Up thrust on Seaplane?

[Mike Hall]

I noticed that on some twin engine seaplanes there is about 3 deg of up thrust. Is this something i should carry over to a model? I would think i would need down thrust like most other models but maybe there is something im missing when it comes to seaplanes. Any help on this topic would be great.

Thanks
Mike

[vintage1]

I thinkits all about high thrust lines to get props and engines clear of spray, leading to nose down pitching moments under power.

If there is not a lot of decalage, I'd be inclined to leave it in.

[Mike Hall]

The wing has 4 deg positive (same as engine) and, the stab is set at 0 or maybe .5 deg down. So i guess this makes the Decalage 4 to 4.5 deg. Should i leave the upthrust or what?

Thanks
Mike

[raptor22]

Probably helps lift it onto step.

--Alex

[feihu]

The wing has 4 deg positive (same as engine) and, the stab is set at 0 or maybe .5 deg down. So i guess this makes the Decalage 4 to 4.5 deg. Should i leave the upthrust or what?

Thanks
Mike

What airplane is this?

feihu

[vintage1]

I think the reality of seaplanes is begiinning to reveal itself. They HAVE to take off and land with the hull level. Very little chance of hauling back the stick until the hull is clear of the water. So they are rigged with positive incidence on both wing and engine, and then in flight one assumes thet fly 4 degrees nose down or so.

Most of the flying boats seem to have been relatively slow anyway, so would have flown with a reasonable angle of incidence on the wing even in cruise.

My guess is with 'model' power they will be relatively overpowered, and need some down trim mixed in with the throttle, but I'd keep the angles as per full size, and a handy thumb on the down stick on the first flight. Forward CG probably not a bad idea either.

[tim hooper]

I'm no expert so could well be wrong, but I gather that quite a few high-engined landplanes need upthrust to stop them from pitching forwards (eg MPX Twinstar). The fact that high-engined planes tend to make good seaplanes could just be coincidental!

tim

[Bill Glover]

The Twin Star doesn't actually need that upthrust (I removed it from mine, and it made no noticeable difference).

With props only a small distance from the CG you need an awful lot of upthrust to give any significant pitch effect (look at seaplanes with pylon-mounted motors, or gliders with power pods above the wing ... they often have what looks like 10-15 degrees or more of upthrust). Whereas a prop on the nose (or tail) only needs a few degrees offset to make a difference - a much longer moment arm.

As already hinted at, if the wing and motors are both at +4 degrees relative to the fus. then you don't really have any upthrust (because the plane will fly nose-down anyway).

But you will have an upward thrust component when the fus is level that will help 'lift' the plane at full power and low speed, as suggested possibly helping it to get onto the step. Think of this as 'vectored thrust' (like a Harrier with the nozzles tilted down a little from the horizontal). Maybe this is intended to help the Twin Star (in stock configuration) hand launch more easily.

[vintage1]

Totally agree, and remember that with the hull in the water, there is a huge drag when first accelerating up to takeoff spped.

I suspect the full size designers had loads of problems with 'snow ploughing' and eventually settled on this rigging layoiut as the only way to get the brutes into the air!

[MikeL72]

Don't visit this forum much but was doing some searching for airfoil info and saw your thread. My Twin Pond Master has 0 degrees in both wing and stab with 1.5 down in the twin speed 400s that drive this flat hull 43" span waterplane. Sorry for the poor quality of pics but I'm still trying to figure the best and most economical way to get vhs video into my PC. These are taken with my old digital camera from stills on my TV.
As you can see -- the plane jumps out of the water at full throttle. The batteries are 8 cells of 1900 NiMH pumping about 25 amps into the 400s that have Graupner 6.5x4 props. It does plow water which gets into the props but gets on step pretty quick and only a touch of up is required to get it to break away.
I'm very new to waterplanes but flying this makes me think that downthrust affects the flying characteristic vs throttle moreso than the ability to get on step and to break away. And of course -- having the power.

[raptor22]

The fact that high-engined planes tend to make good seaplanes could just be coincidental!

tim

It's to keep the prop and engine from wear caused by spray.

--Alex