Why the enlarged tail surfaces?

[jpuke]

Can anyone explain to me why all the Guillows kits (and maybe other brands, all I have are Guillows at the moment) have enlarged/non-scale tail surfaces? What would be the result if I was to build one with a more scale size rudder and stab? I always hear that planes come out tail heavy and I'm wondering if I can avoid that with smaller tail parts.

Thanks,
Jeremy

[Wildpig]

Jeremy,
That's an interesting question. The technical answer is called: scale effect. Here is my 2 cent definition: "you can scale down an airplane but, you can't scale down the air." This gets into boundary layer aerodynamics, pitching moments, etc.
The short answer is: the larger tail is needed for stability in flight. Usually it's just the horizontal stabilizer that needs to be enlarged by a small amount. That's all I have time for right now. Maybe somebody else will elaborate with an answer.

[BillParker]

The other answer is that they don't fly worth a durn on a rubber motor with scale tail feathers . .049 U-Control works okay with scale on the warbirds, anyway...

[kittyfritters]

Few real airplanes, especially high performance airplanes, have stabilizers large enough to fly stably without a pilot. Most free flight scale models require that the tail surfaces be slightly enlarged to achieve this level of stability.

For the Beaver and Porter kits I designed both a scale and an oversize (115%) stabilizer and both sets of parts are supplied in the kits. In test flying I could get the models with scale stabilizers to fly, but they were twitchy little beasts, difficult to get into trim and sensitive to the slightest turbulence in the air. With the slightly oversize stabilizer either model is much easier to trim and much better able to cope with rough air. If you are going to fly free flight outdoors definitely use the oversize stabilizer. Indoor scale models are generally easier to trim with scale tail surfaces because they are operating in relatively stable air.

[Steve Blanchard]

Full scale aircraft need to be on the edge of instability for the sake of maneuverability. Smaller tail surfaces allow the aircraft to be turned tighter. For free flight models you need greater stability to fly predictably. That is why the tail area is increased Note that if you look at three views of fighters from WWII you will see that the tails are much smaller than what you would need for a successful free flight model. A lot of the earlier kits from Comet and Megow and Guillows did not have sufficient tail surface areas to make for stable flight. Usually an increase in stabilizer of around 10 - 15% would fix this. Also, a vertical tail that is too large can be as bad as too small as the tail gets caught in the prop wash. That's a whole other discussion. As far as tail heavy goes it's really because of the fact that most models have a much longer tail moment than nose moment. You may find that you will begin to build tails from much lighter wood than the rest of the plane and therefore help with the tail heavy issue. It is just another trade off for stable flight.

I hope this helps,

Steve

[flash52]

For a good flying free flight model the stab should be around 1/3 the size of the wing

[Bill Gaylord]

I would worry more about the weight of the material used in tail feather construction, more than the size. A bit of area increase will not add much weight, and is better off to have. The tail heavyness issues depend mostly on the airplane. For rubber, the obvious problem is that you don't have an engine up front, but even with electric, planes like the 190 (I've done both sizes) really need all the gear up front, when sheeted. As for it, the small scale like tail performs fine, with the longer tail moment. I would imagine a number of these planes have issues, as the builder simply refuses to add enough ballast to balance properly, taking the load off of the tail. I realize that's difficult, with minimal rubber power.

Another thing not talked about much is the specific airfoil used. Some Guillows planes have airfoils with far rearward max camber points, such as the FW190 and other flat bottom airfoiled Guillows warbirds. They look awkward to me, but this airfoil allows for a further rearward CG, which really helps with planes like the 190. I would have never tried the CG point that works well for rc, if a friend had not tried it first. This plane is sheeted, and has a CG of 47mm from the wing LE, if I remember correctly. Video on YouTube:
http://www.youtube.com/watch?v=UVSIrZ_tph8&list=UUtk2LreLH_IhNe3uS3fLSoA&index=8&feature=plcp

[jpuke]

Thanks for the answers!