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Thrust pitch sea duck

Hai-Lee

Old and Bold RC PILOT
#2
Has any of you messed with you thrust pitch on your sea duck? I have had to put almost all of my down sub trim in it to get it to fly level
Seriously I would recheck my CG and the wing and tail incidence from the build first.

If the bird is tail heavy then it will require significant trim input to just fly level. The same can happen if the tail incidence angle is to positive and again if the wing incidence is far too positive.

Just a few thoughts!

Have fun!
 
#3
Seriously I would recheck my CG and the wing and tail incidence from the build first.

If the bird is tail heavy then it will require significant trim input to just fly level. The same can happen if the tail incidence angle is to positive and again if the wing incidence is far too positive.

Just a few thoughts!

Have fun!
My cg is perfect with a little nose down and it climbs hard with any throttle
 

quorneng

Well-known member
#6
The indicated CofG is only "correct" if the wing and tail plane incidence angles are exactly as indicated. The angles required are small so just a degree error can have a major effect.
The first thing is is to trim the plane to a constant minimum speed glide. If the elevator is still trimmed down (and the CofG is correct) then the angle between the wing and tail plane (the decalage) is too big. Lift the nose of the tail plane by a mm and try a glide again. When the glide does not require any down elevator trim the decalage angle is correct. Only then should you consider adding down thrust although the tendency to climb on power is likely to be much reduced.
Bare in mind that any stable plane will tend to climb as the power (speed) is increased. It just comes down to a matter of taste as to what you do about it either by adjusting the elevator trim (as they do in full size) or adding motor down thrust if practical.
 

Headbang

Well-known member
#7
I have built quite a few seaducks. If you built the power pod correctly (it would be difficult to build it wrong as it is super simple and only fits one way) the thrust angles will be perfect. Possible issues could be the booms are not level with the bottom of the wing, tail heavy, ailerons acting as flaps (not in line with wings at center travel). If the plane is tail heavy it will still fly well, I balance mine way tail heavy for flat spins and hovering. It does not sound so much like a cg issue.
The seaduck tends to fly well even if warped all to hell and wrinkled with half broken wings.
 

Sero

Well-known member
#8
As said check your incidence angles, thrust angles and control surfaces including the ailerons.

Bare in mind that any stable plane will tend to climb as the power (speed) is increased. It just comes down to a matter of taste as to what you do about it either by adjusting the elevator trim (as they do in full size) or adding motor down thrust if practical.
This is so true especially with the SeaDuck. The motors centre of thrust is above the wing chord which means with throttle, a lot of airflow will be above the wing creating artificial lift.
It can be minimized with experimenting with variations of incidence, thrust angles, trim, CG and adding a throttle/trim mix.
 

Headbang

Well-known member
#9
As said check your incidence angles, thrust angles and control surfaces including the ailerons.



This is so true especially with the SeaDuck. The motors centre of thrust is above the wing chord which means with throttle, a lot of airflow will be above the wing creating artificial lift.
It can be minimized with experimenting with variations of incidence, thrust angles, trim, CG and adding a throttle/trim mix.
The seaduck will climb, but it does not pitch up under throttle, the engines being mounted high tend to cause a bit of down pitch which cancels out a lot of the extra lift.
 

Sero

Well-known member
#10
The seaduck will climb, but it does not pitch up under throttle, the engines being mounted high tend to cause a bit of down pitch which cancels out a lot of the extra lift.
The op is experiencing climb with throttle, and from my experience and from what I hear, it is a common theme with the SeaDuck.

There are many variables that can affect this which I believe have been covered.

Which direction are your props spinning? Top in or top out?
 
#11
The op is experiencing climb with throttle, and from my experience and from what I hear, it is a common theme with the SeaDuck.

There are many variables that can affect this which I believe have been covered.

Which direction are your props spinning? Top in or top out?
If your looking at the props the left goes left and the right goes right over the top of the wing ?
 

Sero

Well-known member
#12
If your looking at the props the left goes left and the right goes right over the top of the wing ?
Top of the blades out/away from the fuse.

That's how I set-up my twins and I've read that it can create more lift as the prop wash spiral goes over the top of the wing. The ailerons have more authority for the same reason. I haven't verified that theory but it's easy enough to try.
 

quorneng

Well-known member
#13
The conventional argument is over the top towards the fuselage.
This is done on most high powered full size twins as in the case of engine failure the engine torque on the good engine works to lift the dead wing. Further on take off the ground effect on the prop wash tends to direct it towards a central fin increasing the effectiveness of the rudder.

How much this effects models is open to question but the benefit of counter rotation (no net torque) is very positive.
 

Sero

Well-known member
#14
The conventional argument is over the top towards the fuselage.
This is done on most high powered full size twins as in the case of engine failure the engine torque on the good engine works to lift the dead wing. Further on take off the ground effect on the prop wash tends to direct it towards a central fin increasing the effectiveness of the rudder.

How much this effects models is open to question but the benefit of counter rotation (no net torque) is very positive.
Yes you are right, most full scale plane when having counter rotating props go top in. For fin effectiveness but it also eliminates the "critical engine" or at least reduces its effect. For the RC world that is generally not of a concern. Especially for the SeaDuck since it has 2 fins right behind each motor and no rudder.

Also keep in mind that not all full scale planes have top in counter rotating props, the P-38 had top out. It was also one of the few (maybe only) wartime plane to have counter rotating props, most multi engine had same rotation to help keep interchangeability and therefore more planes in the air. The British switched their P-38s to same rotation for this reason.
 

quorneng

Well-known member
#15
Sero
The 'British' P38 was a low powered 'export only' version that did not have turbo superchargers which at the time were on the US 'no export' list. As a result it had limited performance and only at low altitude (the P38 was specifically designed as a high altitude escort fighter). It was only used for training and never for combat.
The prototype P38 had inward rotating props but it caused tail turbulence which was eventually cured by changing to outward. This lead to a number of accidents as the P38 entered service as an engine failure shortly after takeoff inevitably resulted in a roll over fatal crash. After much testing Lockheed eventually altered the pilot notes to instruct the pilot that if an engine failed on take off to reduce the throttle on the good engine to maintain aileron control, stop climbing and let the speed gradually increase reach to the minimum single engine manoeuvring speed. Only then could a turn be made to return to base.
The late war DH Hornet had inward rotating props as did the post war F82 Twin Mustang.
 

Sero

Well-known member
#16
Sero
The 'British' P38 was a low powered 'export only' version that did not have turbo superchargers which at the time were on the US 'no export' list. As a result it had limited performance and only at low altitude (the P38 was specifically designed as a high altitude escort fighter). It was only used for training and never for combat.
The prototype P38 had inward rotating props but it caused tail turbulence which was eventually cured by changing to outward. This lead to a number of accidents as the P38 entered service as an engine failure shortly after takeoff inevitably resulted in a roll over fatal crash. After much testing Lockheed eventually altered the pilot notes to instruct the pilot that if an engine failed on take off to reduce the throttle on the good engine to maintain aileron control, stop climbing and let the speed gradually increase reach to the minimum single engine manoeuvring speed. Only then could a turn be made to return to base.
The late war DH Hornet had inward rotating props as did the post war F82 Twin Mustang.
Thanks! I had read most of that previously at some point, a few new things new me.