Offsetting my Center of Thrust

[Arya]

I'm working on a Seaplane. I've got a working and stable airframe that floats, but to keep my intakes dry I've had to offset the engines almost a metre above the airframe. How should I direct my thrust to counteract the tendency to pitch-down under throttle? And is there anything else I should watch out for with this kind of design?

[jcewazhere]

A picture is worth a thousand words, however if the engines are behind the center of mass pointing them so they thrust slightly up could counter the down pitch. Like pushing down on one end of a seesaw.

[jcewazhere]

Changing the engines could harm your overall lift a bit. Maybe you could add some small static (un-moving) fins to the nose to help counter the pitching?

[maj.solo]

It can't think of a simple solution , if thust is off axis then it is. One need to look to real world for inspiration and do a number of things that will balance the craft but if the thrust is off axis then it always will be doing the pitch down and to counter that you need to have pitch up forces.

There DEFINITELY is a difference between normal planes and sea planes. The following guy tries to exhaust the subject, I didn't check all he wrote though:
https://omorfiasophias.wordpress.com/2011/06/14/principle-aerodynamic-difference-between-land-plane-and-seaplane-and-trying-to-understand-it-all/

Problem is your engines can be full throttle or no throttle. And the other forces are dependent on speed so you can be at zero speed or cruise speed. The plane will always be a ugly duckling.

You could try this.

1) Instead of a mid body wing move the win high up. Then at least the drag from the wings will be along center of thrust.
2) Larger control surfaces that simply fight against the pitch down brute force.
3) Move center of gravity a little more aft.

all I can think off. But it is difficult to trim such a plane when nothing is truly at the center of body,

Anyways such a plane will always fly uggly and things dynamically will be changing depending on speed and thrust at the moment. It will then not have a wide range speeds but only a typical cruise speed at which all forces balance each other and the craft fly the cleanest with minimum drag.

Maybe moving CG aft is the best. When at cruise speed and normal throttle it be level. And if you throttle down it will pitch nose up a little and rear down but I hope you have a "flying tail" to rest upon.

[Arya]

Thanks guys. I've got it flying quite stably now through brute counterforce. I'm now working on a more elegant, more permanent solution.

I also tried a plane with a fuselage based engine, and that negated the problem completely.

[jcewazhere]

Gratz

[Arya]

I've taken a slightly odd approach to learning how thrust-offset works; I've made a boat, something I'm a lot more familiar with than an aeroplane and with much simpler forces at work.

The boat has an offset central engine, and I've been experiementing to see how changing the thrust angle makes the boat sit in the water. Unorthodox, but it's yielded good info so far. And having a jet-powered 200+ KPH speedboat to play with is a nice bonus.

[Arya]

Thanks a LOT Roofcat. Building a hydroplane felt like the next logical step, and you've given me everything I need to start developing one.

[maj.solo]

heh heheheh try make this .... and please some pictures if you do ....
have 2 Mk3 cargo bays on it and fill them with Ore to TEST KSP physics if it is any good ....
This is what you should try and make

https://en.wikipedia.org/wiki/Caspian_Sea_Monster

[maculator]

I don't think ground effect exists in KSP. But well it's odd how low your speed with a plane needs to be to take off from water, so maybe it does exist?

[Arya]

Originally posted by maj.solo:

heh heheheh try make this .... and please some pictures if you do ....
have 2 Mk3 cargo bays on it and fill them with Ore to TEST KSP physics if it is any good ....
This is what you should try and make

https://en.wikipedia.org/wiki/Caspian_Sea_Monster

I have to echo Maculator's comment here. I'd love to make an Ekranoplan, but without Ground Effects I don't think it would fly.

[Hitori]

Center of thrust might not be important at all, but center of lift need to match as close to it as possible/needed.

If center of lift is in front/behind the center of thrust, then you should be fine even if the center of mass is a bit off.

Either that or add a thruster aimed uppward but with a reduced thrust limit so it scales perfectly with the main thrust, not recomended tho as it wastes fuel.