UAV Propulsion Tech – Post #2 – Get more thrust with a propeller reduction drive

reduction drive

This is my second UAV blog post and today I am going to focus on the advantages of a propeller reduction drive system to improve thrust in a UAV.  This will be detailed further after the key UAV news section.

Key UAV News for the week:

Get more Thrust with a Propeller Reduction Drive:

Reduction drive systems have been around a long time in the consumer aircraft industry (since the early 1900’s) but are not that common yet in the UAV industry.  Most systems are direct drive but some UAV companies are seeing the benefits in using a reduction drive to spin a larger propeller slower to increase thrust and reduce fuel consumption.

Commonly called propeller speed reduction units, these are either gear boxes or pulley belt drives that reduce the propeller speed compared to the engine output speed.  This helps optimize the propeller speed for the application.  Piston aircraft engines (both 4-stroke and 2-strokes) typically generate maximum power from 4,500 – 6,500 rpm but the most efficient speed for the propeller is typically 2,500-3000 rpm.  A reduction drive allows the engine to operate at peak power while spinning the propeller at its most efficient speed.

In optimization of the propeller, you do have to keep the propeller tip speed below the speed of sound.  I did find a website with a tip speed calculator.  http://www.hoverhawk.com/propspd.html This site notes the optimum speed is between Mach 0.80-0.92.  I found another site that can calculate static thrust based on propeller speed, pitch and diameter.  You can use this to see how a change in speed and diameter can improve thrust.  http://personal.osi.hu/fuzesisz/strc_eng/index.htm.

Reduction drives aren’t typically used on small engines (say <10hp) but can be effective on engines 15hp to 100hp+.  On one application, we used a 1:1.7 belt reduction drive on a 15hp engine and doubled the propeller diameter.  The result was the static thrust was doubled.  A couple things you have to keep in mind is there could be propeller diameter limitations due to the launcher (if there is a launcher), or ground clearance if it is a runway take-off/landing application.

Aero engineers should evaluate this alternative to achieve the desired thrust with the smallest engine possible in order to maximize fuel consumption and flight endurance.  This will also minimize overall propulsion weight.

Industry Events for the Week:

Quote for the week:

“If the facts don’t fit the theory, change the facts.” – Albert Einstein

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