Lateral area distribution for acrobatic sailplane

[surfimp]

I've got a question concerning lateral area distribution for acrobatic sailplanes. I understand that a sailplane will never have the sort of knife-edge capabilities that a powered aircraft has, but I'm just wondering about the manner in which lateral area is distributed in acrobatic sailplanes versus acrobatic power aircraft.

In most acrobatic sailplanes, the majority of fuselage lateral area seems to be concentrated from about the 1/4 chord line forward, resulting in a large and somewhat bulbous forward fuselage area. The Fox is a good example:

http://www.manfred-schadl.de/fox3seit.jpg

Now obviously the pilots have to sit somewhere, but even on acrobatic model sailplanes, this forward distribution of lateral side area is maintained. The Voltij is said to have some of the best knife edge performance of any sailplane on the market, and it definitely has a forward bias to its lateral area distribution:

http://perso.wanadoo.fr/aeromod.concept/Couleurs_FichiersPartages/voltij12.jpg

By comparison, the lateral area distribution on most powered acrobatic aircraft seems to be the opposite; the majority of lateral area is located behind the leading edge, even on designs with relatively long noses. Here's a Sukhoi:

http://home.nordnet.fr/~dapierre/images/sukhoi.jpg

Powered acrobatic models follow this same general distribution.

Thus, my question is why the difference? It would seem that there would be a given "optimal" distribution for lateral area which would provide the best possible knife edge performance, but this seems to be different for powered and unpowered aircraft, but I can't quite sort out why. Any help most appreciated!

Steve

[biber]

Hi Steve!
A prop in front of the CoG destabilises the fuselage. It works similar to a lateral (and of course horizontal too) area as long as it makes thrust. The more thrust the more effect on the lateral area distribution. And the longer the nose the more influence gets the prop. This is one reason why the powered planes need less lateral area in front of the CoG. And because of the heavy engine the pilot has to sit behind the wing to achive the right CoG. This also leads to a bigger lateral area behind the CoG. Besides that note that the fox, swift, kobuz and other acrobatic gliders have far more lateral area behind the wing then any other modern fullscale glider designs do.

[surfimp]

Thank you biber! The influence of the prop was something that I knew was important, but couldn't quite put my finger on.

So: for an aerobatic sailplane, is it necessary from an aerodynamic (as opposed to aesthetic) point of view to have the majority of the lateral area biased in front of the CG, to provide a functional equivalent amount of lateral area as does the prop on a powered plane? In other words, for successful knife edge flight is it important to have the majority of you lateral area in front of the CG, whether in the form of a tall fuselage (sailplane) or a large and powerful motor and prop (powered plane)?

I just don't quite understand the necessary dynamics of knife edge flight, and this is what is leading to the confusion. Thank you for the help!

Steve

[Sparky Paul]

Although tractor props are destabilizing, the final distribution of the lateral area must be behind the c.g. or the back end will attempt to become the front end.
Fuselage area contributes the most to lift in KE flight, but there is still the aerodynamic requirement for the neutral point of the entire airframe being behind the c.g, with a static margin of some value although it may be slight.

[surfimp]

So to ensure that the neutral point of the airframe in knife edge flight remains behind the CG, does that mean you need to distribute most of your lateral area in front of the CG? Sorry if your above response answered this clearly, I'm just trying to be 100% sure.

Thanks SP!

Steve

[biber]

You need not to have the majority of lateral area in front of the CG. And it is no full instability needed for good knife edge. But the yawing stability must not be to strong. It has to be weak enough for the rudder to be effective. Thats is similar to side slipping, wich is much easier if there is no big yawing stability counteracting the rudder.

[surfimp]

Ah ha! Now I think I'm onto something. So by making the boom of the fuselage relatively small, and making the rudder large, and making the area in front of the fuselage large (all in terms of lateral area), you can best facilitate the aircraft being able to "slip" (if you pardon the pun) into knife edge flight and hold itself there with rudder. Is this correct?

Thanks!

Steve

[biber]

I think you've got it right. In KE the fuselage has to play the part of the wing and the rudder/vertikal stab has to play the horizontal stabs part. Unfortunately the fuselage has a rather poor aspect ratio :(

[surfimp]

Hehe, yes the fuselage does indeed. I guess this is why they make them so tall! Thank you for your help, I have gained a small insight today.

Steve

[biber]

Ok, here in germany it's about 2:00 am and i'm going to bed. But nice discussion so far on an interesting topic i think. Will check it again tomorow.