Hi guys,

I am just seeking some assistance with mass balance and arm distance/control effectiveness. I've come across this question, what would the correct answer be?
An aileron with a mass balance is fitted on an upward reflex wing (dihedral). In relation to the aileron centre of gravity and the hinge line, this would give a:
a. greater arm and greater aileron deflection
b. lesser arm and lesser aileron deflection
c. lesser arm and greater aileron deflection
d. greater arm and lesser aileron deflection

I understand that a mass balance is used to reduce the arm about the hinge however what effect would this have on control surface deflection?
Thank you!

Hi Bob,

Interesting question and I would go for "(c) lesser arm and greater deflection".

My argument would be: "Lesser arm" because the CofG of the aileron would move towards the hinge line and "Greater aileron deflection" because the dihedral is going to provide a degree of lateral stability. This lateral stability would need to be overcome with forces generated by the aileron. As far as I am aware, the presence of mass balancing does not affect control deflections required so the question wording seems odd to me.

Looking forward to some other opinions on this one

Cheers,

Rich

Thanks for your response. I personally would have gone for that option as well however I did find the question a bit ambiguous. I do have a few more questions if you don't mind...
What is the difference between lift and weight and thrust and drag in a powered descent vs non powered descent? I am under the impression that since an aircraft is descending, that weight would be greater then lift and since the aircraft is traveling forward (and not plummeting straight down to the ground) thrust would be greater then drag for both cases. Can you confirm that this is correct?

Also a question on range and endurance. I understand that a piston engined aircraft should fly 'low and slow' to achieve best endurance and that an aircraft should fly at its full throttle height to achieve max range. Are these also correct for a supercharged/turbocharge engine? Am I right in believing that a turbine engined aircraft will have to fly at a higher altitude to achieve its best range when compared with a piston engined aircraft?

Thanks!

Hi Bob,

In a constant descent (i.e. a descent in equilibrium), the forces of lift and drag combine together to balance the combination of thrust and weight. In an un-powered descent, lift and drag are balancing weight only. In this case, the forward motion is being driven by the forward component of weight.

If you add thrust, you will start to accelerate downwards as the thrust and the forward component of weight will overcome the drag and the aircraft will accelerate to a new equilibrium (assuming the ground doesn't get in the way first!). If you want to maintain the same speed but have some thrust coming from the engine, you would need to raise the nose so as to reduce the forward component of weight acting in the direction of motion, parallel to the thrust.

In both cases though, while an aircraft is in equilibrium in a steady descent, lift will be less than weight (because the vertical component of drag is helping to support the weight) and thrust will be less than drag (because the forward component of weight is providing force in the direction of motion and therefore helping thrust to counteract drag).

As for the maximum range question, maximum range for the turbo-charged aircraft will occur at the full throttle height which results in the indicated airspeed for minimum drag/best range. Because of the turbo-charging, this height will be higher than without turbo-charging. For best endurance, low and slow still applies. You want thickest air for the best lift potential combined with the lowest practical power setting so the fuel lasts as long as possible.

Cheers,

Rich