Designing a Propeller for Very High Altitude is Harder than you Think

[Jonfliesgoats]

At lower atmospheric densities, any surface has to interact with a volume of air in order to generate equivalent forces.  This means you either need to get faster (SR-71) or have bigger surfaces (U-2 or Canberra).  When we start making propellers for high altitude we get into a series challenges.

1.) The speed of sound is slower up high (due to low temperature).  But the propeller has to turn faster.  Exceeding the speed of sound drastically reduces propeller efficiency.

2.) making a larger diameter propeller allows us to operate with lower RPM, but the higher diameter also puts the tips close to Mach 1.

3.) Even with problems 1 and 2 worked out, an airplane flying at high altitude has a higher true airspeed for a given dynamic pressure.  Keeping a high altitude propeller at sustainable RPM means the propeller pitch (the bite the blades take out of the air) is often so high that a large amount of thrust is lost and lots of force is vectored against the tongue generated by a motor.

4.) Propeller blades which don't interfere with each other at low altitude start to have significant blade to blade interference at altitude.

High altitude propellers exist, and can sustain flight as high as 90000 feet!!!  It took a lot of very smart people a lot of effort to get blade design just right.  All of that started with some guys eyeballing wood in the early 1900s.  Here's a link to a NASA publication regarding a prop design for an 85hp motor to fly at 85000 feet and .4 Mach.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980017535.pdf

[Jonfliesgoats]

At lower atmospheric densities, any surface has to interact with a volume of air in order to generate equivalent forces.  This means you either need to get faster (SR-71) or have bigger surfaces (U-2 or Canberra).  When we start making propellers for high altitude we get into a series challenges.

1.) The speed of sound is slower up high (due to low temperature).  But the propeller has to turn faster.  Exceeding the speed of sound drastically reduces propeller efficiency.

2.) making a larger diameter propeller allows us to operate with lower RPM, but the higher diameter also puts the tips close to Mach 1.

3.) Even with problems 1 and 2 worked out, an airplane flying at high altitude has a higher true airspeed for a given dynamic pressure.  Keeping a high altitude propeller at sustainable RPM means the propeller pitch (the bite the blades take out of the air) is often so high that a large amount of thrust is lost and lots of force is vectored against the tongue generated by a motor.

4.) Propeller blades which don't interfere with each other at low altitude start to have significant blade to blade interference at altitude.

High altitude propellers exist, and can sustain flight as high as 90000 feet!!!  It took a lot of very smart people a lot of effort to get blade design just right.  All of that started with some guys eyeballing wood in the early 1900s.  Here's a link to a NASA publication regarding a prop design for an 85hp motor to fly at 85000 feet and .4 Mach.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980017535.pdf

[benzman]

Realising the problems with propellers at high speeds and high altitudes is precisely why people developed jet engines.