Thrustline question for over-the-wing engines

[Lon Enloe]

I'm working up a design for a specific situation. I've got access to a nice, flat field with healthy grass, but it's almost too healthy for flying--any plane with landing gear gets caught up in the grass and flips at least 80% of the time. I'm also sick of breaking props--so I'm seriously considering a gear-less design with the (small brushless electric) motor over the wing, like a seaplane. I'm especially eyeing the pod coming out of the tail a la the Seawind that Tower Hobbies has been advertising (picture attached, from their Web site.)

My question is, what does one do about the thrustline on such a configuration? I'm used to the idea of pointing the thrust down a few degrees on a high-wing ship, for example--so does this get a little nudge up to counter the torque that's trying to pitch the nose over?

Thanks!

[BMatthews]

Normally it takes quite a bit of upthrust to counter the high thrust line.

But what about other options for your field? Larger and fatter wheels to float over the grass better? Hand launch and belly land? What sort of and size of models are you flying?

[Tom Harper]

Lon,

I use 4 degrees of upthrust to compensate for the high thrust line vector. That is an arbitrary value but it seems to work. Details are on the LD4 design and build thread.

[Terry S]

Use a big elevator, at take off with full power I have had the plane nose in due to low flying speed and high power.

Terry

[HELModels]

Take a real good look at that picture! The motor is mounted neutral relative to the horizontal stab's chordline.

[HELModels]

Here is a picture of the full scale that shows the thrust line. The yoke is probably locked in the down position while sitting on the ramp, but you can see the leading edge and get a feel for when the elevator is neutral.

Thrustline peek (http://www.seawind.biz/images/Fast-Business-Trip.jpg)

[Rodney]

I've only tried two of this type design and never reached a satisfactory way of setting up the thrust lines. The best approach was to mix throttle and elevator with a computer radio with increasing throttle setting resulting in an increase in up elevator trim. Even that left a lot to be desired.

[Lon Enloe]

I was actually thinking about using this on a flying wing configuration. Is that sounding like mission impossible? I've got an airfoil picked out for this design, and a planform, and until very recently I was convinced that I was going to make this a pusher like many wings are, but after an unfortunate encounter with a prop two weekends ago launching a pusher design, I will never, ever build anything where the prop isn't either forward of or above my hand..which is why this over-the-wing concept seemed interesting.

[Tom Harper]

Lon,

My first attempt at a flying wing was a scaled down (167 sq in) version of Wainfan's Cyrano with a small electric motor mounted above the wing. Couldn't get it to fly at all until I added up thrust. First time I'd tried such a thing. I was rewarded with a nice level flight.

[Thomas B]

4 degrees of upthrust is a good place to start. It is not a bad idea to make some provision for manually adjusting the thrustline on a model like this.

[Lon Enloe]

Yes, I was thinking of making a motor mount that clamped around the vertical stab and holding off on any sort of fairing behind the motor until I got the thrustline working and fixed in place.

Sounds like +4 degrees is the starting point--thanks!

[Lon Enloe]

Interestingly enough, in the middle of my thinking about this, my son comes home from the library with "The Concise History of Aviation", in which one finds reference to the SIAI Marchetti SM-55 series of seaplanes (drawing, from another source, attached). I guess there's more wiggle-room in the "up" direction than one might imagine.