Downthrust Eq's

[raptor22]

Well, it pissed me off when everyone in another thread said it couldn't be done (I wanted it so I don't have that annoying trial and error), so I made some equations to calculate downthrust in a pitch neutral (not in a overly high incidence plane like a trainer) aircraft with a high wing or low thrustline.

M= (total thrust)(distance of thrustline below CG) + (total drag)(dist of AC above CG)
Deg Downthrust= arcsin((M/(dist motor from CG)/total thrust))

If anyone is qurious, I derived these using the concept that the CG is the pivot in space, and the AC is the center of total drag. I then used the concept of moments and vectors to produce these equations.

So, Let's see what everything thinks is wrong with em. I can give you exactly the dirivation if anyone is interested.

--Alex

[Majortomski]

Good first step to a static solution. Now add in the negative pitching moment from the airfoil and the postitive pitching moment from the horiz. stab for each airspeed and you'll have the whole tool.

tom

[raptor22]

I probably should do that (I will in the future), but I was trying to make a tool for aircraft that have virtually zero pitching moment (like hotliners or pylon racers); not, like I had said, trainers and others that tend to pitch up at speed due to a highly positive net pitching coeffecient.

--Alex

[feihu]

Alex -

My comments for what it's worth:

What you're really looking for is zero pitching moment when power is applied. This condition will exist when the thrust line of the engine goes thru the vertical CG (that is the CG as projected on the side view of the airplane) of the airplane. In simple trigonometry, it is the angle whose tangent is the distance of the engine above (-) or below (+) the vertical CG divided by the horizontal distance between the engine prop and the vertical CG. Sometimes a slight compensation is in order for lift caused by velocity, so many flyers set the thrust angle of the engine for cruise flight or at the power setting that is used for the most part of the flight.

To acquire lift and frontal drag values to apply to your equation is not a practical practice.

feihu

[Kwok_Yu]

Have you actually done a calculation and gotten a reasonable number? Seems kind of strange to derive a calculation for down thrust, only for planes that don't need any.

[raptor22]

yes. And you do need it if you have a underslung motor mount.

--Alex