How to fix high Angle of Attack
 

I recently test flew a World Model Groovy 50 3D. The plane flew well overall. However, the angle of attack during straight and level flight seemed excessive. You could easily see that the plane's nose was pointed around 20 degrees above where the plane was actually heading. This problem increased during landing where the angle of attack was around 40 degrees and caused the tail gear to touch down before the main gear.

My question is: What can I do to fix this large angle of attack? Would moving the CG forward have an impact on this or will I have to change the incidence angle?

Thank you for you help.

RE: How to fix high Angle of Attack
 

hi

it sound you have tail heavy problem.you might want to ckeck your CofG IF OK add some lead to the nose.

RE: How to fix high Angle of Attack
 

Yup, ditto on the CG. try that first....

RE: How to fix high Angle of Attack
 

I disagree with this being due to tail heaviness. The angle of attack has to be whatever angle of attack generates the needed lift, regardless of CG. Flying faster will reduce the AOA needed to produce the nessesary lift. A higher wing angle of incidence will give the wing the nessesary angle of attack while keeping the fusilage aligned with the plane's direction of travel.
Reflexed ailerons could cause this by cambering the wing causing the zero-lift AOA to shift positive. You might experiment with moving both ailerons down slightly to shift the wing's zero-lift AOA to a negative direction, like flaps do.

RE: How to fix high Angle of Attack
 

Sounds great, but check your CG first. An aft CG can cause a slightly tail-down attitude in level flight. If that does not solve your problem, THEN you can argue about the math. It really doesn't need to be this complicated. These are TOYS. Expensive, high performance and potentially dangerous toys, but toys nonetheless.

Good Luck!!

RE: How to fix high Angle of Attack
 

I've never seen a tail heavy model fly nose up unless the angle was needed in order to provide the lift just like BLE says. Tail heavy has NOTHING in any way to do with the angle of attack the model will fly at. It only affects the pitch stability of the model.

Now if by some chance you had a design that was using an exceptionally small stabilizer combined with the tail heavy CG then perhaps it would not be able to generate the lift needed to control the pitching up but if that was the case then the model would not last long in any event. But this is a pattern style design with adequite stabilizer area so this doesn't apply in any case.

Either the model's wing and tail surfaces are rigged wrong or the model is flying too slowly and the result is the need for a high angle of attack to generate the lift needed. The specs on their site at...
http://www.theworldmodels.com/para/i...ion/html/A189/
indicate a flying weight of just over 5 lbs. Did your copy come out at around that or did it come out a lot heavier? Are you flying it with a pattern type prop pitch or did you go with a 3D style prop with a very flat pitch?

RE: How to fix high Angle of Attack
 

In my airplanes with a long-ish tail moment, it's quite noticable when I move the CG aft. The thing just increases it's nose-high attitude the further back I go. I'm not saying that it's necessarily the case here, but it's a lot easier to adjust than incidence.

Relax man, they're just toys.:D

RE: How to fix high Angle of Attack
 

I haven't checked the weight of my aircraft since I don't have a weighing scale to weigh it. I'll look around the kitchen to see if I can find something to weigh it on. I believe that my model's weight should be alright as I am using a 2-stroke engine and the instructions call for a 4-stroke.

The model is currently slightly tail heavy. I will add nose weight to see if that solves the problem. Speed wasn't an issue since I was using a fastish 12x6 wide blade prop on an OS 55AX.

The wing is shoulder mounted and the mounting holes had been pre-drilled into the fuse. I couldn't have screwed up wing installation if I wanted to. I checked all measurements again and the wing and stab is perfectly aligned with the fuse.

I will do the following on my next flight and report back:
1) Move CG forward.
2) Setup flaperons and see if giving a small amount of flaperon trim impacts the angle of attack.
3) Try a faster Topflight Powerpoint 12x8 prop.

Thank you all for your answers so far.

RE: How to fix high Angle of Attack
 

I have to agree with BMatt on this one. CG has nothing to do with AoA. I would recheck the wing incidence. Also, doucle check tail incidence and thrustline.

RE: How to fix high Angle of Attack
 

You guys are funny!!

agexpert... I'm with you

RE: How to fix high Angle of Attack
 

I really doubt that plane flies in the attitude claimed.
Those angles are only possible with control inputs and power.

RE: How to fix high Angle of Attack
 

Tall Paul, My thoughts exactly when I first read it.

Falcon7a, if those angles are truly the case what type of ground speeds are you getting? Sounds like its being flown behind the power curve in a Harrier. The way to fix that is more airspeed.

RE: How to fix high Angle of Attack
 

weigh yourself on a bathroom scale with and without the plane; if you're nearsighted have someone else read the scale.

RE: How to fix high Angle of Attack
 

Will I see you in Vegas? chase/phat

RE: How to fix high Angle of Attack
 

Nope...I'm way up in MI...


and I do agree that the 20 degrees and 40 degrees are probably a bit excessive... but a plane can certainly fly in a tail down attitude

RE: How to fix high Angle of Attack
 

I'm with TallPaul and others. I've never seen ANY airplane fly with the nose 20 degrees up in straight and level flight unless it was flying near stall speed. The 40 degrees mentiond during landing is a good harrier landing - no way the wing wasn't stalled if the nose really was that high on landing approach. Something is odd about this. falcon7a, Is it possible you are over-estimating the angle? It is difficult to estimate deck angle accurately, unless you are looking at the airplane directly from the side. If you like the way it flies otherwise, I wouldn't mess with it.

RE: How to fix high Angle of Attack
 

I think this is definitely not a CG issue. I would expect an aft CG to actually reduce trim AOA. By shifting the CG aft, you "load up" the tail (or reduce the download on a tail that pushes down in level flight). By loading up the tail (or reducing download), you reduce the weight that the wing has to carry. This will in turn reduce the trim AOA.

I have flown straight and level at 20+ degrees AOA without being anywhere near stall. The attitude was stable and the aircraft was fully controllable in all 3 axes. The aircraft was also well below the AOA for maximum lift coefficient (about 34 degrees). I have flown the same aircraft straight and level beyond 40 degrees AOA. The aircraft was still stable and controllable, but you could make the case that it was stalled because it was beyond the AOA for CL max. That said, the aspect ratio was <3 which significantly increases the stall AOA. I would expect the Groovy 50 (AR about 4.5) to be uncontrollable at either 20 or 40 degrees AOA.

RE: How to fix high Angle of Attack
 

It's the wing load that determines what angle of attack is needed to fly level at a given airspeed. Changing the CG changes the elevator trim needed to maintain that angle of attack.

RE: How to fix high Angle of Attack
 

I stand corrected, after looking at the pictures posted by Tall Paul, the AoA on Groovy is not 20 degrees. I would say its more like 10 degrees in normal flight and around 15 degrees during landing. The plane flew very well overall and I had no problems with control on landings whatsoever.

I'm wondering if engine thrust line has anything to do with AoA. The engine is side mounted and I may have inadvertently added some up or down thrust while mounting the engine. If the AoA seems higher than normal, is this a sign of upthrust or down thrust?

Thanks.

RE: How to fix high Angle of Attack
 

Something has to be out of alignment.
A tall-heavy airplane that far out of balance would be uncontrollable for the few seconds it was in the air.
A small offset in thrust line won't create that serious amount of nose up flight.

RE: How to fix high Angle of Attack
 

I got a Lanier ARF Q500 plane that flew terribly right out of the box. The bottom of the fuselage is designed parallel to the zero line of the stab and 90 degrees to the thrust line, so it is an easy plane to check on a table. The design is high wing, but not so far off the thrust line to cause the plane to fly as poorly as this. The culprit was the wings' incidence, it needed to be jacked up in back to counteract the nose down and "tucking" tendencies that it had. After setting the wing to "zero", the model looks like hell, but flies the course with ease. This kind of a design error had nothing to do with warping, it was just cut and assembled by folks who didn't know what they were doing, they might as well have been throwing together incense burners.

RE: How to fix high Angle of Attack
 

Yep that would be more like a chopper would it not? ;)

Now an RC model is a tad different than a jet fighter with boundary layer control which delays airflow separation into much higher angles of attack thus delaying stall characteristics.

RC models respond to standard physical characteristics of sub-sonic convergent airflow. For the normal airfoils that usually happens at around 16 to 20 degrees of AOA.
An airplane can be maintained at an observed angle appearing to be performing at higher AOA than such when using the thrust vector/s to produce lift to create desired performance parameters.
Center of Gravity has no effect on anything other than stability when displaced from the Center of Lift point.
When a model, in convergent airflow is flying in straight and level in a somewhat tail-low attitude it is simply because it has insufficient airspeed to produce lift for the weight at the given incidence and/or thrust settings. If controlled for straight and level, then it will be at the controlled speed the AOA as required for that Air Speed.
Most modelers either have their so-called symmetrical wings set wrong, or those ungodly things called strip-ailerons out of alignment by a degree or more, (bad incidence) or the stabilizer out of alignment or the thrust line out.
In reality the HORIZONTAL STABILIZER is always ZERO. It sets the incidence line for everything. The airflow does not have a clue as to what the "ENGINE-EAR" / "Draftsman" drew as a straight line and started setting as positive / negative incidences and thrust lines. For example a +1 stab. incidence, a +3 wing incidence and a -2 thrust line reference a straight center line on a plan is simply 2* positive incidence wing and -3* downthrust. With bad incidence settings the pilot must hold the AOA for whatever speed he holds. Leaving out the frills which he doesn't control, the pilot's control of the lift equation is Lift = 1/2 of Velocity(IAS) squared X Coefficient of Lift (AOA)

Now one can get behind the Power Curve in a model just as in a 1:1 scale where one has full power and the nose is too high to accelerate. To accelerate one must go down. Bad situation but it can happen, however not seen with the usual overpowered Q-500 or 3D stuff. :D To the inexperienced that may appear as a tail heavy situation but it isn't. Just some bad piloting, yet one that can happen when flying heavy warbird types, and especially if trying to go around from a poor approach and pulling up prior to getting the power up. Don't ask me how I know that. [:o]

Just a story for fun and education. I built a small Spitfire years ago for Scale War Bird Racing. It was fast and stable in the air and easy enough to land but a bear on take-off. It took a year to finally crank the ailerons up enough to trim out the take-off snap tendencies to make it a sport flier on take-off also. I just hated to have it sitting there with both ailerons almost 3/8" above the TE of the inboard wing. Looked bad but almost no reduction on speed and flew like a dream. Then one race I was scooting past a P-51 and he pulled up and cleaned the tail feathers off right behind the wing. I never thought I was so close. Just broke his prop!!! [:@] But adjusting things can make a model FLY! Don't be afraid to tweak things.

RE: How to fix high Angle of Attack
 

Well, I finally got a chance to fly Groovy again this weekend. I adjusted the CG and moved it about 2/3" forward. As soon an I did that, the high AoA tendency was gone. The plane now flies as it should and is a joy in the air. I tried different manuvers to check for proper trim & thrust angle and the thrust angle seems ok so far. :D

After this experience, I went home and fired up Real Flight 3.5 to evaluate the effects of CG on angle of attack. I played around with the Great Planes Yak 54's CG. Sure enough, as soon as I moved the CG to the rear on the Yak, the AoA increased. It wasn't as noticable during straight and level flight as it was on Groovy but it was very noticable during slow speeds and landings.

RE: How to fix high Angle of Attack
 

Sorry, Just sometimes it's difficult to understand just how basic one needs to be when explaining the basic theories. With a symmetrical airfoil, and even more so with a rather blunt leading edge, CG can be instrumental to offset the normal very small amount of up elevator trim which provides the down force used to load the wing needed to provide the positive AOA to produce adequate lift to fly the machine, and this can be very noticeable at slower speeds.
Pattern fliers try to balance rather tail heavy, near 40% MAC, and get away with it with their very lightly wing-loaded, long tail moment, highly powered designs, using very minimal control movements. The aft CG then assists their trim changes about the lateral axis when transitioning between inverted and normal maneuvering.
Your experiences during the slow flight regime proves my point as the AOA increased, you became somewhat behind the power curve and the aircraft could not fly out of the extra drag due to the higher AOA which also produced the lift required for the level flight or slight descent. The aft CG replaced up elevator which produced the situation only because of a given speed in a given condition. Drag is also a function of the square of the airspeed as well as lift.
No big problem, many have the same misconceptions about those factors and all seem to work OK. H-ll they're just toy airplanes . :D

RE: How to fix high Angle of Attack
 

Are you sure that the extra nose weight isn't just forcing the model to fly faster which in turn reduces the AOA needed to provide the lift? A nose heavy plane will automatically pitch down when the airspeed goes down and that in turn increases the airspeed, a self correcting feed back mechanism in other words.