Capt Pit Bull

People get fixated on the idea that there has to be some thrust somewhere.

Stop looking for the thrust. There isn't any. You don't need to find it, its not gone AWOL

I can understand what you're thinking. You understand that if you resolve forces along the flight path then Drag = W Sin (Glide angle). You're trying to relate the situation to something the student already knows - always a good plan. But when you refer to it as 'Thrust' you're being incorrect and you aren't actually doing the student any favors.

The particular danger is that you'll get someone that's shaky on newton 1 and is still latched onto the (incorrect) idea that if an object is moving there must be a force acting in the direction of movement. This is the sort of student thats happy with the idea that you found the 'thrust' because that fits right in with their misconception.

If you want to prove the derivation of optimum glide angle, there is basically two ways.

a) Resolve the weight, Write down some equations about lift and drag. Each will involve some trig. Then set yourself the task of combining those equations and finding the condition that makes the glide angle smallest. This requires either a knowledge of trigonometry and algebra or some rote learning.

or

b) Establish that in a steady glide (T=0) Weight is equal and opposite to total aerodynamic reaction. Then recall (the student should already know) how lift and drag are defined in relation to direction of moving. It is then immediately obvious (with a couple of sketchs, one for good L/D and one for bad) that the L/D ratio determines the direction of movement.

Although (a) is a very commonly used method, (b) Simply requires the application of things already known with no need for algebra or trigonometry.

I've been using (b) for years and its quick and works a charm because its visual rather than equations. But you have to be happy to let go of thrust

Bottom line is that you should teach it a way that you're happy with, and if the students get to the end result, its all good.

Think of option B as a Dr Pepper moment... try it... you might like it

Sorry for banging on about this, but I enjoy it and don't get much student contact these days

pb

Cows getting bigger

DFC, thanks for the lesson in stability. However, I think you are confusing thrust with power.

DFC

Capt. P. B.,

Yes I see your point about Thrust being AWOL. I should have spoken about the effect of gravity along the flight path.

The problem is not contact with students - the student can't really debate the issues. It is contact with other instructors. Even the "refresher" seminars lack any serious discussion on matters such as these.

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C. G. B.,

Not at all.

The total drag curve show the total drag at various speeds.

To maintain a constant speed in a powered aircraft, one has in simple terms to make thrust equal to the drag.

In a jet aircraft it is (relatively) simple - make thrust equal to drag and you are a happy bunny.

However, in a propeller aircraft as you have quite correctly pointed out, the power required curve is not the same shape and in fact can be very different from the total drag curve due to propeller efficiency etc.

That is why I have tried all along to use Thrust and have tried to only use B747 (jet) or glider as examples.

So let's forget about "power" since that is a different debate.

Regards,

DFC