Pusher Prop Thrust Line Offset from Datum Line
 

I'm am totally novice when it comes to aerodynamics so please be kind. http://www.rcuniverse.com/forum/js/f...n/drowning.gif

Hypothetically this is a 25 - 30 lb plane. The pusher engine develops maybe 19 lb of thrust. Generally speaking, what is the affect of having the thrust line well above (9"maybe)above the datum line? Assume that the wing, stab, datum and thrust line have the same incidence.

TIA,

RE: Pusher Prop Thrust Line Offset from Datum Line
 

There is a nose down pitch that is induced by the moment or torque of the thrust force times the distance of the perpendicular to the CG.

For the airplane to be stable in horizontal flight, the sum of the moments and forces respect to the CG must be zero; hence, the moment that is induced by the thrust must be balanced by an equivalent pitch up moment.

Generally, that balancing moment is achieved by a combination of several gravitational (from CG locations) and aerodynamic (from airflow) forces and their respective arms.

Downthrust and upthrust angles tend to reduce the arm of the thrust.

RE: Pusher Prop Thrust Line Offset from Datum Line
 

It will drive you nuts- especially on launch-
the salvation is to instantly gain enough speed to get the wing and stab working -otherwise -it noses over fast.
I just junked a similar ,slower setup and went to a setup with the thrustline (pusher ) almost centered on the wing -the design is for camera work.
the problem is that the drag is all lower than the thrust . It is exactly like trying to push a carton across a rug-pushing on the top edge.
Lots of goofball flying boat designs used that same engine up high setup
lousy concept- forces are all working at bad angles.

RE: Pusher Prop Thrust Line Offset from Datum Line
 

Got it. Seems that trimming this kind of package at different throttle settings would be problematic.

Em, if it's big enough - perhaps onboard computing with a ton of sensors ...

rmh - Iposted this before I saw your post ...

Are you saying that you dropped the thrustline like Ihave in my pic 1? Or pic 2?

RE: Pusher Prop Thrust Line Offset from Datum Line
 

Not an impossible layout, though.:)

In this design, the drag of the tail group, combined with the lack of landing gear, induces a nose up that helps.
Also, the range of the recommended location for the CG is very small.

RE: Pusher Prop Thrust Line Offset from Datum Line
 

The vertical chunk at the rear in the location typically reserved for the vertical stab is a 12" diameter ducted fan assembly with a 4 blade prop spinning at a low low 13K. The vertical stab will go down from the fuse rather than up. The center of the fuse is reserved for a vertically oriented ducted fan for VTOL capability. Just thinking out loud.

RE: Pusher Prop Thrust Line Offset from Datum Line
 

With your original diagram you'd want the thrust line to be angled somewhat down towards the nose. Likely something like 3 to 4 degrees.

Before you actually take on this rather massive sounding project I'd suggest a series of half size models powered by smaller glow or electric engines/motors to test each aspect. With a quickly built model you could fine tune the amount of "downthrust" angle to achieve reasonably neutrally trimmed throttle changes.

RE: Pusher Prop Thrust Line Offset from Datum Line
 

Uh, this IS the small model - 3rd gen in fact. The 1st two - may they RIP. Trust me - I am NOT the wizard behind the curtain. Just might be the builder of the next iteration though. The latest "maiden" failed during take off. I was not present but am sharing what I was told. The roll out was nice-n-smooth. Lift off was gradual. Up elevator gave a nice gradual climb rate. More up elevator and then, after climbing to about 40', the craft abruptly nosed over - full throttle - into a vertical down line and stopped when it met with the concrete runway. Thinking that a combination of added pitch adjustment with building thrust just may have been toooooo much. [:@]

Thus my question about the effect of the thrust line well above the datum line. Had to smile with rmh's comments ...

Hey - it's just a hobby - isn't it?

RE: Pusher Prop Thrust Line Offset from Datum Line
 

The desire to build something different -exists in most of us -
I designed some stuff years back -which were patented and sold by my employers.
One of the issues here is "just how practical is this aircraft design?"
Flying boats are the magnet for plenty of "great ideas"
The setup used on the SeaBee was probably one of the most practical-single engine setups.
I am finishing up a varient of that design -for my new camera model
There are quite a few small rtf models - using this same arrangement - -it is easy to do and resolves most of the problems with pushers.