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        Discussion Twin Boom Pusher Plane - neutrally unstable?

#1 pardus Nov 10, 2012 06:53 AM

Twin Boom Pusher Plane - neutrally unstable?
 
A question for experienced designers/aerodynamacists:

I have built and flown my third own design twin boom pusher plane. All 3 are very similar in dimension and proportion.1.7m span, 1.3m length, 1.3kg weight, Clark-Y airfoil.

However from the previous experience I determined that I needed about 3 decalage to prevent "tuck-under" (pitch down with increasing speed) in a dive test (no power, nose down 45, release sticks, see what plane does). The others would gently pull out of the dive which was desirable.

Currently on this one my CG is at 26% from leading edge at the root, (straight leading edge wing) but I still am getting mild "tuck-under", it's expectedly worse if I move the CG back. If I go to 25% (feels a bit nose heavy) then it's almost right as then it flies just straight, I haven't yet tried further forward. The others flew happily at 28-30% CG.

Because of the decalage there's quite a strong pitch up moment with full throttle, but motor thrustline is 0,0 as it was on the others.

So I want to lessen the pitch up on full throttle AND stop the tuck-under in the dive test AND possibly move the CG to ~27%.

What can I change to get this right?

http://i49.tinypic.com/2n2dyq.jpg

http://i46.tinypic.com/207ss5f.jpg

#2 richard hanson Nov 10, 2012 07:09 AM

Titlt your ailerons (both) UP a little bit judging from your picture you have a bit of droop in em
this causes an undercambeed wing - good for slow speed but a problem if trying to trim at speeds .

#3 pardus Nov 10, 2012 08:34 AM

Quote:

Originally Posted by richard hanson (Post 23232199)
Titlt your ailerons (both) UP a little bit judging from your picture you have a bit of droop in em
this causes an undercambeed wing - good for slow speed but a problem if trying to trim at speeds .

They're just like that as there's no battery plugged in, when powered up they are both up very slightly.

#4 richard hanson Nov 10, 2012 08:55 AM

3 Attachment(s)
You may want to also change thrust - -tho that layout is not particularly sensitive due to short moment arm - point the spinner down 5 degrees - and se if that helps
also tape on a rounded block of foam to the nose -to cancel the angle shown.
My camera plane is similar in force setup but the higher motor mounting produes less climbing effect as speed is increased .

#5 pardus Nov 10, 2012 09:26 AM

The nose angle is actually a clear window for my FPV camera so not much I can do about that. I will try thrustline changes but as the motor is not powered during the dive test it won't help with that. Re: the thrustline, surely the spinner must be pointed up rather than down to stop the pitchup on power?

#6 richard hanson Nov 10, 2012 09:41 AM

Quote:

Originally Posted by pardus (Post 23232854)
The nose angle is actually a clear window for my FPV camera so not much I can do about that. I will try thrustline changes but as the motor is not powered during the dive test it won't help with that. Re: the thrustline, surely the spinner must be pointed up rather than down to stop the pitchup on power?

Yes -
but real issue is wing reflex/incidence
If the cg is furthe r aft the problem may also reduce - On this arrangement - somewhre between 25-30 % should work
I opt for more aft setups and use expo to soften pitch response -
My backgrounfd of competition aerobatics has shown me that overall on a slow flyer -this works best -for me

#7 pardus Nov 10, 2012 10:37 AM

From other threads:
you have to think at it like a balance. The plane is hanging at lift center (about 30% of the wing, depends on wing profile). If you want a stable plane, GC has to be in front of it (not so much). A pusher engine act on the balance so if it is tilted down it does not push the plane upwards, it makes the plane turning around the "balance" point (center of lift), diving then instead. For this reason engine has to be tilted upwards.

And:
Draw a sketch of a pusher, side view. Add the direction the air is blown by the prop (back and up or down)
If, ..., you angle the prop up, you blow the air up, and push the tail down = upthrust!
Angle the prop down, thrust goes down, tail goes up = downthrust!


But these don't take into account the up or down air pressure on the horizontal stab, so angle "prop up" causes up pressure on the underneath of the horizontal stab causing nose down? (And it does depend obviously if your motor is above or below the thrust line.)

Confusing, but I will try to test both ways and check the results.

#8 ShoeDLG Nov 10, 2012 10:44 AM

Another option (not an easy change), would be to mount the horizontal stabilizer/elevator on the top of the vertical stabilizers. This would largely move it out of the prop wash and reduce the throttle-to-pitch coupling you're getting. This is a fairly common pusher/boom configuration.

#9 kcaldwel Nov 10, 2012 11:15 AM

The pitch-up under power is because the CG is too far forward, with the resulting requirement for too much incidence in the wing for trim. You have to work from the zero lift line of the airfoil for setting the incidence, not the chord line. The zero lift line of a Clark Y is about 3.5 degrees from the chord line already. If you add 3 degrees on difference between the Clark Y chord line and your stab setting, you actually have about 7.5 degrees of incidence difference. This is way too much, and will make the airplane change pitch trim with throttle quite severely.

The tuck under in a 45 degree dive is not from the CG setting.You will have quite a lrge static margin with the CG at 28 to 30%. I suspect you have an aero-elastic problem - something is flexing at higher speeds, like in the dive test. The booms are very small, and that is the first area I would suspect. If they deflect under load, you will have strange pitch effects at speed. I don't know what your wing construction is, but with a cambered airfoil the wing can have increasing washout at speed which will also make it tuck in a dive.

Calculate your neutral point, and find the CG location for a small positive static margin, like 10%. Adjust your elevator setting or incidence for trim with this CG. This will minimize the pitch changes with power. Even smaller static margins should be perfectly flyable (5%), and will pitch less with speed differences. A symmetrical airfoil will also remove the wing pitching moment from the equation, requiring less incidence difference between the wing zero lift line and the tail zero lift line, at the expense of a slightly higher stall speed.

Simple CG calculator:
http://adamone.rchomepage.com/cg_calc.htm

More precise CG calculator:
http://www.rcgroups.com/forums/showthread.php?t=1106300


Reinforce the booms so they do not deflect under load. Make sure the wing does not twist under load. That will fix your tuck under issue, which is not CG related.


Kevin

#10 pardus Nov 10, 2012 11:21 AM

Quote:

Originally Posted by ShoeDLG (Post 23233345)
Another option (not an easy change), would be to mount the horizontal stabilizer/elevator on the top of the vertical stabilizers. This would largely move it out of the prop wash and reduce the throttle-to-pitch coupling you're getting. This is a fairly common pusher/boom configuration.

You're right, that's not an easy change at this point.
I think sorting out the "tuck under" is my priority as that bothers me the most. As I understand decalage, in the dive test as the plane accelerates, the faster airflow over the -3 horizontal stab (relative to the wing) pushes the stab down causing a pitch up, mine however dives as it accelerates which can cured by moving the CG forward but it pretty far forward anyway.
I'll try changing the thrust line in the morning when I fly again.

#11 pardus Nov 10, 2012 11:37 AM

Quote:

Originally Posted by kcaldwel (Post 23233512)
The pitch-up under power is because the CG is too far forward, ..............

Reinforce the booms so they do not deflect under load. Make sure the wing does not twist under load. That will fix your tuck under issue, which is not CG related.


Kevin

Thank you Kevin, this is my result below (in mm) so will try it in the morning.

My booms are carbon tow/cyano reinforced but there still is some flex in one area so will rectify that too.
http://i46.tinypic.com/ekomfq.png

#12 kcaldwel Nov 10, 2012 12:09 PM

Quote:

Originally Posted by pardus (Post 23233548)
You're right, that's not an easy change at this point.
I think sorting out the "tuck under" is my priority as that bothers me the most. As I understand decalage, in the dive test as the plane accelerates, the faster airflow over the -3 horizontal stab (relative to the wing) pushes the stab down causing a pitch up, mine however dives as it accelerates which can cured by moving the CG forward but it pretty far forward anyway.
I'll try changing the thrust line in the morning when I fly again.


That isn't exactly right. Incidence angles are only to set the trim speed, and have nothing to do with stability. You usually set the wing incidence so the fuse drag is minimum when the wing is at cruise angle of attack. The CG position is set for the desired stability level - elevator power, landing flare, pilot preference, etc. The stab incidence is then set to trim the airplane at the desired trim speed, with the wing airfoil Cm and CG position.

The airplane will be pitch stable as long as it has a positive static margin, no matter what the stab incidence. The trim speed might not be the desired one though, unless the stab incidence is correct. Low Reynolds number effects (poor low Re airfoils, etc.) can muddy the waters a bit at model scales, but that shouldn't be an issue with the Clark Y.

Your booms are very small diameter, and they will flex even with carbon fibre. It only takes a few degrees to start doing strange things at speed.

Kevin

#13 kcaldwel Nov 10, 2012 05:26 PM

A couple more thoughts:

The spreadsheet CG calculator I linked to will give more reliable results. It takes into account the tail position in the wing wake, rather than using a fixed tail effectiveness like the simple on-line calculator. It can also take the fuselage into account. It is a great help designing and building as well, since you can locate all the components, and will calculate where the CG is, and where it should be.

A little thought experiment will demonstrate that stability is not a function on "decalage". If you have an airplane with a full-flying stabilizer, giving a pitch command to move the stabilizer incidence is effectively changing the "decalage". The trim angle of attack of the airplane changes, but the airplane does not become unstable, even with negative "decalage".

The same effect occurs with a fixed stab and elevator. The load on the stab is typically quite small, and stays pretty constant with aircraft AoA. What changes when you give an elevator control is the zero lift line of the stab's angle with respect to the wing. You are effectively changing the aircraft "decalage" when you give an elevator command.The aircraft just changes trim AoA, and the aircraft stability, set by the CG distance ahead of the neutral point, remains unchanged.

There are some small effects from wing height above the CG and dihedral moves the wing MAC higher too, so those have to be taken into account for a more detailed stability analysis.

Kevin

#14 pardus Nov 12, 2012 02:11 AM

Thank you for the suggestions.

Put a camera on the horizontal stab over the weekend, no noticable deflection.

Upthrust on the motor has seemed to cure most of the ills, as now need much less down trim to maintain gentle climb with power which has eliminated the increasing dive in the dive test.

#15 pardus Nov 12, 2012 03:46 AM

DiveTests (2 min 0 sec)


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