Expert: Paul Soderman - 7/23/2010

Question
QUESTION: Sir,

I would like to know about a specific configuration which is mostly used in the fighter aircrafts.

the Canard configuration is commonly used in the fighter aircrafts for control purpouse, in essence to replace the rear wing and elevators for the control.

Here the sole purpouse of this canard is to control the vertical stability of the plane as that of the elevator.

Recently for one of the design competetions i ve built the lifting canard configuration with an intention of optimising the lift characterstics of the model. The tractor propeller is mounted on the top of the rudder.

Glide characterstics:
The glide characterstics for this model are observed to be outstanding than that of a conventional model.

Powered model characterstics:
The main problem came in this field when it is powered. It has this tendency to to glide leftward and also to suppress itself and thus not allowing it to fly.

This is what has perplexed me the most. After careful calculations and achieving good gliding characterstics the powered dynamic characterstics have proved that the model is unstable.

Can you please add some suggestions as to what improvements i can make to achieve the same and also if there are such configuration airplanes in the field.

Thanks for reading,

Anudeep Palanki

ANSWER: Without seeing the airplane it is difficult to say.  But an aircraft that glides well and fails to track under power suggests that the propeller is putting a torque in the aircraft and probably a pitching moment that must be countered by control surfaces.  The control surface movement is inducing a yawing or rolling moment or both.  You need to study aircraft stability and control to estimate what moments and forces are being introduced by the control surface deflections.  A change in wing or canard dihedral may help.  It is also possible that the propeller is changing the flow over the wing and/or canard.

---------- FOLLOW-UP ----------

QUESTION: I accept that the prop is putting unwanted thrust on the plane. But the problem which i am facing is that its pushing down the plane while creating an thrust because of which after attaining a certain velocity its tending to steer leftward and goes out of control..

My main wing has an dihedral angle of 20 degrees sir to provide roll stability and its the only thing which prevented our model to crash..

Since in glide none of these dynamic characterstics take over so its pretty stable in flight.

Can we check these characterstics by simulating in ANSYS 12.0 or anyother software sir??

I am also attaching a picture of our model can you please suggest some modifications.
This is the photo taken when around 80% building is completed.

ANSWER: I don't see any control surfaces; i.e., ailerons, rudder, elevator.  How do you counter the moments introduced by the propeller ?  I am not familiar with your software.

---------- FOLLOW-UP ----------

QUESTION: There are no ailerons. For this competetion we are allowed to have only 2 servos for which we control the pitch and yaw. The elivator is on the forward canard while the rudder is on the vertical tail plane sir. Its yet to be attached in this picture.

Can you suggest some software to simulate these conditions sir??

Answer
No, I an not familiar with such software.  It seems to me that if you introduce a torque on an airplane from the propeller you have to have ailerons or some way to introduce a counter moment in roll.  I suppose you could somehow twist the wing tips to introduce a rolling moment.  But movable surfaces would be much better.  The rolling moment of a twisted wing tip would be small until the forward velocity is high enough; the propeller torque would be present at all times.  Maybe you need to swap yaw control for roll control if you can only have two servos.