Dear Bob and Richard,

Lift, weight, thrust and drag revision. Let me see if I have got this right.

1. In straight and level flight at cruise speed lift equals weight and thrust equals drag:
L = W, T = D.

2. In straight-and-level flight at slow speeds, the actual thrust is greater than drag, but wing lift is less than weight, so:
L < W, T > D

3. A climb is just the same except the thrust is even greater:
L < W, T > D

4. In a descent lift is still less than weight, but now drag exceeds thrust:
L < W, T < D

If this much is correct, please help me by correcting the following:

5. In a balanced level turn lift must now exceed weight as it as a horizontal component as well. So I think:
L > W, T = D

6. In a climbing turn thrust again exceeds drag, but what happens to lift? I think lift still exceeds weight:
L > W, T > D


Cheers, rubber

Anybody care to weigh in on this question?

All OK except:

2) I think you meant to say "wing lift is less than weight".

As for (6) here's something else to think about: in a climbing turn, the inclined lift vector would mean that lift has to be greater than weight but the thrust is also inclined and a component of it is supporting the weight as well. Therefore, you can't say lift will always be greater than weight. Taking it to an extreme, an aircraft climbing in a near vertical spiral is almost totally dependent on its thrust. The lift is definitely less than the weight in this case.

Cheers,

Rich

Thanks Rich, ah yes, now I have corrected my mistake in (2).

What you say in (6) makes sense. It depends on the amount of thrust.

I don't think there is such ambiguity in (5) however, as the turn isn't a climbing turn. I hope it is safe to say that in every case:
Thanks again for your valuable advice and assistance!
rubber

Why is point 2 not the same as point 1?

Hi basketball,

My understanding is wing lift is less at slow speeds than at cruise speeds but it doesn't fall out of the sky because the vertical component of thrust is helping to support the airplane. A slower aircraft is inclined a little bit upwards (tilting backwards) even though it is maintaining same altitude.

rubber

cheers rubber, now that you mention it, I do recall reading the same in Bob's book! Cheers!

I guess it all depends on your definition of lift. It can be confusing considering only a wing in a wing tunnel. The only meaningful definition of lift for a real aircraft in flight is to consider all of the forces acting at right angles to the direction of the relative airflow.

Lift would include the vertical component of thrust and any lifting force generated by the tailplane, fuselage and, in some cases the canopy! This would also mean that the centre of pressure of the whole aircraft would not be at the centre of pressure of the wing alone.

Thrust would be that component of the engine's thrust which acts in the direction of motion and drag would be all of the forces which act opposite to the direction of flight.

In other words it is much more complicated than a simple diagram of a wing in a wind tunnel suggests. Better not to think too hard about it because, as Shakespeare said in King Lear "That way lies madness"......

Point taken Bob. Though it is exactly the kind of question CASA asks in the exam.
I think Shakespeare was referring to the exam when he wrote that.

rubber