Help with new plane design

[evermax]

Hi all,
I am working on a new photoplane and have a pretty extreem nose down when I power off, Once trimmed with power off it will fly OK, but it is a bit of a handfull.

I moved the cg back a bit and it seemed to fly a bit better but still has a major nose drop when I power down, and nose up way up when powered on.
When trimmed for power off, a 45degree dive test tells me it is still a bit nose heavy but not much.
I originally flew it as a pusher with camera weight it was like 60oz.
And it flew like 60oz ... BAD, I have got it down to around 30 to get the airframe setup and will eventually add another 10oz of a/v gear.
It is no longer a pusher but I may revert once I get it figured out a bit.

Main problem is landing, it seems to drop hard and elevator that is sensitive at speed is very unresponsive when time to flair.
Flaps are taped off and not connected.

Im posting so see if anyone sees anything I am not. Im rebuilding the tail to make it lighter and 110degrees.

I come from a sailplane building background and have limited thrust knowledge.

Here are some pics.

Wing Span 56" wing area 654 Sq. in , standard ClarkY (not sure what the %s mean)
Powered by a PJS800 and cc35 ESC /3s1p Lipoly
wt approx 30 oz.
Wing mounts are GWS SS parts, Leading edge mount is 3/8 higher than the trailing edge mount. I used this same setup for the center motor pod. So that it is very similar to the SS behind it.
I have flown the wing on the standard SS and it flys fine but still seems to land hard at slow speed. Balancew points were 3 1/2-4" on a 12" cord.

Thanks for any help.
Evermax

[hul]

it seems to drop hard and elevator that is sensitive at speed is very unresponsive when time to flair.
sounds nose heavy to me. Where is your C/G?

Hans

[evermax]

Hans
Balance point now is 4" back from leading edge on a 12" cord.
33%.

[Sparky Paul]

You're probably flying too slowly, and stall it when flareing.

[evermax]

Yes I do probably land it too slow, which is good for me because I can fix that, but what about the nose down when motor off at higher speed, is that a balance issue?
I thought so, but I thought 33% was about right. Maybe not.
I guess I'll keep moving CG back 1/4" at a time and see how it does.
Also when I do the 45 dive test should It be with or without motor on, or does it not matter as long as it is trimmed for level hands off flight when I do it? I had the motor off when I did it the other day but it might have been old sailplane instinct.

Sir
Thank You
Sir
:)

[hul]

I thought 33% was about right.
the formula in post #12 of this thread is said to give a reasonably accurate CG position: http://www.rcgroups.com/forums/showthread.php?t=339401
Dive test with or without motor, but it must pick up speed

Hans

[Sparky Paul]

The descent with power off is an indication of too much drag.
Aerodynamically, that is a "dirty" airplane.. it requires more power than a smoother design would use, and can't glide as fast as it should because of the drag.

[theKM]

I don't agree with anyone :)

Balance the plane so that in a glide with the power off and centered control surfaces it's nice and steady, without wanting to stall. This means that the airframe is now set up to fly, everything else becomes the application of power.

Now, add down thrust to the motor so that when you power on, it's not wanting to pull a loop or pick its nose up in a drastic way. A slight want to pick its nose up is OK, but you more or less just want it to power up without any pitch changes.

Now if it's wanting to torque around into a turn when you power up also, you will probably want to add side thrust in the other direction.


...with this done, you've satisfied three things:
1: the airframe and blanace is set up so the plane can fly
2: thrust is set up so it simply provides forward thrust for the plane, and the plane can then do something with it.


The plane in gliding mode really is the best judge for setting up the airframe. This is why Otto Lilienthal was the real pioneer of flight... he was gettin it done in the late 1800's way before anyone else.

[fhhuber506771]

I disagree with all of the above... (almost)

With the CG at 33%... I'd hesitate to go further back. I HAVE had CG as far back as 43% on my Goldberg Tiger 60... but I was correcting the flight characteristics for Pattern maneuvers... and used a LOT of expo on elevator to cure the excess pitch sensitivity. this is not something you want to do on a regular basis. (and your model would be a beast to recover from a spin if the CG is too far back..)

I believe the angles of incidences are off.

Looking at the pictures: I THINK I am seeing a significant angle of incidence in the wing mounting. It may be too much. That could cause 'balooning" under power which if trimmed out would cause the nose to suddenly drop on loss of power.

Aerodynamicly "dirty" models loose speed fast requiring the nose to be lowered to maintain speed... if they need speed...that looks like a very slow flying model. The slower the model will fly.. the less it matters if its aerdynamicly dirty. Power needed to maintain any speed is a function of the cupe of the speed. fr the same model to maintain 10 mph requires 1/8 the power it takes to maintain 20 mph. IF the model maintains the same AOA of the wing...(which would result in a climb at the higher speed) A model that "cruses" at 10 mph has 1/8 the effect from adding any dragy item (like rubber band wing mounts in stead of bolts) as a modelthat cruises at 20 mph.

[evermax]

THanks You for all your input!!
Will be testing today.
I have set up so that I can easily adjust cd and have made a number of shims to alter the incidence angles. I will start with a trimmed glide dive test, and then check the cg after happy with it Ill trim and test cg under power.
The fomulas hul pointed out for cg (TY) tell me CG should be 3.75 on the 12" cord so I am in the right ballpark as far as the numbers.
I will then alter the incidence using shims to note changes.
Right now the pusher motor thrust line is in line with the wing incidence.

Ill let you all know how it goes.
Thanks Again for all you inputs.
Evermax

[BMatthews]

....Balance the plane so that in a glide with the power off and centered control surfaces it's nice and steady.....

This assumes that the built in incidence angles are optimized to begin with. A perfect occurance that very seldom happens. Adding elevator trim is a perfectly valid way to compensate for the poor guesses made on the design table. As such this statement is not really correct. But I agree with the rest of the trimming info.

Sparky has a part of it for sure. It IS a very dirty model and the drag WILL make it so that it needs to have a rather steep glide slope to maintain the flying speed needed even though the speed in this steep glide is probably on the slow side.

I also see a model with a very short tail moment and wide wing chord. in conjunction this is a problem. Wide chords should have large stabs and longer tail moments so the stab has the leverage to control the wing's instability. Your stab is large enough but your tail is too short. I'd make it about another 4 to 5 inches longer and it'll probably help a LOT.

Beyond that I suspect you've trimmed the elevator for power on and do not have enough, if any, downthrust. This probably means you've trimmed the elevator to offer level flight under power and it's quite normal to have it dive into a relatively steep glide angle when you shut off the motor. The model is trying to reach the trimmed flight speed and fly at the same speed in the glide as it did under power. To force it to fly at a slower speed you need to add some up trim. The model will then tend to fly at the new speed based on the elevator trim. Adding or removing power (within limits) will alter the climb or glide but the model will try to stay at the same speed within a range of about 10 degree nose down to nose up. Adding downthrust is a "trick" that let's you convert some of the built in power related pitch response to airspeed by reducing the nose up auto-speed response and converts some of the power into airspeed instead of climb.

[theKM]

Please throw "it should be 30%...." and "I like 40%" comments out the window. As well as "I had to correct for pattern maneuvers". They are chalk and cheese, and don't address the aircraft that you have built.

...point one... you need to trim your model. It will tell you everything you need to know. Get the model trimmed and blanaced so that it will glide as well as it can without stalling (i.e. a nice glide angle). This is how your plane, in its current setup wants to fly, including all the drag, wing size everything. A happy gliding plane is a plane that's happy to fly.

...then fix the thrust so that it doesn't baloon or anything when you apply power. With this correct, you will now have a nice flying plane! It wont balloon when you lose power.... why?... because you trimmed it to glide properly before you trimmed it for powered flight! ....it wont nose down when you lose power... why?.... because you trimmed it for a proper glide without power! ...it wont ballooon or nose down when you add power... why?... because you've trimmed the motor after it was flying correctly so that it wouldn't do this.

This is the basic process to get a nice flying plane, of any discipline. You can use this to trim free flight models, or high powered RC models. It doesn't matter. Trim the plane how it wants to be flown. You know why they made little gliders of SpaceShipOne?... so they could have a sanity check and see how it was wanting to glide, to confirm the CofG and things like that. They did glide test after glide test... and then they fired the rocket. Because in any bad power-on situation (and this goes for your plane too), you can cut the motor and know that you have a nice gliding plane ready to be flown.

...and then people can say "oh, but my aerobatic plane has all these different requirements"... this is because aerobatic planes are unstable. But you could trim them all for "stable" flight using the above procedure.

[theKM]

This assumes that the built in incidence angles are optimized to begin with. A perfect occurance that very seldom happens. Adding elevator trim is a perfectly valid way to compensate for the poor guesses made on the design table. As such this statement is not really correct. But I agree with the rest of the trimming info.
...sure, but when you have a bit of trimming experience, you can fiddle the other things. My description is for a simple process that will work. And it does work.

Sparky has a part of it for sure. It IS a very dirty model and the drag WILL make it so that it needs to have a rather steep glide slope to maintain the flying speed needed even though the speed in this steep glide is probably on the slow side.
absolutely. But my description is for any glide that does have it wanting to stall... whatever glide angle this takes. It may be steeper than others... doesn't matter in the least. Trim for the glide angle that has this beast on a steady glide path.

[theKM]

...penetration may be improved by removal of wing incidence.

[Sparky Paul]

There's nothing exceptional about this modified Slow Stick.. other than the enlarged wing, which is also thick.
The angle it's gum-banded to the fuselage rails is normal for the type.
What it is looking for in the steep glide is a balance between the lift available, and the drag.
The drag comingfrom the large area, thick wing, and combining with the low weight place an upper limit on the glide speed.
At the incidence it has, the descent in the glide can only be cured by adding incidence. Which will slow the plane down, with a better relation between wing incidence and tail incidence/trim. But the penalty is the plane will be speed-limited. The maximum speed in the glide will go down, and make the difference between glide speed and stall speed even less.
And curtail the ability to fly in any wind.
Upwind penetration is always nice to have, to bring the thing back from downwind.
A slow gliding plane can't do that.
Going to a different wing.. still cambered, but not as thick as a Clark-Y.. which tend to run about 12%, to a 9% thickness.. taper the outer panels.. no need for all that stuff out there. Tapering improves the efficiency anyway..
With the current plane at level 1, a thinner wing at the same angle to the booms/tail, tapered, should up the efficiency to 2, or at least 1.5. And the wind handling will be improved.
When the a/v equipment is added, the speed in the glide will go up.
The power needed to fly will go up.
This is something to be worked out; the flight regime for the imaging, power on or power off will determine which needs to be worked on, the climb performance, or level flight-powered, or level flight-glide.
Climb shouldn't be penalized to optimize the level flight. Use trim for climb, as altitude is usually a nifty feature for AP. :)
Need more climb, add a cell or two.
The trim between powered level flight and power-off level flight should be worked out with incidence, if it's important. Trim usually works there also.

[adam_one]

...Main problem is landing, it seems to drop hard and elevator that is sensitive at speed is very unresponsive when time to flair.
...
That's due to the short tail moment. Try using dual rates or increasing the tail arm.
It's normal for a powered model to dive when the power is cut off, how fast it dives depents on several factors, such as wingloading, airfoil type, thrust angle, how draggy it is, CG location and so on.

[hul]

..get the model trimmed and balanced so that it will glide as well as it can without stalling (i.e. a nice glide angle)...
agreed. But this does not give you one "correct" combination of CG and incidence. CG can still be moved around if incidence is adjusted to suit. Moving it back (and reducing incidence) will give less need for down thrust, less trim change for a change of speed and less stability. It's a matter of taste.
...then fix the thrust so that it doesn't balloon or anything when you apply power....

agreed

Hans

[Sparky Paul]

Looking at the photos, the problem is the use of the Slow Stick incidence fixture.
With that excessive (for this purpose) incidence, AND the incidence in the new wing, the plane has way too much longitudinal dihedral.
Take out the front incidence blocks and put the wing on the bar, with the rear bottom part parallel to the bar. This will put the wing at about 1 to 1-1/2 degrees incidence, a normal setting for a Clark-Y.
It will fly faster with the same power, and won't drop out of the air due to the excessive trim drag.
When I did the "Anna-Nicole" version of my Yard Stik, I dropped the kit incidence 50 %. With the full fuselage (hence the name) and the same power it flew faster and better than the bare-bones YS carrying the camera out in the breeze..

[evermax]

WOW Im so glad I posted my question here!
Everyones input has been extreemly appreciated.

Almost windless, except for the thermal fill.

Todays testing went very well, by end of day she was flying better than I have ever had here fly.
I set her up as a pusher and added 7oz to simulate camera drag/weight.

After 3 divetest flights and adjustments to the cg (motor off) she dove at 45 and stayed right on line. The CG is at 4 1/8" back from leading edge.
With motor on she rose just slightly.

I tried a couple flights adding shims to the trailing edge, I did not notice any improvement in the power on power off trim differance. And a few times it felt is if she was sinking faster during glide, but there were thermals so I may have been in some sink.

I setup the flaps for about 40 degrees and tested in the air using down elevator via stick to compensate. First test without compensation put her straight up as expected. She leveled at approx 1/3 down elevator and I will mix it in for next round. She landed well with the flaps, and the other landings were fine without flaps as long as I kept her moving.

So the main issue I need to work on is the trim differance between power and glide. So shimming the center pod to alter the engine thrust angle will be next.
Right now the center pod is mounted with same size SS mounts.
As a pusher, Im thinking of adding shims to the rear, adding a bit of down thrust and flight testing, I also will slide the wing forward on the booms to increase the tail moment from 22 to 26" and retweak the CG as needed.

I am also going to cut a new wing with differant airfoil. I have flown the tripped eppler214 on sailplanes and love it. Best airfiol I have ever flown was a Weston Aerodesign Custom used on the 3 meter MAGIC sailplanes, but not sure it would be good in this application, since a super glide ratio is not what I'm after. Short take off and short landings are what I need for photography since I don't always have ideal locations and conditions, catchable hence the pusher which was common when I would show off with my 3meter glider.


Ill be vacume bagging the new wing platform, and open to any suggestions on a good airfoil to replace the stock clarkY. I see lots of clarky variations but not sure what the 10% thickness means, Is it reduced 10% in thickness?
Either way Ill be cutting templates and cores and bagging with very light glass.

One pick shows the "warped via tape" washout. Ill be cutting this washout into the outer panels and reducing it Alot, Im used to flying without any for drag reasons but figured Id bend some in for the design tests. I tried to tip stall it and I could not. It might be a contributing factor in the dirtyness.


http://www.rem-studios.com/About%20Us/McKeever99/blaster.GIF
FYI here is my tripped E214 scratch design, Glass fuse, bagged wings.
Full camber control,

Below are some pics in its post flight configutation.

THanks Again
Evermax

[evermax]

more pics

[hul]

but not sure it would be good in this application, since a super glide ratio is not what I'm after. Short take off and short landings are what I need
you need a high lift airfoil, which means high camber, and one that tolerates low Reynolds numbers well. Can't see much wrong with the Clark Y, why not just stick with it?
More drag helps with the landings, it makes the glide steeper. That helps on a small field with trees or buildings around.
I see lots of clarky variations but not sure what the 10% thickness means, Is it reduced 10% in thickness?
reduced to 10% thickness, the original Clark Y is about 12%. Less thickness means less lift though, not what you want.

Hans

[fhhuber506771]

Based on the new pictures:

IF.. the rudder-vators are at thier neutral position in those pictures... I THINK they look like they are drooped a little.

That is an idication of one of 2 things (maybe both) CG back towhere you need a lifting tail. And/Or wing incidence higher than needed.

Since this is based on the views I have.. I'm partly guessing.

*****

BTW.. I mentioned the MISTRIM on purpose of my Goldberg Tiger 60 (moving the cg back that far is dangerous) as an example of how you can get the desired performance from a model that really wasn't made for the task. (A Tiger 60 really isn't a Pattern model... and needs major changes to do well at the task with the CG where it should be.) It also shows that yuo can correct for excess decalage (difference between wing and horizontal stab incidences) with CG.

I ended up with my elevator drooped kind of like yours LOOKS (to me, from here) now... because FOR MY PURPOSE.. the degalage (in my case stab incidence too far negative) was too much.. and I pushed the CG way back almost to the point of instability. (I have since put the CG back where it belongs and just use the model for sport flying.. and my elvator isn't "drooped" in neutral.)

[biber]

Evermax, I would recommend to use the clark y leaned down to 10% thickness (may be even 9%). At the low RE your model is operating that won't take away much lift but is less likely to suffer of laminar separation bubbles and thus may improve the handling/flight characteristics and overall performance (especially for windy conditions). Look at sparky paul's yard stick, it is even more thin airfoiled and that's certainly for reason ;)

biber

[Sparky Paul]

The trim difference between power and glide, is due to the excessive incidence of the wing.
Power on, you need down elevator.
When you cut the power, this pushes the nose down.
Once the wing incidence is fixed, the trim difference power-on to power-off will diminish or go away.
To thin an airfoil, find the percentage difference between what you have and what you want.
To thin a 12% to 9%, that's a 25% difference.
In your paint/CAD program, keep the same x scale for your airfoil, and re-scale the y-axis to 75%.
The result will be a 9% thick airfoil.
For the airspeed you're flying at, one will be as good as the next, for similar shapes.
To see where the front wing braces -are- needed on a twin boom SS, look at the White Knight Space Ship One thread in Aerial Photography. There he uses the stock SS wings, which are severely undercambered, and need all that incidence.
Clark-Ys don't.

[evermax]

Based on the new pictures:

IF.. the rudder-vators are at thier neutral position in those pictures...

The pics are not in a trim state. Propably should have shot them that way but Ill check them. TY

Paul
Thanks for clearing up the airfoil %, I imagine the clark y standard foil is 12% ( meaning 12% thickness (y) in relation to the 100% cord (X). It now make perfect sense.

I need some servo room so I'll swap out the rear ss mounts with a set of fronts and then test fly and adjust with shimms.

On the new wing Ill add some length and tapper to the tip panel and a may transition to a lower % at the tips.

Thanks Again

Evermax

[Sparky Paul]

Ever, I use a lot of standard 2M glider wings on my AP planes.. most of these are semi-symmetrical, with very little incidence between the wing and the tail.
Basically I setup the fuselage/pod-tail to be close to the conditions of the plane which donated the wing.
The incidence runs between 1 and 2 degrees most frequently.
The best wings I've built use the Selig 7035 airfoil. It's for long duration TD gliders, and adapts well to AP, with it's heavier than usual loads.
The GP Spirit Elite uses the Selig 7032, another very good shape, and quite useable in windy conditions when the higher drag Clark-Y isn't a good choice.
As the wind blows most of the time out here, upwind penetration is a nifty feature.. :)

[hul]

The trim difference between power and glide, is due to the excessive incidence of the wing.
Power on, you need down elevator.
When you cut the power, this pushes the nose down.
Once the wing incidence is fixed, the trim difference power-on to power-off will diminish or go away.
That's correct, but you can't reduce incidence without moving CG back.
To thin an airfoil, find the percentage difference between what you have and what you want.
To thin a 12% to 9%, that's a 25% difference.
In your paint/CAD program, keep the same x scale for your airfoil, and re-scale the y-axis to 75%.
The result will be a 9% thick airfoil.
Doing it this way reduces camber by 25% as well. The "standard" way leaves camber alone and only reduces thickness. Profili can do that for you.

Hans

[Sparky Paul]

"you can't reduce incidence without moving CG back."
??
C.g.s for the usual Clark-Y airplane, or any similar airfoil tend to cluster around the 30% mark, for average handling qualities when everything is proportioned reasonably.
Average handling qualities are what an AP airframe needs, so the pilot isn't paying more attention to the flying than the photographing, which is the purpose of the airplane.
Reducing camber reduces the pitching moment, which relaxes the requirement for incidence and tail volume, again a plus for a straight-and-levei airplane.
These are not finely tuned precision aerodynamic tools. They're somewhat crude tools (its father is a Slow Stick, not an F-104) to carry cameras with as little fuss as can be arranged.

[hul]

"you can't reduce incidence without moving CG back."
??

isn't reducing incidence the same as down elevator? That will make it fly down and faster.

This is how I come up with a CG position:

Practical approach:
plane has too much incidence; pitches up under power or when dive testing. Go and move CG back and fly again. You'll now find that down trim is necessary. Reduce incidence to remove down trim.
Or retain the high incidence (to retain the stability it gives) and use down-thrust instead.

Theoretical approach (somewhat simplified :) ):
To fly straight and level, forces and moments acting on the plane must be equalized.
1. lift must be the same as weight. This means a certain angle of attack at a given (or desired) flying speed in a given plane.
2. this angle of attack gives a certain pitching moment of the wing.
3. the CG is in front of the neutral point, that gives a pitching down moment
4. horizontal stabilizer's downforce equalizes these moments (all 3 add up to zero)
Reducing incidence alone will unbalance the moments (less down force from the stab). To balance them again you'd have to fly faster or move the CG back.

Hans

[Sparky Paul]

Hans, the plane is flying too slowly as it is.
This is due to the incidence from the Slow Stick wing mount cascaded with the incidence in the Clark-Y wing.
The climb under power most likely requires a fair amount of down elevator to keep the plane from stalling, due to the excessive lift from the incidence.
When the power is cut then, this down elevator pitches the plane down.
And power off, trimming the nose back up results in serious drag, due to the difference between the wing and the tail's incidence.
This drag in both conditions prevents the plane from flying at a reasonable speed. It's really close to stall most of the time.
Lowering the incidence on the wing will speed the plane up, reducing the amount of down-trim needed for power, and power-off, a more drag free glide.. more speed to manuver with before stalling.
Paul

[hul]

the plane is flying too slowly as it is.
Paul, guess that's a possibility. If that's the problem your fix will work, as I said 2 posts up
To balance them again you'd have to fly faster...

Hans