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Nov 21, 2004, 02:41 PM
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Thread OP

Reflex model scale question (Burkhard?).


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
Last edited by HankF; Nov 21, 2004 at 02:44 PM.
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Nov 21, 2004, 03:14 PM
Reflex sim model builder
BurkhardE's Avatar
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.
Nov 21, 2004, 05:39 PM
Registered User
CustomPC's Avatar
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.
Nov 21, 2004, 07:58 PM
Registered User
Thread OP
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
Last edited by HankF; Nov 21, 2004 at 08:02 PM.
Nov 22, 2004, 03:37 AM
Reflex sim model builder
BurkhardE's Avatar
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...
Last edited by BurkhardE; Nov 22, 2004 at 06:58 AM.
Nov 22, 2004, 10:06 AM
(aka Cliff Lawson)
Wright Flyer's Avatar
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
Nov 22, 2004, 11:00 AM
Reflex sim model builder
BurkhardE's Avatar
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? )

Burkhard
Nov 22, 2004, 01:44 PM
Registered User
Thread OP
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
Last edited by HankF; Nov 22, 2004 at 05:47 PM.
Nov 23, 2004, 06:48 AM
Registered User
max z's Avatar
I have TRIED to fly the real thing
Nov 23, 2004, 09:13 AM
Reflex sim model builder
BurkhardE's Avatar
Max, you tried, what was the result? (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
Nov 23, 2004, 12:49 PM
Registered User
max z's Avatar
Quote:
Originally Posted by BurkhardE
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 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.
Nov 23, 2004, 02:04 PM
Reflex sim model builder
BurkhardE's Avatar
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
Nov 23, 2004, 02:16 PM
Registered User
Thread OP
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
Last edited by HankF; Nov 23, 2004 at 02:22 PM.
Nov 24, 2004, 09:01 AM
(aka Cliff Lawson)
Wright Flyer's Avatar
Quote:
Originally Posted by HankF
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
Last edited by Wright Flyer; Nov 24, 2004 at 09:08 AM.
Nov 24, 2004, 09:08 AM
Reflex sim model builder
BurkhardE's Avatar
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


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