerned about the parameters. Since they are all abstract, aggregate parameters it's even possible to define a horizontal tail overlapping the wing, as I did. Reflex obviously doesn't notice. ;)

So you could also stick to the 'old' ailerons over the whole span. It's funny that you changed to 50-100% ailerons and I the other way round. Now your model is a bit roll insensitive while mine may be even too sensitive (but I reduced deflection).

I don't think we need to set same span to compare. Our versions are so close to each other in behaviour that I think it's the wing loading what makes them comparable (and geometric similarity). But I wonder why you increased drag?

I reduced elevator travel in my par file since I found it too sensitive and the model couldn't be pitched up fully anyway without power. (Tried 'loops' with full elevator? Looks funny!)

The landing gear is much sturdier now, though not indestructible. If it crashes too easy increase 'spring rate', that should help now.

I added all our par files to the zip only for 'historical' reasons but will of course delete the older versions if you prefer. And I'm waiting for agreement on a single common par file called V-173.par. The txt file is in preparation for the 'rollout' of the model. Any suggestions for the content?

Burkhard

[HankF]

But I wonder why you increased drag?
The drag areas we had been working with weren't even close to what is actually there (unless I don't understand what Reflex is refering to). What it does is force more power for flight so the model ends up at a much higher angle of attack. Max said the real thing could be fully controlled at 45 degrees. We have not been able to come close to achieving that previously.

The sweep back in my latest model relates to the leading edge and trailing edge of the ailevons. It is the closest I can come to duplicating the real airplanes plan form.

It sure seems to fly realistically and aileron planform doesn't seem to matter much.

Too bad Max doesn't have XTR, we could have him decide which is the best .par file (even though his experience was limited). Of course I like mine, naturally. Maybe you know somebody who might be familiar enough.

WOULD ANY OF YOU ONLOOKERS LIKE TO HELP US DECIDE?

Hank

[max z]

Too bad Max doesn't have XTR, we could have him decide which is the best .par file (even though his experience was limited)
Not likely, flights were too short or too erratic.
Max.

[BurkhardE]

WOULD ANY OF YOU ONLOOKERS LIKE TO HELP US DECIDE?Silence... :(

Seems heli-nek was the only one who downloaded our par files at all (regarding the download counter). But do we really need help to decide? I don't think so, since we are not yet finished anyway.


The drag areas we had been working with weren't even close to what is actually there (unless I don't understand what Reflex is refering to). What it does is force more power for flight so the model ends up at a much higher angle of attack.
I seem to be a bit dense, I always need some time to understand your intentions. Hope I got that now: The high drag prevents the model from being 'pulled up' by the prop, while there is still much propwash for more lift and better control.

I tried to estimate the drag wit the A.C. Calculator program. In the top view, there are only the black prop shaft tubes/fairings and spinners. In the front view, there are the spinners and the canopy. All other areas are wing or tail and don't belong to the 'detrimental drag areas'. That is an inappropriate translation from German, may be it should better be 'parasitic drag'. It is an area perpendicular to airflow multiplied by a drag coefficient, hence the name 'area' (and unit m2). It's applied to fuselages, nacelles, struts, and other parts of the airplane which only produce drag but (nearly) no lateral force like lift. There are typical drag coefficients for certain parts, e.g. 0.3 to 0.4 for landing gear wheels without pants.


Max said the real thing could be fully controlled at 45 degrees. We have not been able to come close to achieving that previously.True, but seems we were only one step before! I tinkered with the parameters and perhaps found out how to achieve (as far as I understand).


The sweep back in my latest model relates to the leading edge and trailing edge of the ailevons. It is the closest I can come to duplicating the real airplanes plan form.

It sure seems to fly realistically and aileron planform doesn't seem to matter much.O.K., but I still believe the wing is completely unswept (see the thin red line in the drawing). For the Reflex model, sweep is important to increase directional stability. But now it seems to me we had no idea where instability comes from.

It's simply the wing stalling! I tried with your model first, since a swept wing even tip stalls, and set substantial downwash. That helped, but not completely. Still the model tended to go sideways at high pitch.

Then I checked my theory regarding propwash (that Reflex simulates prop position from mod file, but only one centerline downwash with diameter from mod file and speed from par file). I had to change the mod file to one single centered prop with 1 m diameter to cover the whole span. (Collision is no problem since the prop object was not enlarged, only prop disk diameter.)

Now the whole wing is hit by propwash, ailerons are efficient, lift is increased, and high-pitch flight is (nearly) stable. Additionally, I set pretty much 'decalage' (down horizontal tail) and forward c/g to increase longitudinal stability. But c/g is so much back that the model flies at high pitch automatically. I had to reduce the tail airflow speed from (real) 12 m/s to (well-tried) 7 m/s to avoid too much lift.

I did the same with my model and found no real difference, so the whole wing planform doesn't really matter. When applied full power, the model takes off in high-pitch attitude without roll (the V-173 is designated V/STOL) and will establish on a high-pitch horizontal flight when power is reduced properly and smoothly. Even in glide the model will maintain it's high-pitch attitude when supported with some elevator (and go down like a leaf). So the big single prop may be not absolutely necessary.


Too bad Max doesn't have XTR, we could have him decide which is the best .par file (even though his experience was limited). Of course I like mine, naturally. Maybe you know somebody who might be familiar enough.Maybe, but we're just at the point where Max's experience was limited. We are adjusting the flight behaviour to our will, and we're across the limits of Reflex. Maybe we shouldn't decide at all.

Reflex (as the other sims, too?) obviously doesn't render the propwash effect on wing airflow correctly (since it doesn't allow for really high pitch of about 45 deg, stall will come before in any case). We're observing an approximation, I think. The approximation isn't bad at all, but we need the single prop mod file and that looks really bad. Are only wing downwash, big decalage, forward c/g, and sweep good enough for your practical purposes? Or do you intend to build a model of the V-173 (like Max) based upon our experiences, and would the single prop version do well?

Next step in our argument! :)

Burkhard

P.S.: For testing you need the new zip file (http://www.fh-augsburg.de/~erd/privat/Modellflug/download/V-173Reflex.zip) containing the single prop mod file and both par files (my 5th, 'your' 4th).

P.S. 2: Maybe we (at least me) overlooked or misinterpreted the ailevator deflection seen in the pictures. The pitch of the machine is not so high but deflection is substantial. That doesn't mean the ailevators gave down lift, only they gave less lift than the wing (relatively). And it's the same now on our models, high deflection even in normal flight! Are we on the right track now? (And should we render the 'default' deflection in the mod file?)

[HankF]

Burkhard,

I have to say that the single propeller models don’t impress me as an improvement over the twin models. Maybe you could put a “phantom” prop on it and leave the others visible. I don’t see that flyability is improved. Not only that but it's introduced more complications, for example, wing washout. I've been trying to keep as many parameters at 0 as I can. I've even abandoned defining the flap in the .par file as being basically unusable.

I’m trying to reconcile our twin prop versions. Below are the plan-forms we’ve been working with (the latest twin rotor .par files.). What I’ve found is that sweep doesn’t make much of a difference if you keep the CG in proper relation. For example your 2 degree sweep requires an 18% (of wing span)CG where my 9 degree of sweep requires a 25% CG. There is also a trade-off of elevator area and tail moment arm and CG. Your extreme forward CG and short tail moment arm force you to use a large elevator area. And lastly, I set my aileron chord and span the same as yours (and what I had originally) since I couldn’t tell any difference.

As far as choosing one over the other (since they are essentially equivalent flight wise)? I think the ailevon area should be included in the wing area for Reflex. That’s the reason for the 9 degree sweep-back. Also, my plan-form is a little closer in length to the actual machine. I’m not sure that makes much of a difference though. I’m trying not to be stubborn as is my inclination (must be my Germanic heritage: last name is Fritze).

The problem with our models (both single and twin prop) as I see it is that they seem to float too much (if the values for parasitic drag are as you have them set). It doesn’t seem like Reflex handles the lift generated by the props very well. I’d like to see the model settle gently to a landing at just over idle power with an attitude of about 22 degrees. Is there some drag source we are missing?
(And should we render the 'default' deflection in the mod file?)
I think the ailevators provide some of the lift of the wing (as I think you said) so, no, I think they're set just fine the way they are.

[BurkhardE]

Hank,

I surely agree to abandon the one-prop trial, it's useless since Reflex doesn't simulate the lift effect anyway. I feel we agree too that wing planform (aspect ratio, sweep, ailerons) doesn't really matter. But I'm again surprised that you have a completely different impression of flight behaviour. (Two stubborn Germanics is too much... ;) )

I'm very happy especially with sweep now and I feel the model floats exactly as it should. As I now adjusted the parameters, at idle power the plane glides slowly without elevator input. You may flatten the glide with a bit power, but she will speed up then. With a bit elevator she slows down, and descent rate increases a lot. That's what I meant with the falling leaf, the steep glide. I wouldn't call it float! There is not enough energy in the model for landing so it would plop down. Apply elevator and power (both very little) at the same time and it will settle gently and roll a very short distance.

So we 'need' no drag we could be missing. I think the designer of the V-173 had drag reduction in mind when choosing a flying wing configuration. The main drag we are working with is induced drag. Slow the model down and it will glide steeply, a short push on the elevator will flatten the glide and speed up.

Meanwhile I tried several times and generated an additional mod file with deflected elevator (8deg) since I'm so wild about. It's in the zip file now. But should we also exchange dmo files of our test flights?

Burkhard