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4 Answers

Ground speed increase as altitude increase

Asked by: 11556 views Aerodynamics

As most of us know that general rule of thumb TAS increase 2% per 1000 ft

And as altitude increase, air gets less denser, hence density altitude increase

Question:

Next, as altitude increase, air is less than, less oxygen for combustion, therefore we lean the mixture, thus engine performance will decrease, am i right? Therefore would't it be ideal to just fly at a lower altitude?

 

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4 Answers



  1. Nathan Parker on Jan 13, 2012

    “Therefore would’t it be ideal to just fly at a lower altitude?”
     
    What really matters is how quickly thrust goes down compared with how quickly drag goes down as altitude increases.  Given that performance charts show an increase in TAS up to some critical altitude, it appears that drag reduces faster than thrust, up to a point.
     

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  2. John D. Collins on Jan 13, 2012

    You really have to define what you mean by ideal.  The best L/D speed is independent of altitude, so if you want to achieve best range in still wind conditions, the IAS is independent of the altitude (assuming a IAS well below the speed of sound so that compressibility doesn’t need to be factored in).  Unfortunately the best L/D speed is slower than most want to operate their airplane at as it is also the best glide speed. If you select a constant power setting, then a higher altitude will result in a faster TAS.  Professor Carson of the US Naval Academy developed an efficiency speed that he defined as the fastest cruise speed obtained by the addition of a unit measure of fuel expended.  It works out to be equal to the speed for best L/D divided by .76, or multiply the speed for best L/D times 1.32.  For my Bonanza, the best L/D is approximately 105 Kts and the Carson efficiency speed is approximately 140 Kts.

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  3. Best Answer


    Bill Trussell on Jan 13, 2012

    Your assumption of lower engine performance as altiude increases assumes that there is no turbo charging involved.  Every reciprocating engine has an altitude above which it is unable to produce the same amount of horsepower that it did at sea level.  Leaning the mixture helps to achieve the best performance possible, but that may be lower than that available at sea level.  This can be seen by looking at the power setting tables or graphs in the POH.
    The impact of lower drag at higher altitudes also contributes to better overall aircraft performance to a point.  Remember the phrase ” you don’t get something for nothing” and use the POH to determine the optimum altitude for the proposed operation.

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  4. Brian on Jan 16, 2012

    “Every reciprocating engine has an altitude above which it is unable to produce the same amount of horsepower that it did at sea level.”

    Assuming normal reciprocating engine this would begin at 1 foot off the ground. Now, the typical max cruise power setting is 75%; which I think is what you’re referring to. But the engine, non the less, begins losing power the moment it leaves the ground. Not at some arbitrary altitude as I read the quoted text to suggest. (Maybe I’ve misread? If so, oops. 🙂 )

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