Wing Wobble Thrust Angle

[HELModels]

As my pack voltage drops and RPM drops below throttle settingfor straight and level, my wings start to wobble just above stall. Do I need to further adjust thrust angle so that there is no pitch change from less than cruise throttle to power off? As it is, I get great climb from 1/2 throttle on up
without power on stalls, which leads me to think it is o.k.. It is when I slow fly and the pack voltage drops that I notice a problem. Power off glide is good and solid. CG is fine, self recovers with 20 + feet of altitude.

[Ollie]

As your battery voltage drops and RPM drops, you could advance the throttle to maintain the need of RPM. The drag of the prop makes the airspeed near stall when the RPM slows compared to power off glide. If you don't like manual throttle advance, try a prop with more pitch.

[HELModels]

Thanks Ollie! I didnt consider the prop pitch. Advancing the throttle definately works to get above stall and also draws more juice. I'll try a 6x5.5, since that is the only other prop I have and has 1.5 more pitch and 1 less diameter. When I tested static amp draw with those 2 props, the 5.5 pitch drew a few tenths more. It will be interesting to test it in the air.

[HELModels]

Yup, it handled alot better at lower throttle setting with the 6x5.5. Only problem was climb rate was less and the lowest throttle setting for straight and level was a notch higher. Maybe a 7x5? If the 7x4 was great for climb and only was a problem at lowest throttle, then the 7x5 might fix that.

Maybe I'll just build a glider and forget about motors for awhile.

[HELModels]

Well, it doesnt matter now. It needs a new wing - cracked the top spar. Flew in 20-25mph winds and got "shot down" on the 4th flight at lcaa. Funny, 2 folks with strange attitudes and CASA shirts were johnny-on-the-spot with the explanation that I got shot down. I think doth do offer up an explanation way too quick. They left in a hurry too. If I'm free, I'm flying. If they dont want me to live free, then they know what I think of them. Sure, be careful what you say because some malicious SOB's will hold you to your last stinking word.

Freedom isnt free? Price fixing is illegal!

AMA 811113

In Memorial to AMA 83339

[Ollie]

"Yup, it handled alot better at lower throttle setting with the 6x5.5. Only problem was climb rate was less and the lowest throttle setting for straight and level was a notch higher. Maybe a 7x5? If the 7x4 was great for climb and only was a problem at lowest throttle, then the 7x5 might fix that."

When you increase prop diameter, you get more current from the battery. The flight time decreases. You might want a bigger battery. You might want a better motor with higher efficiency. The new motor might be more costly. On and on, etc..

"Maybe I'll just build a glider and forget about motors for awhile."

Enjoy building a glider. Be patience with lift conditions. Enjoy increasing your flying skills.

[Ollie]

If you fly with malicious SOB's conditions, use an EPP glider that lives after being shot down. It might take out some fun from SOB's.

[HELModels]

It Survived! Had to cut the covering to place a splint and reglue the boom.

AMA 83339 was a person, not a plane.

The Malicious SOB's wore CASA shirts, but that doesnt mean that is who they represent. That is who they want me to believe would do that. I'm tired of being manipulated by rotten lying people.

I've met some very good people at CASA, those impostors were at LCAA and most likely dont represent them either.

[HELModels]

CG is right!
Thrust angle is questionable!

I found a broken spar! thought it was fixed, flew it and was erratic unbelievably hard to control.

Is a variable dihedral wing, erratic and hard to control?

My guess is yes, but it took very close inspection to find it and everything I have to fly it while it was broken.

the break was around the root and hard to see.

[HELModels]

That was part of the problem, variable dihedral caused by broken spar. First it was motor mount was loose. Man, try controlling that. Oh yeah, forgot about the loose tail. Boy, it was all over the place, snapping, looping. Got me considering all kinds of new ways to control a plane. Fixed the motor mount, fixed the spar correctly, fixed the twisting boom. Flies like a new plane with totally new problems. Now, when full throttle is applied it climbs straight up in a right hand corkscrew pattern. That is fine until I try to change that to a left turn and then let off the stick - more thrills and excitement. Throttle back for straight and level and wings rock back and forth like there is an upset. Looks a little like those stability drawings with the ball rocking back and forth, but it barely stops oscillating. My conclusion to that is that there is something continually upsetting it and the lower throttle setting almost eliminates it but not completely. What is the cause of the upset?

The motor is not permanently mounted to allow experimentation with thrust angle and CG, but after it lands hard it will sometimes shift around. The motor was not mounted in the center of the wing. The side view thrust angle was fine but it was not down the centerline of the plane. It was about a half inch over to the left side. the prop is 7 inches in diameter and 3.5 inches should be on either side of the centerline. When corkscrewing upward at full throttle it had 4 inches of prop on the left wing and 3 inches on the right.

At full throttle I produce 7000 RPM with a 7x4 prop. Can someone kindly offer a formula for determining how much differential thrust I introduced?

[HELModels]

That's funny, no formulas needed.

Took it out in the backyard and with the motor mounted along the centerline of the fuse, applied full power and watched it climb out nice and straight. No corkscrew, but the CG was wrong again. Hilarious! The Wobbles were back at low throttle. Previous landing at the field was decent, but the wing shifted just enough forward to shift the CG a directly proportional amount, i.e. 1:1 - 5mm wing forward = 5mm CG back. The final version of this contraption will have nylon wing and motor mount mounting bolts so that each flight will produce pretty much the same character.

[HELModels]

A click of downtrim cures the wobbles at very low throttle. It is the difference in drag between very low throttle and no throttle. I wonder if having a thrust angle on a pusher that cants the prop down contributes to the wobbles at low throttle but keeps the nose from hovering attitude at high throttle settings. As I see it, the drag of a large diameter prop at low throttle is high and makes a good brake, but the thrust angle introduces a pitching moment. If the prop is canted downward, the plane will pitch to balance the forces and at low throttle that means pitch up. A higher pitch prop reduced this effect, but it was also lower diameter and required more throttle/watts than usual prop. Why fly around at low throttle? Slow down in lift, speed up in sink.

Feel free to contribute or I will monologue forever.

[Ollie]

You have a good attiude about adjustments.

"Slow down in lift, speed up in sink."

That's why some planes cut off the motor and use a folding prop.

[HELModels]

O.K. Ollie I'll try to hold my heading and my airspeed in the green.

I thought some more on how the prop diameter and thrust angle at low throttle is affecting AOA, so correct me if I've suddenly entered an unusual attitude. It is the prop disc diameter and pitch and not so much the thrust angle. The motor is behind the A.C., the prop is large, slow the motor enough and the pitch speed is very close to plane's stall speed. The profile drag caused by the large prop is behind the A.C. and so when airspeed is low enough, the drag of prop is more than thrust and causes a pitch up. Stop the prop spinning and the profile drag of the prop disc goes way down and the pitch up moment goes away which is why I see nice solid glides at motor off. If anything, the thrust angled down would actually reduce the profile drag of the prop disc some tiny amount.

[Ollie]

The total drag has three parts, induced, profile and parasite. At slow speeds (under the maximum L/D speed) the induced drag is more than half the total drag. The induced drag is proportional to Cl^2. So the induced drag is zero at zero lift. The drag vector and the thrust vectors form a pitching moment dual. The moment arm distance between the vectors chances with the position of the drag vector. The induced drag component (tip wing) is above or below the thrust vector.

Be careful you assume simple reasoning. Study aerodynamics, not only special cases. I'm 74 years old and I'm still learning.

[HELModels]

Right, that is what I'm seeing. This low thrust/low speed prop drag duels with the baseline aerodynamics and prop disc wins, causing a pitch up. Throttle up or throttle off or trim and this special case goes away. I'm studying this special case because that is the spot in the envelope that needs understanding. I dont have a wind tunnel, transducers or an extensive library, so what I learn must come from what I observe.

[Ollie]

Study aerodynamics is free on the internet! Your effort might be long and hard. It depends on your motives and your smarts. You can learn by studying aerodynamics.

"I dont have a wind tunnel, transducers or an extensive library, so what I learn must come from what I observe."

I don't accept your excuses. Ignorance is a better excuse. I am still working on reducing my ignorance. How about your approach?

My point view is from my whole life time. Maybe your point of view is way too short.

[HELModels]

Thanks Ollie, I'll consider them words of encouragement.

[Ollie]

I'm not helping ElectroStorch with my advise.

You other Guys could help him, please.

[HELModels]

You're definitely not the problem Ollie, I apoligize for making it seem that way.

I have plenty of problems here that need attention, misdirecting my anger at you or anybody else on here only leads to more problems locally. since I'm no longer discussing anything relevant to this forum, I'll shut up.

[HELModels]

"How about your approach?"

Well, I'm approaching all this without the ability to do line integrals or conformal mapping or solve differential equations by approximations. Actually, I think building, flying, testing, diagnosing might qualify as an approximate solution of differential equations. Every text I've ever been interested in relies on knowing what the $#%@ those crazy symbols mean - it is always differential equations or that crazy sigma. Approximating a solution by doing and seeing is my approach right now. sure, it is crude, but you use the tools at your disposal.

As for the low speed part of the envelope, I've accepted that there is a low RPM with that prop that pitches the nose up and causes little teeny tiny stalls that wobble the wing depending on where in rotation is the prop- this due to more prop over the wing and prop blanked by fuselage. I was able to reduce this effect somewhat further by adding another zip tie to the motor mount. I was getting gyroscopic precession in every plane and adding another zip tie stopped that. It would oppose almost every input briefly. even with that fix, at very low speed, it actually needs some down in turns initially. Steepen the turn, needs some up. Exit the turn, needs some more down.

It flies nicely overall, I'm just trying to use as few electrons as possible. With a 340 mah pack, I should be able to fly for 20 minutes since 1/2 throttle static is less than 1 amp.