high stabs? full scale airfoils?

[Bam Bam]

I was wondering why the stabs on most of the full size aerobatic palnes are much higher than what they are on models? if you look at most full scale edge's extra's yaks and sukoi's the tail sits pretty high in the turtle deck but on our rc sized planes it is more in line with the wings and thrust.

I though it was better for everything to be in line so you dont need as much coupling etc. i actually look for it on our models!
But why dont they do this on full scale's?

Also, knowing next to nothing about airfoils, why is the airfoil on full size so much different to our models?

[extra330ss]

if you think about it, there's less obstruction from other CS's..... study the mig 15, and you'll understand it better.
you get higher performance by the unobstructed airflow on the tail... others could probably add to this.... i'm not going to get into detail... but the coupling thing is correct....

-extra

[Mithrandir]

the tails are placed high 90% because the steel tube fuselages accomodate it well on top....
full size planes don't have springs that restore the stick to a certain and rigid "Center" like an RC transmitter.

exception is the yak 55

[Bam Bam]

so it is actually better performance wise to have everything line as on our models?

What about full scale air foils comapared to model scale? why so different?
Wouldn't the theory behind lift of the wings be the same as full scale?

[Mitsu1]

	Quote: Originally Posted by Bam Bam
	I was wondering why the stabs on most of the full size aerobatic palnes are much higher than what they are on models? if you look at most full scale edge's extra's yaks and sukoi's the tail sits pretty high in the turtle deck but on our rc sized planes it is more in line with the wings and thrust.

You ask why the full scales don't have stabs in the thrust line, .. whereas I ask why replica models don't have stabs aesthetically correct to the full scale position? Actually, I know the answer, to help them fly pure and true with little coupling, but I still wish they were placed where they belong, especially since we all have computer radios that can mix out most bad characteristics. I have an EF Extra 300, and they are super flying planes, but that stab is sooo low on the tail, the cables and control horn have to be mounted on the TOP ... of the elevator instead of below. Man, ..that just don't seem right. .. but I do understand.

[mmcconville]

The aifoils used on a lot of full scale aerobatic airplanes such as the Extra/Cap/Edge simply dont perform well on models. The reynolds numbers are very different, so the same theory cant really be applied.
When used on a model, they tend to snap very poorly.

As for tail placement and coupling, the answers above are accurate. We place the stabs lower to uncouple the airplane and give us a LOT better performance, The full scales normally have hi stabs for structural reasons.

I have to disagree with the statement that coupling can be mixed out with computer radios. True to a point, but it will seldom to never yield an airplane that flies as true as a properly designed one. Simple explanation is coupling is always non-linear and is also speed and attitude sensitive, so to mix it out completely is virtually impossible.

[extra330ss]

	Quote: Originally Posted by mmcconville
	The aifoils used on a lot of full scale aerobatic airplanes such as the Extra/Cap/Edge simply dont perform well on models. The reynolds numbers are very different, so the same theory cant really be applied. When used on a model, they tend to snap very poorly. As for tail placement and coupling, the answers above are accurate. We place the stabs lower to uncouple the airplane and give us a LOT better performance, The full scales normally have hi stabs for structural reasons. I have to disagree with the statement that coupling can be mixed out with computer radios. True to a point, but it will seldom to never yield an airplane that flies as true as a properly designed one. Simple explanation is coupling is always non-linear and is also speed and attitude sensitive, so to mix it out completely is virtually impossible.

listen to him...... he designed the plane i fly

[Mithrandir]

Ever see the airfoil sections of an extra or a sukhoi???

They have an extremely fwd max thick point and extremely blunt leading edges...

the aerodynamic consequences of this are actually positive good characteristics if your arse is in the plane....

but for models, the problems are because of the following...

Consider a 40% Extra.... the "Scale Weight" of the plane should be about 135 pounds.... Typically our 40% planes weight 38 to 42 pounds....
so compared to a real extra or sukhoi, we have verylight weight planes with very very low inertias......

the scale airfoils, require a very large pitch angle before the airfoil seperates and stalls.... this fact means that to do a snap on an upline for example, you gotta pitch the plane a lot before it departs. DOing so kills any speed you had and can put the plane way off heading.... remember.. our models are relatively light and low inertia, so the big pitch departure sends the plane off course rather then the plane "Skidding to a Stall" like the real plane with that airfoil (but essentially flying along the original flight path)
.... if our models weighed over 100 pounds... we might want this type of section!!!

Even with the planes and airfoils we have now... there are "Tricks" people use to get good snaps... like.. as soon as the plane departs.. get off the elevator and finish the snap with ail & rudder only... any elevator just gets the plane deeper and deeper into the pitch and it washes out and is piggy!!! lol

Another consequence of the light weight plane....
Difficult to do clean spins...
.... With a full size plane, to do a spin, they ease off the power... hold a straight and level heading, as the plane slows down, they pull back more until the wing stalls and autorotation is initiated....

With my Compy SX, if I try that, it will slow down, I keep pulling more back stick... it slows more, I pull more back stick.... and I keep pulling more back stick... and more and more... until I am at full elev... like 45 degrees of full elev... the wing never definitively "Lets Go".... it just gets mushy and descends with the fuse level in a nice harrier....
Point is... the wing never crisply "Lets Go".. or departs...

Maybe if I had a more forward CG it would... I dunno.... I have heard of people trying several different things to try and synthesize a crisp stall... or the appearance of a crisp stall.

[Bam Bam]

Thanks guys, that is the kind of info i wanted! in the case of a plane flying more pure on our models, why not put the stab directly in line with the wings and also the thrust line on most planes is a little higher (on extra's) than the wing.

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 for one asked these questions as i found that on my models with everthing more in line, they typically flew more true than the planes that have higher stabs and low wings. i found that it is nearly impossible to get hands off knife edge flight when you have to use alot of mixing as it changes depending on speed etc. i have a comp-arf 330l 2.6m that i have spent a lot of flights getting the best i can, and i still need to control ail and elevator quite alot.

I think planes look meaner when everything is lined up anywhoo! (design me a new bird mike!)

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!

[Edge 540]

	Quote: Originally Posted by Mitsu1
	You ask why the full scales don't have stabs in the thrust line, .. whereas I ask why replica models don't have stabs aesthetically correct to the full scale position? Actually, I know the answer, to help them fly pure and true with little coupling, but I still wish they were placed where they belong, especially since we all have computer radios that can mix out most bad characteristics. I have an EF Extra 300, and they are super flying planes, but that stab is sooo low on the tail, the cables and control horn have to be mounted on the TOP ... of the elevator instead of below. Man, ..that just don't seem right. .. but I do understand.

Really?!? You'd honestly rather buy an aircraft with poor coupling characteristics so that you can use the mixes in your transmitter?!???

[Mitsu1]

	Quote: Originally Posted by Edge 540
	Really?!? You'd honestly rather buy an aircraft with poor coupling characteristics so that you can use the mixes in your transmitter?!???

It's not that I'd rather have a plane with poor coupling characteristics, ... it's just that I am such a stickler for accurate scale appearance. I'm not into the Scale Masters thing ... but I really dig an aerobatic model that LOOKS just like the real thing. So much so ... that I'd be willing to give a little trade-off.
... A little example, .. At the field ... I'd rather pack up and just watch others than to fly my Extra 300 without a spinner or wheel pants ... arrgghhh

[Mitsu1]

	Quote: Originally Posted by mmcconville
	I have to disagree with the statement that coupling can be mixed out with computer radios. True to a point, but it will seldom to never yield an airplane that flies as true as a properly designed one. Simple explanation is coupling is always non-linear and is also speed and attitude sensitive, so to mix it out completely is virtually impossible.

Great point on the speed and attitude sensitive aspect. I agree its impossible to mix out all bad characteristics, that why I said most of them, but I concede that as well. I guess I just want to have my cake ... and eat it too