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Endurance vs altitude

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PHILIP ARTHUR created the topic: Endurance vs altitude

Hi

One of my KDRs for CPL performance is about endurance. I thought endurance (as opposed to range) is unrelated to altitude, as it has more to do with power setting. I got it wrong it appears. Can you explain how altitude affects endurance and how an aeroplane will achieve maximum endurance by changing altitude? Lower is better or higher? Note this is not about range, where I understand the TAS will be higher at higher altitude due to decreased air density so range will be greater.

Another one has to do with comparing LDA and TORA. I thought they are the same, being the distance from the runway threshold to the end of the runway strip (not including clearway or stopway). When would LDA be different from TORA? I'm not talking about TODA here, just TORA.

Thanks
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  • John.Heddles
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John.Heddles replied the topic: Endurance vs altitude

Endurance requires us to minimise the rate of fuel burn which, for a prop aircraft, is related to power. The detailed analysis gets a little involved so it is probably better that I leave fleshing out the answer to Bob/Stuart so that you get whatever answer the examiner seeks rather than the engineering story.

So far as LDA and TORA are concerned, they should be the same if approach gradients don't present a problem. If there is tiger country on the approach, then you may have a displaced threshold for landing and then the two certainly will differ.

There used to be a nice little CAAP which told the story but now it's in MOS 139 www.legislation.gov.au/Details/F2017C000...lume_1#_Toc473551580 - see para 5.2.2.2

Engineering specialist in aircraft performance and weight control.
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PHILIP ARTHUR replied the topic: Endurance vs altitude

Thanks John. I suppose with a displaced threshold the LDA could be shorter than the TORA. Maybe that's what they were getting at. Regarding fuel burn, I know that you throttle back to increase endurance but the question was specifically about whether changing altitude would increase the endurance. I can imagine at higher altitudes that the engine is less efficient as there is less oxygen but will that reduce the endurance?
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John.Heddles replied the topic: Endurance vs altitude

I suppose with a displaced threshold the LDA could be shorter than the TORA.

Did you have a read of the link I provided ? Suppose becomes will be.

Maybe that's what they were getting at.

A near guarantee on that .

Regarding fuel burn ...

There is a number of competing considerations. Better that Bob/Stuart indicate just what the examiner wants in the answer these days rather than my complicating the subject for little point. You could have a read of the following

people.clarkson.edu/~pmarzocc/AE429/AE-429-11.pdf
my.fit.edu/~dkirk/3241/Lectures/3241%20Lecture%2041.ppt (copy and paste into your browser)

However, the general guidance is that we go for low for endurance in a piston/prop combination. This can be seen from the two links if you have a wade through them.

Engineering specialist in aircraft performance and weight control.
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PHILIP ARTHUR replied the topic: Endurance vs altitude

Thanks again. I've just referred to Bob's CPL Aerodynamics book and the explanation is there. TAS determines the power required for a piston engine aeroplane. At higher altitude you need a greater TAS for a given IAS so more power is required to achieve the same IAS. So for greatest endurance fly as low as possible with engine RPM as slow as possible.
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