A nose dive problem in self made balsa pusher configuration model.

Dear All,
                I had made a balsa mosel of pusher configuration with the poluhideeral wing span of 1.6m.
I had just done a flight test today and recorded some observations.
1. I did a hand launch , plane dives when throttle is more than 3/4. otherwise it climb up smoothly on less throttle .
2. its gliding is excellent ,  so i can say that C.G. balanced correctly on model.

3. As i increase throttle more than 3/4, i t does a nose dive. Can anyone suggest me what cud be the problem.
I checked the angle of incidence on wing and tail , Tail=0 degree and wing = 2 degree(which gives better results on the earlier made models).
The motor angle is 1degree down. Please suggest what cud be the problem

Thanks in advance.

 

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Increase the motor angle to 3 and see. Can you share some photos of the airframe? good luck
Viky,

on pusher planes, motor incidence has to be positive !
Say propeller toward sky.

Anyway where is motor mounted, in line with fuselage or higher like EZ* etc ? This last one is causing pitch down even more.

About CG a good glide is not an indicator of right CG. You can try CG playing with throttle while climbing. You can do that watching the behavior in inverted. You can put the plane gliding in a 45° dive; when you center the sticks if it goes straight it is well centered, if it climbs it is nose heavy (yes that's right !) if it dives more it is tail heavy. But remember that all this is related to wing and motor incidence too.

Best regards,

Ric
Oh thanks Morli, Ric and Rana,
Initially i tried with a zero degree motor angle, there was a huge thrust of motor towards down side, so i just change the motor angle to one degree down and i was able to fly my model at 3/4 throttle but not good as i increase throttle it pushes it towards down (nose dive). So tomorrow morning i will try again by increasing motor angle 2 degree downwards, so that it can push the plane straight /up . Lets see what will be the results.
Please suggest if i m going wrong.

With Regards
Vikram
As Ric mentioned, for pushers the the motor should be tilted up, not down.
Hi Rana,

no, just for that I pointed it between parenthesis. If the plane is nose heavy it is trimmed pitch up. If you increase speed (diving) this is becoming more noticeable (climbing). This is more true on sailplanes.
For this reason if you want a real neutral plane you have to have all well tuned: wing incidence, CG, and motor incidence, especially on pattern planes.

What you mind is true for free flight planes.

Hope I was able to explain it.

Best regards,

Ric
Viky,

you have to think at it like a balance. The plane is hanging at lift center (about 30% of the wing, depends on wing profile). If you want a stable plane, GC has to be in front of it (not so much). A pusher engine act on the balance so if it is tilted down it does not push the plane upwards, it makes the plane turning around the "balance" point (center of lift), diving then instead. For this reason engine has to be tilted upwards.

For a 3D plane forget all this ;P

Best regards,

Ric
Hi Rana,
you have to read Ric's point again. He said inverted , if so he is right( actually both of you are right since you assume plane in normal attitude ).
@ Ric,
I also did not understand when you said propeller "towards sky". Propeller towards sky has different meaning in pusher and tractor config In pusher config , the prop is behind the CG point and therefore if the "angle of thrust" is positive ( prop is pushing air down )then the plane is going to dive when power is applied. But if it is negative ( prop pushing air slightly upwards in pusher config) then plane will climb when power/throttle is applied. This is what I see in EZ* and similar models. Is this correct? if yes then , what do we call this thrust line/ motor firewall setting, Positive or negative? Thanks for the explanation. I am with you regarding checking the CG and trims in inverted attitude.
Has to test it still, i will update as i get chance to test it again... may be i will do on this weekend.
Hi,
you should check thrust line to be zero angle and to put nearly wind chord.
How it?
Hi viky's,

good experience! I thinks you should check the thrust line location .You need to change the thrust line position near the wing chord line with zero angle.
thanks
Oh thanks hawk , it was very old problem we were facing and we were able to resolve it by advise of few experts like you had given. And you r right hawk we had concentrated on thrust line. to confirm our exact thrust line setting , We hang the plane tied with thread on cg with a football pole and apply thrust with different motor angles to confirm the thrust line. Atlast we have got quite good stable thrust line by these tests which helped us to save some risky flying tests. Now model is flying good. Thanks for ur valuable advise. I was looking for some free or cheap software for analyzing aircraft thrust line or structure stability . Do you know any.
Yes viky,

I know you are very concentrate ur Design.good luck with your job.I use Aeropack and AeroSim software using MathLab .u can downlaod free.
thanks

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