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Engine inlet icing - design papers?

Hi everyone

Here’s a seasonal one !!! Picking your brains on icing on a hot July day!

I am working on the redesign of engine air inlets and soliciting the knowledge of the experts here – again!

I see that many small GA aircraft use the submerged NACA inlet. And yet NACA themselves say the inlet is inadequate to feed a cooler or an air filter.

The only benefit of the NACA inlet( to the best of my knowledge) is its low drag (ok ok and sporty looks).
And yet, it is used by respectable names like Columbia/Cessna for the 350 and 400 who were definitely interested in maximum efficiency too.

But I also observed that recent inlet designs (see for example how Diamond did it on the DA62 and other Austro Engine equipped aircraft) use a semi-submerged opening that has vanes on the sides to guide the airflow in. These inlets obviously “swalllow” the boundary layer, which supposedly causes a loss of efficiency.

In comparison, racers (F1 racers for example) use a protruding pitot style inlet with a sophisticated expansion funnel. I understand that this is by far the best way to go from a pure efficiency perspective AND in flight.
Pipistrel (who do not have any experience with icing) have put a pitot style inlet on the Panthera. And they made it so thin that it would probably take a nanosecond to freeze it shut in icing…

Aircraft like the JetProp or the TBB also use such pitot sty le inlets, but their lip is seriously de-iced.

From the above I would tend to conclude that all designers KNEW that the most efficient/lowest drag solution is the pitot, but that other solutions were adopted to avoid ice accretion on the inlet or other undesirable side effects of the pitot inlet.

Hence my questions:

1. Are you aware of research documents comparing the resistance to icing of various inlet solutions.
2. Do you think it would be a good idea to make a pitot style (protruding) inlet and permanently circulate warm air inside its skin to prevent ice accretion?
3. Can you think of other reasons for not using a pitot style inlet (for example bad performance on idling). Ugly looks is not an acceptable argument :-)

Thanks a have a sunny day

Last Edited by Flyingfish at 06 Jul 06:27
LSGG, LFEY, Switzerland

One one-liner claim that a NACA inlet avoids icing is here. That incident was never resolved BTW because the aircraft was very quickly sold by its owner (who was not the poster)

This is a great topic because there is much evidence that cowlings are mostly sub-optimally designed, and create a lot of drag. Previous thread.

Administrator
Shoreham EGKA, United Kingdom

Yes, NACA inlets are chosen in situations where the designers want to avoid (for simplicity of use, cost or weight reasons) installing anti- or de-ice systems. The cost is lack of efficiency. A typical example is the inlet of the Meridian (Turbine PA46 with a NACA inlet) vs that of the Jetprop (retrofit Turbine PA46 with a scoop). The Meridian has a much bigger version of the PT6 but its performance is heavily constrained by the inlet in comparison to the Jetprop’s smaller engine.

EGTF, LFTF

denopa wrote:

A typical example is the inlet of the Meridian (Turbine PA46 with a NACA inlet) vs that of the Jetprop (retrofit Turbine PA46 with a scoop). The Meridian has a much bigger version of the PT6 but its performance is heavily constrained by the inlet in comparison to the Jetprop’s smaller engine.

I think the permanently open inertial separator vs the pilot selectable one in the Jetprop along with the derating of the engine is the reason for the weaker performance down low.

EGTK Oxford

They are also factors, but I have it from John M that the inlet is a factor :-)

EGTF, LFTF

Not a research document, but anyway: Mooney changes the air inlet of their turbo charged model to the NACA style sunk duct with the 252. Never did I experience inlet icing with our plane, nor did I hear of anybody who has with this model, while this problem is very common with the older 231 model.

EDFM (Mannheim), Germany

I experienced once inlet icing at FL250 in a Cirrus SR22 Turbo. The alternate air door in the SR22T cannot be opened manually and did not open automatically creating a loss of engine power. So I would look closely at the design (of course depending on your flying profile).

EHRD, Netherlands
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