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I tried looking online, but all I can find is what pitch ratio is, butI can't find anything related to the question above. Help will be appreciated.

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  • $\begingroup$ Different aircraft need different size propellors due to engine size, load capacity etc and the pitch needs to change for cruise, climb etc... $\endgroup$ – Solar Mike Jul 29 '17 at 15:34
  • $\begingroup$ If you want to control the amount of thrust produced by a propeller, there are two options - change the RPM or change the pitch ratio. For the "simplest" small aircraft, varying the engine RPM works well enough, but for larger propellers the maximum RPM is limited by the tip speed of the blades remaining subsonic for efficiency and noise reduction, so running at (approximately) constant RPM and changing the pitch ratio is more practical. It also means you can generate reverse thrust from the props for braking after landing, feather the prop on a failed engine to reduce drag, etc. $\endgroup$ – alephzero Jul 29 '17 at 16:04
  • $\begingroup$ Thanks for the help guys, one more question, does a higher or lower pitch ratio increase the thrust produced by a prop? Thanks again in advance. $\endgroup$ – Lin Ko Jul 31 '17 at 3:24
  • $\begingroup$ A higher pitch will increase the angle of attack fo the blades, and thus their lift and drag. So if the engine has enough excess power to keep the rpm constant, higher pitch will mean higher thrust. Look up blade pitch on wikipedia for a decent article. $\endgroup$ – AEhere supports Monica Jul 31 '17 at 13:00
  • $\begingroup$ Which part of the control system of the drone would be responsible for sensing and changing the blade pitch? $\endgroup$ – Lin Ko Aug 1 '17 at 8:14
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Pitch ratio is important because it is a key parameter in propeller performance. As you have probably read already, it reflects the angle of attack of the blades, and thus their lift. Many propellers can very their pitch in flight to adapt to different requirements.

Look at the entire system: an engine produces torque, the amount of which depends on its rpm. The propeller converts said torque into thrust for the aircraft. The rpm at which the prop-engine system will settle is the one at which the torque from the engine matches the torque exerted by the airflow on the prop. This can be varied by altering the propeller pitch.

An additional use of pitch control is to "feather" the propeller, which is to place the blades in a least-resistance configuration when the engine fails, so the now useless propeller will generate the least possible drag.

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