Designing an airframe? Heres some notes about evaluating performance

Above are the lecture notes for my aircraft performance course.  If you are serious about developing an airframe then I would recommend you read through these, its all very useful and interesting.  They're notes written to prepare a future aeronautical engineer!

they basically cover calculating and predicting (for different altitudes, payloads and drag coefficients):
  • rate of climb
  • service cieling
  • finding optimum and economic flight regimes
  • takeoff, ascent, cruise, descent, and landing performance
  • turning performance
And just some interesting stuff really.  You may think that varying altitudes are irrelevant to DIY UAV's, however hotter or colder conditions affect density as much as altitude does (changes in density are quite influential on an aircraft).

A very interesting and easy to miss part is the solution to question 3 of section 5.  It basically describes how to experimentally determine your drag coefficients !

That is, as long as you have a way of measuring:
  • shaft power ( Power [W] = torque being produced by motor [Nm] x RPM [rad/s] )
  • your propeller efficiency
  • airspeed ( using a pitot static tube, GPS speed is NNNOOOOTTT good enough)
  • the aircrafts weight (the only easy one :])
  • density  ( density [kg/m^3] = pressure [Pa]/ ( 237 x temperature [K] )  )

If you do look through these notes, please note that at least half of the solutions to the examples have numerical errors somewhere, but their methods all hold true! Also there is some stuff about Mach numbers which you can definitley ignore, I don't think any DIY UAV will ever be even near Mach 0.7+

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So you're thinking ' what the hell man, how am i going to measure my motor tourque in mid air !?!? '

you could buy an expensive tourque cell OR set up you motor+prop+esc+battery on the bench like this:

And measure your rpm however you want
Then also measure the power being supplied to your ESC ( Power = Voltage x current ), and find the ratio between tese powers, ( ie. the efficiency of your motor+ESC )

This allows you to find the shaft power in flight by doing 'electrical power x motorESC efficiency'
This means you will need a voltmeter and ammeter in flight though.

I haven't tried or tested any of the stuff becasue of uni exams, but over chrimbo i'll be trying all this stuff out !

Thanks for posting this - It looks like I too have some reading to do over the break ;)

Yeah, some interesting stuff in there if you're into design and optimisation !

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