Hi All
I´m debugging my new hydro, wich is my own design. It´s basic design is to be an easy to build electric model with high wing (really parasol).
There are some minor problems wich are not hard to sove, but ther´s another problem quite hard to understand for me and then to repair.
On It´s original configuration l never tried to take off from water, only some hand launch .
She flew normally unless at less than half of power, if power was full, nose pointed down and need all elevator UP to keep it leveled. Motor and wing were 0º incidence in relation witn fuse top.

Due to water intake for the sides of fuse and some difficulties handling receiver & batteries and pilon fragility decided to change the configuration making a "canopy" and the motor was moved to the trailing edge of the wing. Again incidence was set to 0º related to the wing wich was +2º related with de fuse top.

Again whit full power its nose goes down and with just a touch of power she flies normally and very easy.

It seem ther´s a motor incidence problem ..but really I´m not sure how to set up a pusher motor in such position. Anybody can help ?


Add two images, on land is original configuration, and second is on water

Two things might help (1) move the CG back a half inch or so and retrim. That will make your airplane less likely to climb when you add power. (2) Put down thrust in the engine, maybe as much as 10 degrees. The downthrust will be more effective the more power you apply.

Ican think of two probable causes for the nose down problem:

-The wing generate a strong nose down moment at full speed.
-The propellor and motor provides thrust at the end of a long moment arm.

If the problem is due to the thrust moment arm, it should have improved when you modified the hull/fuselage with a higher wing position.

To check if it is the wing isgenerating a nose down moment, try reflexing the ailerons. (Adjust the linkage to raise both alierons, maybe 3mm up.)

Second try adjusting the motor thrust angle. There are two possibilities, both might work.

Adjust the motor thrust angle to make the slipstream blow down against the stabilizer. With full power the stabilizer should then generate a stronger down force at the tail, holding the nose up. I would try this first.

Adjust the motor thurst angle to push the nose down, in other words oposite to the above.

Enlarge the stabiliser with larger area, not a cure but the pitch down might become less severe.
Change to a T-tail layout, this put the stabilizer up in the slipstream, making it more effictive.

Himat

The wing is a foam copy of that normally build for SPAD (Corroplast models) wich I build in may models and never have such kind of problems. It has not a normal airfoil, but never my SPADs have any kind of nose down. The attached file shows the wing style of construction
In my opinion problem is related with thrust angle, why?...cause even al low speed if accelerate suddenly nose come down inmediately, not when speed increses, just when power is increased.
Even hand launchig at half throttle is safest than full power cause the model flies on rails at that level of power.

T tail is a must, cause in water test the elevator has problems touching water making almost impossible to take off from water.
So next changes are, T Tail (working on it now), adding some kind of extension at tail for longer tail moment, but need a best idea wich is te best thrust angle

You said
Don´t understand why...it can make my problem worst !?

Jim problem is just the opposite, nose comes DOWN whe throttle goes UP, model enter in a soft dive, not climb.

When the thrust line is above the center of drag for the model you need to compensate for that. On the first version with the pylon the motor needs to use a goodly upthrust angle in order to avoid the nose down issue at full throttle. Sometimes even as much as 7 to 10 degrees.

On the rear mounted version in the second picture you're thrust line is down closer to the center of drag but now your prop is behind the CG. For this setup you want to use some upthrust angle so there it lifts the nose at full throttle. I'd start with about 2 or 3 degrees and alter if from there for more or less upthrust. And in this case, just so we're all clear, upthrust for a pusher model is when the prop is more forward at the top of the circle than at the bottom and the axis of the motor is angled down as it extends to the nose. The same angle for a motor and prop on the nose would be DOWNthrust but because your prop is behind the CG it's now UPthrust. Clear?

Thanks for more information about your plane.
As you states, with the nose down problem on aplication of power, it is probably a thrust line problem. Quite common to seaplanes with a high motor and thrustline.

First, T-tail and longer tail moment sound like a good idea.

The devil is in the details.
And sorry, my explanation was not that good.

Thrust angle, I would first try adjusting this to get the propellor slipstream down against the stabilisator. Thus increasing downforce at the tail. This worked on a design I did.
This is actually is angling the motor to push the nose down, but with the motor more or less straight above the centre of gravity/centre of drag the change in thrust angle have little leverage. The aerodynamic effect on the tail have a longer moment arm and more leverage.

Doingitthe other way around, the thrust push the nose down, but have to fight a decreasing tail down force as the slippstream change the stabiliser angle of attack.

It's a matterof balancing two opposite effects. The question is which is the strongest in your design. Tail downforce due to the slippstream changing the stabiliser angle of attack or the thrust force direct pitch moment.

Wooow

This post is becoming very interesting. While reading post I´m waitning for cement to dry, and repairing some minor problems AND discovering very interesting facts

Due to some problem in BEC contacts must test motor and when did [:-] can see clerly that wing is deformed by the thrust !!!!. Show the video please !

[youtube]http://www.youtube.com/watch?v=zsqt_gVrV20[/youtube]


So if agree with Himat, what´s wrong is that the airflow at low rpms / speeds hit the elevator , and at higher rpms the flow becomes paralel to elevator due to the distortion..and then the balance is changed making the nose go down.

At first sight BMatthews opinion seems to be in oposition of Himat´s, but BMatthews is talking only of thrust angle influence, and not of elevator being pushed down by airflow...

I must agree with BOTH , and the next question is:

If modify the tail to a T (really a + ) setup, then the elevator will receive a "centered" airflow and BMatthews "effect" may be more important than Himat´s one. So a good solution for this may be a + tail with more tail moment and the elevator fill moveable wich may be very efficient !!!

Great ! people in this forum is wonderfull !!!

In the air the propellor does not so much throw air as it screws the plane through the air. So while Himat's thoughts make sense while on the ground the elevator won't care about being in the prop wake or not once up to a reasonable speed. Only during ground handling and for the first part of the takeoff does the prop blast become a big factor. Once in the air the stabilizer won't see a prop blast other than somewhat at near the stalling point if you jam on full throttle all of a sudden.

Hold the model at the wingtips with your fingertips at the CG and experiment with different motor thrust angles until you reach equalibrium.
I've done this with scratch built models and it works.

Sorry, my reading comprehension failed me. Do just the reverse of what I advised.

I put a landing gear on this Gentle Lady and used a tractor and pusher power pod.
Adjusted so the plane would take off without any elevator, I had to reduce the downthrust on the pusher motor several degrees, so that the plane would accelerate to a level attitude on the wheels, and then fly off the ground as the speed increased.
The tractor pod needed no changes.

Problem solved using all sugestions posted here and common sense !

-Added some lenght to the fuse for more efficience
-+ Tail to avoid water contact
-Motor incidence changed as posted BMatthews
-Tested as CombatPig

Thanks to every opinion !

See how it works !!!

[youtube]http://www.youtube.com/watch?v=UfXDmtFhNl4[/youtube]

I missed this bit of wisdom until baco mentioned it in his last post. There's a goodly amount of practical common sense to this method since the wing on most models is a very primary source of the overall drag in flight. I gotta keep that one filed away for when I need it myself!

Baco, the video looks great! You're making me think that it's time for a flying boat of my own...