Down-force for Takeoff

posted on October 10th, 2010, 1:42 pm

Well, to start, I'm designing aircraft on the flight simulators and I've run into some very interesting problems with the aircraft.

Down-force - an aerodynamic force that pushes down on an object.  The opposite of lift.
Lift - an aerodynamic force the pushes up on an object.


Down-force is used by race cars to keep them from leaving the ground.  I used the same concept to help my aircraft take off at a lower speed.  Although, instead of applying it to the entire aircraft, I applied it only to the tail of the aircraft.  While I configured the rest of the plane to produce lift.  In doing so, I shaved off about 20 knots on the minimum takeoff speed of the aircraft.  Of course, if I added too much down-force to the plane, it would have disastrous results.

posted on October 11th, 2010, 1:12 pm

Correct me if I'm wrong, but if you apply down-force to the tail section aren't you going to make it harder to get the tail off the ground?

Great, now I've got a mental image of a plane doing a wheelie down the entire length of the runway.

What about if you have different surfaces on the tail section?  So that the top surfaces of the tail promote down-force and the bottom sections give equal lift once you reach a certain speed?

posted on October 11th, 2010, 1:20 pm

Great, now I've got a mental image of a plane doing a wheelie down the entire length of the runway.

Man, now I do too!

....it's a pretty awesome mental image 

posted on October 11th, 2010, 4:52 pm

If I apply too much down-force, not only will the plane do a wheelie, but it'll do a back-flip!  It's a balance between having too much down-force, and not enough down-force.  When done correctly, and given the proper runway length, you can actually let the plane take itself off.  You can control how much down-force is on the tail by using the elevator trim.  Use that to control the speed, then use the engine power to control your altitude.

Using the engines to control your altitude and your elevator trim to control your speed is probably backwards from what you might expect, but it works every time.  Just don't make huge corrections or you'll loose control.

You don't want the plane to do a wheelie on the runway.  You just want enough down-force on the tail to help it get into the air.  And enough down-force so that once you're in the air, you just need to adjust your speed to climb or descend.  The airplane will automatically try to maintain a certain speed depending on your elevator trim setting.