I have a question regarding the placement of the tail. Why is the tail always on top of the model and never underneath? I have done some research and found that a sub-tail can aid in pointing a plane. One old time free flight guy told me that the bottom is the natural place for the tail but due to landing gear interference, it is seldom put there. Any information/references would be appreciated!

-Ross

I saw a video of a aerobatic plane with a vertical fin under the tail.
It would fold down in flight and fold up to land.
As I understand the story, the pilot died before he had a chance to fly the plane.

It would get broken or damaged unless you had someway to fold it, or really tall goofy looking landing gear. Take a look at the predator UAV (with the inverted v-tail), When they switched to the B model, they put the tail on top of the fuselage to reduce the possibility of tail strikes, among other improvements. As far as a model, you could always hand launch it, maybe recover via parachute? I'm not sure that it really matters which side the tail is on(top or bottom), except for the effect of the fuselage blanketing the tail area during nose high manuevers and high-alpha(like climbing out from takeoff), which MAY have an affect on the elevator and rudder and result in less control.(Think J-3 Cub) Most planes don't point downwards too much, unless you're talking about aerobatics stuff.

Actually many planes have small fins on the underside to help maintain roll stability, and possibly(?) increase fuel economy. Take a look at most military fighters(F-16, F-15, F-14, F-18, etc all have 'em) in addition to many of the small buisness jets(gulfstream, etc) have them too. Heck, even my PT-40 had "fins" for a while.

Just my guess - tail on top cause it's easier.

I remember seeing pictures of the predator with the inverted v-tail. I built a small balsa glider with the tail on bottom and it flies really well. There's no concern about a landing gear on it. I can see where ground clearance would be the big concern. If anybody knows anything about yaw effects coming from below rather than above in a real aircraft, that might be interesting. One of my friends said that a downward tail makes a plane unstable/counteracts dihedral in the wings. Not sure if that's got anything to it though.

landing and takeoff are the main concerns with this design feature. A good example of a downward pointing tail would also be the Dornier 335 arrow, which has both a top and a bottom tail. The bottom tail featured explosive bolts as well as the rear pusher prop in the event that the landing gear didn't work, so both could be discarded. if you look at that plane, it is very high off of the ground and the plane can easily rotate without a tail/ prop strike in the back. The price to pay for this? Very long gear.

When practicality meets increased performance, its usually practicality that wins out.

To some extent a low mounted fin will counter the dihedral but it's generally a slight effect. I've made lots of small chuck gliders with low mounted fins as well as high mounted ones. Any difference in the effect of the mounting location has always been lost in the effects from the other design aspects.

And once the model reaches a size where the weight is an issue then it's just a matter of practical handling and operational aspects. Low mounted fins require odd landing gears and do not offer any advantages significant enough to warrant the other oddities that they require.