UAV PropulsionTech Post #6 – COTS Piston Aero Engines for UAV’s

#6 post COTS Piston Aero Engines for UAV’s: A week ago a guy thought it would be a great idea to land a gyrocopter on the Capitol lawn to deliver letters to try to change campaign finance laws.  Pretty crazy idea that highlights some flaws in DC’s security over this type of spectacle.   Luckily he wasn’t there to hurt anyone.  I live in the Tampa area where this guy is from so have seen several articles and stories about this event.   It made me want to focus this week’s topic on the use of COTS piston aircraft engines on UAV’s. Piston engines (both 2-stroke and 4-stroke) have been used in manned aviation for a long time.  2-stroke engines are typically used in sport aviation applications like ultra-lights, paragliders, and experimental aircraft because of their great power to weight ratio.  This engine market is typically served by Rotax, Hirth, MZ and Limbach. 4-stroke engines are used on larger FAA certified aircraft like Cessna, Piper and Cirrus, and this engine market is typically served by Lycoming, Rotax and Continental. Some UAV’s are even trying to use some of these COTS engines for their applications but out of the box, the COTS engines are not the best solution for a UAV.  Even though some of these engines are FAA certified, they have not been optimized for a UAV.  They typically still require some modifications in order to meet the demanding requirements of a UAV.   Typical upgrades include:

  • Generators – Electrical power needs to be increased to meet the power requirements of the UAV.  Most COTS engines have small generators to power the fuel injection system (if available) and some aircraft electronics.  UAV’s require high power density generators that are typically heavily integrated to the engine to minimize weight.  Some also want a start function with the generator for electronic start capability.  Datasheet on Hirth 2kW starter/generator.

generator pic

Figure 1: Hirth Starter/generator integration in HF3503 engine

  • Fuel consumption – Low fuel consumption is a key requirement for UAV’s to improve endurance.  Some 2-strokes use piston porting and carburetors to reduce engine cost which compromises fuel consumption.  UAV’s need to have the lowest fuel consumption at cruise so the engines calibration for best fuel consumption needs to be optimized at this point.  This can be achieved by using direct injection to allow lean stratified operation (low fuel consumption) at cruise and to enable spark ignition operation on heavy fuel









Figure 2: Hirth’s “iPower” direct injection technology

  • Noise/heat signature – The exhaust system needs to be optimized to minimize noise, reduce weight and minimize heat signature (to reduce detection).

S1215 Web

Figure 3: Hirth S1215 UAV engine – Exhaust optimized for weight/noise

  • Electrical noise – The ignition, wiring and engine control unit (ECU) needs to be shielded to minimize electrical noise that can impact payload or communication.  This is achieved thru proper electrical connector selection and harness/ECU/ignition shielding.
  • Diagnostic/communication – The ECU should have a communication method (typically CAN bus) to interface with the flight computer to monitor engine operation.  Electronic fuel injected engines measure several engine parameters to control fuel/air/ignition and these parameters can also indicate engine health and fuel burned.









Figure 4: Hirth ECU with shielded case/connector & CAN or RS485 communication

  • Throttle control – A number of UAV’s are using electronic throttle actuators that are attached directly to the throttle blade.  This provides the best throttle control and doesn’t require the UAV manufacturer to adapt their own servo/linkage system.  Volz is one of the few servo suppliers with engine throttle solutions.

 DA20T clear

Figure 5: Volz engine throttle servo

As noted in this brief summary, COTS aero engines require further optimization to be utilized on a UAV.  This can easily be done and has already been done on some applications flying today.  Hirth’s F23 COTS 50hp air cooled boxer engine is a good example.  The sport aviation engine has been a big success in the sport aviation market because of its power/weight, cost and reliability.  When a UAV customer wanted to use the same engine, Hirth upgraded the engine (now an S1215) to include the items shown in the list above.  This now meets the UAV requirements that weren’t being met with the COTS engine.  This engine will be on display at AUVSI at the Hirth booth #2736 in May.  Stop by and check it out.

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Quote for the Week: “I believe with all my heart that our first priority must be world peace, and that use of force is always and only a last resort, when everything else has failed, and then only with regard to our national security”.  – Ronald Reagan.

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