high stabs? full scale airfoils?

[Mithrandir]

	Quote: Originally Posted by Mitsu1
	........... I guess I just want to have my cake ... and eat it too

then get a Yak 55

It is scale when it is all lined up!!!

Bam Bam.... the coupling is not a function of thrust-wing-stab alignment.... it has to do with very asymetric flow around the fuse, fin and rudder.... There is a fin adjacent to the top side of the stab and only a rounded stubby fuselage bottom surface adjacent to the lower side of the stab...... Often it is hard to completely design out as P-Factor from the prop prolly adds an element of irritation to the mix

[sukhoi26mx]

	Quote: Originally Posted by Mithrandir
	then get a Yak 55 It is scale when it is all lined up!!! Bam Bam.... the coupling is not a function of thrust-wing-stab alignment.... it has to do with very asymetric flow around the fuse, fin and rudder.... There is a fin adjacent to the top side of the stab and only a rounded stubby fuselage bottom surface adjacent to the lower side of the stab...... Often it is hard to completely design out as P-Factor from the prop prolly adds an element of irritation to the mix

Pitch coupling has everything to with with wing/stab/thrust line alignment relative to the center of pressure of the fuse... while roll coupling has everything to do with wing position and effective dihedral/dihedral from the wing/stab placement, and wing/stab taper ratios.

The reality is that full-scale airplanes aren't optimized for coupling - period. Their wings and stab are placed for convenience and structural integrity. For instance, most tails are mounted high as that is the most convenient place to mount them structurally. As a second for instance, most wings are either high or low (think Extra 300 vs. Cap or Sukhoi) so that the pilot has a place for his/her legs (above or below the spar). Those kind of compromises aren't necessary in models, and I agree with Mike that they should be designed out when possible without compromising the fundamental feel and personality of the airplane.

Scott
full-scale Sukhoi 26mx pilot

[sukhoi26mx]

	Quote: Originally Posted by Bam Bam
	if everthing is in line, would that just about get rid of the coupling altogether? i think although it would make them less scale, most would prefere a plane that fly's true. These forums are full of people trying to make things or neutral!

I find that the mid/low wing planform coupling with the mid/high stab works best. It will couple slightly to the gear in KE, but will have a cleaner harrier (with the right airfoil) than a true on datum setup. With the right effective dihedral, roll coupling will be minimal, which is my least favorite to have to mix. Honestly, as Mike and Mith mentioned, it is virtually impossible to remove all coupling but it is possible to minimize it without butchering the personality of the airplanes in many cases.

Scott

[mmcconville]

	Quote: Originally Posted by Bam Bam
	Q for mike, how much higher is the stab on the H9 35% 260? i heard these planes fly very true and am considering getting one if i can find one in oz (distibutor O'reilly doesn't seem to want to get me one for at least six months!!!) i like the nice big wing area!

Bam Bam. Tes the 260 is very true. When the Cg is correct, the pitch coupling is very minimal as is roll coupling.

Going from memory, I believe I ended up putting the stab around 6mm to 12mm above the wing center line. The prototype had a little higher stab, but I lowered it to reduce coupling a little.

[Bam Bam]

i belive the weight of a plane would also make a difference to the way it fly's along with c.g.?
If it's lighter it should fly better with less coupling yeah? or not?

[mmcconville]

Not really. Weight will effect elevator trim slightly, which effects pitch coupling but a slight CG change will eliminate that.

[Mithrandir]

I had a Jekyl years ago and I kinda did an anhydral thing to the stab... it was popular back in the dark ages... (1970's and 80's)...
anyway... the plane was the only plane I ever had that truly had ZERO pitch coupling.... it had adverse roll coupling... but just a little baby amount.... and being a pattern plane, I never got it to like 45 degrees of beta.... (Like my avatar)... so who knows what would happen at that extreme angle....

But worse then coupling, is coupling that is really nonlinear, or very speed dependant, or very "Beta" dependant... or worse yet,.... coupling that changes sign (or direction) as a function of beta... rumor has it that one of the more expensive all composite bad ass planes has a KE coupling that goes positive-negative-positive as you go from zero beta to 50 degrees beta.....

Bottom line in my book... consitant and predictable are virtues to be treasured!!!

[Bam Bam]

i dig that! Very true! thats what i am really chasing i guess, consistancy.
and of all the planes iv'e had, including the 50cc's and 100cc's was the fliton extra 330 freestyle! obviosly not as nice as a 100cc, but it had the best flight characteristis and manners of any plane i have flown! in a .46 size glow!!!

[03fomoco]

One other consideration given in fullscale is ground clearance. Pretty sure there is an FAA requirement for this also?

[kjkimball]

I can attest to the lack of coupling in the full scale model 12 Ti-12. In our test flights, the airplane was rigged for hands and feet free flight level and high cruise speed, say 190 mph. Knife edge flight left or right wing down had zero pitch coupling. It did not want to go to the canopy or to the wheels in level knife edge or in a knife edge climb. Wings level same speed hands off the stick and input full left or right rudder made a flat skidding turn, no roll, no pitch deviation. From 230mph 5 g pull to vertical, hands and feet off controls on the vertical line, the airplane continues straight up until stops and simply flops over pointing vertical down and did not deviate off the vertical downline as it gained speed. Pilot reported he felt it would drive straight in if he did not make control inputs.

So, this is an example of an airplane that has a high stab and some of the arrangement issues pointed out above to allow pesky humans to ride in it that has near zero coupling. This shows that some full size airplanes can fly like a model and do not count on the pilot to "correct" the coupling issues.

Some other notes on this airframe is that the wings and stab are all +2 deg incidence while the engine is zero down and 2 deg left thrust (russian backwards rotation engine). CG at about 27% MAC.

[flying skunks]

well said Kevin , if you are flying a well designed and built plane you should not have any KE coupling issues ..

[Mithrandir]

	Quote: Originally Posted by kjkimball
	.................Some other notes on this airframe is that the wings and stab are all +2 deg incidence while the engine is zero down and 2 deg left thrust (russian backwards rotation engine). CG at about 27% MAC..............

Interesting!!!

How much static margin do you have??? Did you do a broad CG sweep in test flight to find "The Sweet Spot"?

Did you experimentally arrive at the incidence and thrust line???