proper incidence.motor trust angle on Savoia 51

[mistairjoe]

Need advice from aeronautical know it all's as to what the correct incidences for stab, wing and motor should be on this kind of model.The airfoil is a thinned Clark y and the stab is also airfoiled possibly symmetrical.Concern is the motor behind the C.G a pusher.Thanks Joe

[nmasters]

http://www.rcgroups.com/forums/showthread.php?p=11003252#post11003252

You might want to tilt the motor so that a line drawn though the prop hub points at the elevator. Or make your motor mount so that you can adjust it in that direction later if it needs it.

--Norm

[vintage1]

Odd design. I suspect you will get away with all pretty much zero..just a degree or two incidence on the main wings.

There are too many variables to 'get it right' first time. Main wings 2-3 positive wrt to tailplane, and then micro adjustments to thrust line if power on trim is way out.

[fnev]

Vintage1 has probably the best answer as this is a VERY tricky question/problem.

I have dealt with pusher configuration aircrafts for the last 15 years and the BIG problem is the helicoidally generated flow from the prop onto the tail surfaces. This is why most of the more recent designs have vertical and horizontal surfaces outside (as much as it can be possibly and realistically done).

If you are very unlucky (a fairly good possibility with your considered design), you will have trim changes in yaw and pitch for every throttle changes. Furthermore some roll trim issue can occur as well… It is almost impossible to do an aerodynamic analysis on this one. The wind tunnel tests I was able to carry were the best way to get an idea of what to expect in flight. On one particular design we were still battling during the first flights to get the proper compensations.

Not impossible, but very challenging and rewarding at the end if you can make it work. What size are you considering for your model and what engine/propeller would you use? This will have an influence on the ease/feasibility of your project.

[Rodney]

Pitch control will vary greatly with throttle setting as the thrust line is a long distance above the CG which will cause pitch down with increasing throttle settings causing you to constantly have to retrim the elevator when you change throttle settings.

[fnev]

Pitch control will vary greatly with throttle setting as the thrust line is a long distance above the CG which will cause pitch down with increasing throttle settings causing you to constantly have to retrim the elevator when you change throttle settings.


Yes, BUT it is not as simple as this… You have to look as well at your aerodynamic center… And in this particular case it is above the thrust line… As I said in my post: this is NOT an easy scale model BUT a very rewarding one and NOT impossible to make it fly (and fly nicely). Just be ready for a LOT of trial and error!

[nmasters]

… You have to look as well at your aerodynamic center… And in this particular case it is above the thrust line… As I said in my post: this is NOT an easy scale model BUT a very rewarding one and NOT impossible to make it fly (and fly nicely). Just be ready for a LOT of trial and error!

The center of drag is below the thrust line so it will be producing a nose down moment while power is on. The CG is even lower so it will be producing a larger nose down moment but only while the plane is accelerating. This type of layout relies on the interaction of the prop blast on the tail to correct for these pitch/thrust couples. If other scale models of this plane have a problem you may want to make the stab just a bit larger and get its leading edge closer to the center of the prop wash.

--Norm

[mistairjoe]

Thanks Guys.This is exactly the kind of info i was looking for.A fellow modeler from Germany Kuni is the one building the plane. He was asking my advice on the matter because of our common experience with the Savoia 55.I am sure he will chime in on the forum soon. WE kind of agree on the 1.5-2% on the wing with zero on stab and experiment with the motor.Joe

[Kuni]

Hi all,

Kuni here.
I want to build the model with a spann of 1,4m or 55". The wing incidence should be 2°, Stab around 0,5-1° up. My experience with Savoia S.55 would say upthrust, but here is the lift center above the thrustline, so 0-1° thrust-up should do it, right? Airfoil should be a Clark-Y or something from the NACA 24XX line.
Trim changes should be manageable and can later be programmed into the receiver.
I took most of the incidence relations (but not the incidences itself) from a rigging diagram of the Supermarine Walrus.

Thanks for your suggestions,
Kuni

[Thomas B]

The small Macchi M-5 that Dare kitted with this same general arrangement benefited greatly from some noticable upthrust.

With a zero-zero thrustline and the little bit of water drag on the hull, it wanted to plow the nose under water until airspeed built up.

Pointing the thrustline towards the horizontal tail fixed everything.

It is a rare high thrust line model that does not benefit greatly from a thrust line adjustment.

[Wendi Smol]

Hi all, Kuni here.
I want to build the model with a spann of 1,4m or 55". The wing incidence should be 2°, Stab around 0,5-1° up. Kuni

Hello Kunibert :)

the choice of the ClarkY is commendable, but which datum line do you use for the Einstellwinkel?

W.

[nmasters]

What's "Einstellwinkel"?

[Wendi Smol]

Sorry,

angle of incidence - rigging angle

[Kuni]

Hi all,

Thomas,

could you please post the other incidence angles from the Dare M.5? Is the stab 0°?

As there is no straight deck, I have chosen a line parallel to the hull bottom before the step as datum line. This will be the angle when the plane is planing on the water surface just before liftoff.

Regards,
Kuni

[Wendi Smol]

Sorry Kuni

this time I meant the data line you use for the ClarkY, the straight underside or the chord (Sehne). If you use the first (as is often done with that profile) 2deg is fine. Otherwise it's probably too small.

W.

[Kuni]

Hi Wendi,

I used the chord line for measures. BUT: I am not nailed down to the Clark-Y! We can try anything else too...

Regards,
Kuni

[Wendi Smol]

Hi Cuni

in which case I would rather go for about 4 deg. And please don't use another profile, you won't find a better one than the Clark-Y for your application.

Regards

W.

[vintage1]

I love German technical terms.

How about 15 things get bundled up into one word. Germans have the best nouns, the Latins have the best verbs and the Brits have the best adjectives.

The Scandinavians have fish.