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Still new to ailerons, questions about stalls and how best to fly my plane and tweak the design?

Vimana89

Well-known member
#1
I've built and designed few successful RET planes, and flew them a bunch, but I really want to expand to ailerons and elevons and then to the occasional 4ch build. I have built one somewhat successful plane with ailerons in the past, but it had issues similar to the plane I will present here. This design is for the FTFC20, it was an old balsa control line model.
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I built a sloppy prototype with a weak two piece motor mount and electronics taped along the top. Good enough to test the handling qualities and if it will even fly. It flies pretty well, but certain times it just stalls and its difficult or impossible to recover. I noticed it is mostly when banking too drastically without enough speed. Is this normal for a double delta with ailerons or elevons? Is this bad piloting habits from flying very forgiving dihedral RET planes, and a normal limitation of this design, or is this something I can and should address with the build perhaps by adding winglets or some other small change?

Here is the footage. First vid is the issues, second is flying good.



I am in the process of building a cleaner version with a box nose and dimensions more perfectly resembling the original balsa plane. I want to get the build right and not smack it up on maiden. I've come a long way very fast, but I'm still really green when it comes to flying with ailerons. I'm self-taught, and have no real life club or anything that exposes me to new ideas or pushes me to try the next hurdle or trouble shoots my issues, so it's all my choice to push myself to the next step, and whatever inspiration and help I get here and ideas I'm exposed to on this forum. Any help, advice, and constructive criticism is much appreciated.

I'll reiterate my main question and make it clear: Is there a problem with the build or design that can be addressed to decrease or eliminate these stall characteristics, or is this just normal limitations combined with my bad piloting, and I should learn to fly this plane the way it likes, give enough throttle in banks, and not do the stuff that makes it stall?
 

Brett_N

Well-known member
#2
it's a slender delta. It wants to fly fast. Also, are you useing Ailerons or ELEVONS? Should really use elevons on something like that.

Post an actual picture too. To hard to tell what we're looking at from a video.

But I'd bet my 2 cents its either not flying fast enough or you are trying to turn to sharply. Deltas like this like big long patterns.
 

Vimana89

Well-known member
#3
it's a slender delta. It wants to fly fast. Also, are you useing Ailerons or ELEVONS? Should really use elevons on something like that.

Post an actual picture too. To hard to tell what we're looking at from a video.

But I'd bet my 2 cents its either not flying fast enough or you are trying to turn to sharply. Deltas like this like big long patterns.
Ailerons on this to keep their size down relative to the elevator and require less linkage perfection to not roll on climb. May try elevons on a future version. This is completely different than my V Sliver which is a straight slender delta rather than double, and has dihedral tips and RET instead of ailerons. That thing is completely stall proof and has a ridiculous speed envelope including slow flight and high alpha. I think you are right in that the double delta is more a high speed build, especially without the RET and dihedrals.
 

Vimana89

Well-known member
#4
Here's pics of the prototype airframe, the rebuild so far, and the V Sliver, which has none of the nasty stall characteristics on any speed while being skinnier. I think a straight delta has more symmetrical air flow at low speeds than a compound.
 

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Vimana89

Well-known member
#5
So there's probably a lot of minor stuff to tweak but I'll bet you are right. That was my first guess too, a combination of not enough speed and too drastic a bank on turns. She can turn pretty tight on high throttle with no issues but if I'm at a moderate cruise I should turn longer or throttle up. I'm just used to rudder and dihedrals making everything easy.

I'll adjust my flying style with this plane a bit, and that should do. If anyone has any other suggestions let me know.
 
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Hai-Lee

Old and Bold RC PILOT
#6
Firstly the original model was an elevator only, high powered, wing with an interesting shape but poor in lift. Roll and directional stability were maintained by the external control wires and the central anchor of the pilot.

It needs to fly fast aerodynamically as do all deltas and narrow span swept wing designs. Such designs arose out of research into air compressability and lift or efficiency was traded off for gains in speed Vs Drag.

Models do not scale well as the air they fly through does not scale! For a small delta to fly it needs 2 things, extremely light structure and speed. Banking in a turn will cause a rapid loss of speed due to the "High Alpha drag experienced in the bank. Without elevons and using a rudder to initiate the bank will provide an asymmetrical roll force which in turn will provide the roll required with a large amount of yaw, (See Dutch Roll).

Effectively your choice to use RET is causing severe additional drag and yaw when turning. A simple elevon setup with a fixed vertical fin would perform smoother and considerably better assuming you are able to provide enough speed for the planform of the wing to operate efficiently.

Whilst I understand you lack of experience with AET setups, (Yank and Bank), they actually fly far better and easier than RET setups across the board. Control is simple and more precise. to turn you cause the plane to roll to the bank angle required and apply up elevator to drive it around the turn. My students go from RET to AET without any confusion and most do not notice any real difference EXCEPT the loss of the severe Dutch Roll effects of a snap or sharp turn.

If you want to really test the chosen design I would recommend that you go larger, (around 1 metre in fuselage length), keep it light, (even try a simple KFM wing on a profile type prototype), and implement Elevons as the only control surfaces. As this may be difficult to launch without assistance I would recommend a simple ground bungee launcher which would provide plenty of airspeed for not only the launching for flying but also a good amount of speed to allow for glide tests close to the ground.

The larger size would alos allow you to have a power to weight ratio of less than one and still fly very well!

Just a few thoughts!

Have fun!
 

Vimana89

Well-known member
#7
Firstly the original model was an elevator only, high powered, wing with an interesting shape but poor in lift. Roll and directional stability were maintained by the external control wires and the central anchor of the pilot.

It needs to fly fast aerodynamically as do all deltas and narrow span swept wing designs. Such designs arose out of research into air compressability and lift or efficiency was traded off for gains in speed Vs Drag.

Models do not scale well as the air they fly through does not scale! For a small delta to fly it needs 2 things, extremely light structure and speed. Banking in a turn will cause a rapid loss of speed due to the "High Alpha drag experienced in the bank. Without elevons and using a rudder to initiate the bank will provide an asymmetrical roll force which in turn will provide the roll required with a large amount of yaw, (See Dutch Roll).

Effectively your choice to use RET is causing severe additional drag and yaw when turning. A simple elevon setup with a fixed vertical fin would perform smoother and considerably better assuming you are able to provide enough speed for the planform of the wing to operate efficiently.

Whilst I understand you lack of experience with AET setups, (Yank and Bank), they actually fly far better and easier than RET setups across the board. Control is simple and more precise. to turn you cause the plane to roll to the bank angle required and apply up elevator to drive it around the turn. My students go from RET to AET without any confusion and most do not notice any real difference EXCEPT the loss of the severe Dutch Roll effects of a snap or sharp turn.

If you want to really test the chosen design I would recommend that you go larger, (around 1 metre in fuselage length), keep it light, (even try a simple KFM wing on a profile type prototype), and implement Elevons as the only control surfaces. As this may be difficult to launch without assistance I would recommend a simple ground bungee launcher which would provide plenty of airspeed for not only the launching for flying but also a good amount of speed to allow for glide tests close to the ground.

The larger size would alos allow you to have a power to weight ratio of less than one and still fly very well!

Just a few thoughts!

Have fun!
Thanks, this clarifies some things. I'll finish up the cleaner mini build, it has ailerons just like the prototype. as long as I gave the prototype enough throttle when banking, it actually flew very nice if I didn't try to fly it like a RET plane, even with small size. I will be probably bumping up the size and trying a third version though, just with elevons. My V Sliver RET plane is more a fun flier like a Nutball or Flyer that gives you a slender delta feel and high alpha, but at the cost of roll maneuverability and inefficient yaw, but is easy and predictable to fly, stall proof, and has a big speed envelope with three blade prop and RET. It just can't axial roll or bank as graceful.
 
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Vimana89

Well-known member
#8
To get used to elevons, I may actually start by building a modified version of my V Sliver, I like the straight slender delta and high mount pusher setup, just have to switch to elevons and trade the dihedral tips for winglets. This design is meant for more powerful motors like my radial 2205, and the wing is longer while still being made out of one sheet. It's also easier to extend if I scale up. Split the elevator to elevons and move the VS back some, change tips to winglets, should be good to go👍.
 

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Vimana89

Well-known member
#9
Just to clarify, the prototype all wing wonder and the build in progress have ailerons. They are AET. My V Sliver is RET and I was just comparing, because it suffers none of the same low speed stalls. I'll be testing elevons on a modified version of that and then on the All Wing Wonder.
 

Vimana89

Well-known member
#11
The pane in this first video looks tail heavy to me.
Probably was by just a bit. Just a bit is enough with smaller airframes though. I didn't see a CG indicated anywhere I could find it on the original plan, and it looks like a complicated shape to calculate and I'm bad with those things, so I looked up people's builds of the Saab Draken, which has an almost identical wing, and used the CG from that as a basis.
 

Merv

Well-known member
#12
I have found SketchUp Make to be a very useful tool to figure out CG. When you have rough out line of the wing modeled, it is a mater of trial and error to find a point about 25% of the wing area. CG at 25% of the wing area has always been a good stable place for me to start. When I get the plane trimmed out, I will slowly move the CG aft to suit my flying style. Controllable but with just enough instability to have some fun.
 

Piotrsko

Well-known member
#13
Was there a CG Marked on the C/L plans? Control line tends toward nose heavy because that makes some of them slack line bad juju habits go away. Compound deltas have weird stall habits anyways because there are bunches of different things going on all over the wing. You see very few full sized ships unless there is some modulating surface. The video shows what looked like the center section stalling with the tips still flying which would be expected. If the tips stall first, it would violently roll that direction. Yours just kind of stops flying.

One of the hardest things when transitioning to aerobatic gliders is teaching the student to fly faster. It might just not be natural and goes against everything they learned first.

And I thought you are the king of low aspect planes.
 
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Vimana89

Well-known member
#14
Was there a CG Marked on the C/L plans? Control line tends toward nose heavy because that makes some of them slack line bad juju habits go away. Compound deltas have weird stall habits anyways because there are bunches of different things going on all over the wing. You see very few full sized ships unless there is some modulating surface. The video shows what looked like the center section stalling with the tips still flying which would be expected. If the tips stall first, it would violently roll that direction. Yours just kind of stops flying.

One of the hardest things when transitioning to aerobatic gliders is teaching the student to fly faster. It might just not be natural and goes against everything they learned first.

And I thought you are the king of low aspect planes.
No CG marked as far as I can tell. I'm more of a king in training...this is all part of my learning process, I started building and flying RC late last year with no knowledge of anything.Im still learning by trying everything out.I knew in theory that the compound shape has a funky air flow that's not as symmetrical as my regular slender delta. I just didn't know until I built and flew one how noticeable it would be. This compound delta plane likes to go fast and stay nose level, where the straight slender delta is more flexible.
 
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Vimana89

Well-known member
#15
Thanks for all the help and advice. All of your perspectives were helpful, and mostly reinforced what I suspected. The second prototype was aweful. I may revisit this for the FTFC20 but this is not a design I like enough to spend much time on or make a big, nice model of. Not a fan of the double delta. I'm going to experiment by scaling up my normal slender delta with a special KFM wing I've been going over in my head. I'll try my comfortable RET version first, as the design of the wing can be made where the back part only has one layer to crack dihedrals for RET. If that flies well which it should, I want to try elevons on the large version.