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Design Fridays: That’s a big prop

January 28, 2011
The small UAV market is smoking hot. Boeing has a high altitude long endurance (HALE) they’d like to sell to someone. And they’re not exaggerating in taking on the HALE acronym, it can reach an impressive 65,000 ft altitude and supposedly stay aloft for up to four days. It’s doing some final ground testing now before actual flight testing commences so we’ll see the truth in their claims rather soon. Powered by two engines it will also have two really big propellers. 16 ft in diameter big. It dwarfs the not insignificant 2.3L engine that powers it. Here’s a video of their ground testing with the prop:

I’d like to see the prop map on that. What’s a prop map you ask? Well when designing a propeller the contour of the blade lends itself to a certain efficiency at certain airspeeds and altitudes. There is no one blade that will work for every aircraft so you take a known propeller and its efficiency curves and map it out into little efficiency islands.

Then you can take this information and figure out where your aircraft would perform and how efficiently the prop is doing at various airspeeds and altitudes (air density is what’s important here) and compare how efficient your prop is at your max takeoff speed or at your high altitude cruise speed as in the table below.

Given Boeing’s Phantom Ray has two engines and two props I wonder if the redundancy allows it to stay up in the air if one engine fails like many larger military or passenger aircraft. At the least I think this is the start of higher expectations for UAVs where once 20-30 hours was considered an endurance flight I expect that will be too short of flight time going forward.
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