Hello all, I'm new to this thread, but if you will check the thread titled "Berkley B-26 Invader Model" there is a lot of discussion on thrust lines. More in the thread "Pica Duellist." But neither really gets into the reason for downthrust.

A stable, easy flying airplane will normally have the wing set at a positive incidence relative to the stabilizer, usually just the wing positive with the stab set at zero. Sometimes you will find the wing positive, and negative incidence in the rear. Least common has the horizontal fin installed with negative incidence and the wing at zero.

The reason for these incidence angles is purely one of stability. At normal cruising speeds the airplane, slightly nose heavy, has lift from the wing, and a downforce from the stab keeps the plane stable in level flight. If the bird slows, the down force will decrease, the plane pitches down, the speed increases giving more down force, the airplane levels off. If the speed increases in level flight, the down force increases, the nose pitches up, and so forth. Combine this with the wing's dihedral angle and you get positive stability. The airplane will fly itself, and recover from maneuvers with no control input.

Now we get to the engine thrust line. With the wing/stab incidences set for our stable flight, when we add power the plane does not speed up, it climbs. decrease power and it merely descends, but does not slow down. If you read Wolfgang Langsweische's (Spellinng?) book "Stick and Rudder" this is where he gets the statement "The throttle controls altitude, and the elevator controls speed." Simplistically that is true. By adding the right amount of down thrust on the engine you counteract the tendency to climb with power. That is the ONLY reason for down thrust. It is not uncommon to have your trainer climb suddenly when you chop the throttle, now you know why.

All this positive stability is why you can take your fingers off the sticks, and the plane will recover itself, given enough altitude.

Side thrust is to counteract the "P-factor," which will not be discussed here, I've gone on long enough already except for one additional consideration. Aerobatics.

A high performance aerobatic plane will usually have the wing and stab at zero incidence, and the engine set at zero up-down, and zero side thrust. This makes the plane fly the same upright and inverted, slow and fast, and gives the airplane generally neutral stability, that is, it goes where you point it, you have to fly it all the time. Also, the lack of stab downforce requires the center of gravity to be moved rearward, making the airplane more responsive to control inputs.

So, stable or twitchy, make your choice and set your angles. If you want to experiment with the angles, get a copy of Great Planes' "Real Flight Simulator." It lets you alter all these parameters and fly the result. It's also a great tool for scratch building, you can put your numbers in, and alter them to get the flight characteristics you want before you start cutting wood. It really works. My first scratch twin is a dog, didn't use RF on that one. Scratch twin #2 is a super nice airplane. I used RF to check the design. If you are building from plans, that's not scratch building and you don't need RF to check it, the designer has done all the brain work.

Hope this answers some, if not most of the questions.

Build a twin, you'll find you're "IN."

WLR