Performance - difference of opinion

[mad_jock]

It uses the word thrust so its talking about a force pure and simple, nothing at all to do with engines and there various effects
(swatton 13.2 page 96)

You can go from fully dirty spoilers out the whole lot at max thrust and say right well we have max speed now for this configuration so therefore is we are at max drag. Because thrust must equal drag for a steady state system.

But we are dirty so spoilers down.

Speed increases until max drag again equals max thrust and the beast stops acc.

But we are still dirty so up with the gear.

Speed increases again until max drag again equals max thrust and the beast stops acc.

But we are still dirty so away with the flaps

Speed increases again until max drag again equals max thrust and the beast stops acc.

Hang on we are clean now. Can't reduce drag anymore

So we are sitting at max thrust and max drag again.

Through all these cases the value of the max drag and the max thrust hasn't changed only the the speed at which it occurs. Which is the maximum for each of those configurations.


In fact looking at fig 14.6 page 111 in swatton it even has on the graph "Max Speed in Level flight". Where the drag and power curves cross.



Reply to Kieth

To be honest Kieth I don't think it is that badly worded.

My gut feeling is that it was written by an Engineer.

The word "conditions" tells me we are dealing with an ideal system so not to worry about in this case, flaps, spoilers, engine effects etc. So basic principles only.

"in order to achieve" says what in a simplistic system do you need to get x to be y in this case max speed.

And that question wouldn't be out of place on a first year Eng degree paper, but we wouldn't have given them the multiple choice and would have expected them to reproduce the power speed graph along with the prose with the reasoning, proberly worth about 5% of the marks, 10 minutes to answer it.

But as you say who knows what the world of JAR thinks is the answer. But as its written in a very typical engineering format for a question I would be pretty confident with my answer.