The engine scoop/intake on the MQ-9 features a vertical splitter at the inlet face, as well as another smaller NACA duct on the top cowl surface just aft of the inlet lip.

I am guessing the former functions as a vane or diverter to channel airflow to different parts of the engine presumably for cooling, but the additional NACA duct at that location seems unusual (another inlet for an inlet?).

Is that NACA duct meant to modify the airflow within the engine scoop? Or is it specifically routed to another part of the engine altogether?

MQ-9 Reaper on Runway, from https://www.youtube.com/watch?v=X-PPzDwmf3Y


2 Answers 2


Went to do some research after @niels nielsen's answer; the engine being used on the MQ-9 is the Honeywell TPE331-10. As Niels has rightly pointed out, the turboprop engine has its inlet facing the rear, and the turbine exhaust faces forward (I believe this is known as reverse flow).

Screen grabs from the Youtube video below. The left image depicts the aft section in off-white (gold flanges are to the prop driveshaft), with the intake below. The right image depicts the turbine exhaust, which faces forward. enter image description here

Now to answer the question: inferring from this Alamy stock photo, the splitter for the main scoop diverts the airflow into two channels for the main engine intake (the two red kidney-shaped covers). The NACA duct along the centreline is for a separate cooler (the rectangular opening).

Additional photos also show how the air is routed around the engine. The left picture shows a duct that likely comes from the port air channel, which is connected to a plenum(?). The right picture shows a clearer internal view, without the duct. The duct exits are circled in yellow. I am guessing this is the same on the starboard side. View of aft engine without cowl

From the information that I could find, this is my guess on how the internal ducting is done. Please excuse the crudely annotated screenshot: enter image description here


The NACA duct is probably routed to a gearbox oil cooler. The turboprop engine has its air inlet at the prop end of the engine, which means a splitter is needed to divide the incoming air flow into two ducts, one on each side of the engine, in order to conduct the air back to the inlet.

  • $\begingroup$ Thanks for the clarification; it had not occurred to me that the turboprop was a reverse flow type. Did some more research and found several pictures which corroborate with what you mentioned. $\endgroup$
    – tacitrb
    Commented Sep 26, 2019 at 14:14

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