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Airflow under a cowling

Well, it depends how bad the initial design was. In the case of the PA28, it didn’t gain much (if anything) in terms of speed. An Archer III isn’t a single knot faster than an Archer II. Some things merely look fast.

The original cowlings in the Mooneys were a nightmare though, so the Lopresti cowling (standard on Js, Ks, etc.) did quite a lot.

But these were the days when cowling design wasn’t really a scientific process at all. It was more just an enclosure of the engine. Also, manufacturers were usually in haste to get new models out during the heyday of small GA….

If you look at more modern designs (Cirrus, Columbia, Diamond) you wouldn’t gain much if anything by aftermarket speed cowlings, since the original one is already very close to perfect (CAD, wind tunnel testing, etc.). So the market is small nowadays. Also, it feels like obtaining an STC is nowadays much more difficult than 30 or 40 years ago. Put these factors togehter and you see that the market for such mods is slowly dying.

Last Edited by boscomantico at 02 Sep 17:36
Mainz (EDFZ), Germany

So maybe that 20-30% claim is inapplicable except in rare circumstances, because if it was generally true and you could make an aftermarket cowling for say 2k which gives you say 10kt extra, everybody would buy one.

Administrator
Shoreham EGKA, United Kingdom

Aftermarket complete cowlings are usually more like 10-20k. You know how GA works…

Last Edited by boscomantico at 02 Sep 18:45
Mainz (EDFZ), Germany

During preliminary design of piston aircraft you assume 10% of installed power is required for cooling.

boscomantico wrote:

Also, it feels like obtaining an STC is nowadays much more difficult than 30 or 40 years ago.

Well I don’t know about the former times, but the STC process itself isn’t too complex today, provided you do some engineering and not just tinker something in your workshop. And especially if you intend to build a good working cowling, you have to make it purpose-fit for every make/model/engine combination and, of course, show compliance to affected regulations. This means you have to do a lot of work for a specific aircraft. There are a lot of much easier STCs around, meaning you need much less engineering work upfront. Plus, the effect diminished with lower speed, so I figure this is why only certain types have seen cowling modification STCs in the first place. Of course, a higher cruise speed must be your desire upfront, so cowlings for higher cruise speed is a waste of resources for instance for glider tugs or most basic flight training applications.

Peter wrote:

“cooling drag can be 20-30% of overall drag on a certified aircraft”

(accentuation by me). Well yes, some people in the kitplane scene think that certification would matter for basic physics. (Like they assume electronics can work magic, once installed into an engine… ). But there are some fundamental limits you just can’t cross and some ideas, especially in magazines, are formulated with a great deal of neglect towards the underlying working principles – but that is not only related to aviation, let alone kitplanes.

mh
Inside the sky.
EDXE, EDXF, Germany

Peter wrote:

So maybe that 20-30% claim is inapplicable except in rare circumstances, because if it was generally true and you could make an aftermarket cowling for say 2k which gives you say 10kt extra, everybody would buy one.

I don’t want to support the claimed numbers, but you seem to be assuming that just because it’s a large component, it’s easy to reduce. AFAIK drag caused by cooling system (air passing through radiators, but also engine compartment) is a major player also in cars. And that’s why versions with less powerful engines or reduced consumption have almost all holes in the front plugged up these days and sometimes use slats in the front mask to vary the amount of air coming in.

I recall there is a modified cowling for the Mirage (Malibu as well). Goal was to improve cooling as the engine can get hot at higher altitudes. So you either baby it and can’t exploit full potential or push it and probably shorten its lifespan (especially of turbos). They claim it was done without increasing drag. And it’s even less than 2k IIRC. :-)

mh wrote:

Well yes, some people in the kitplane scene think that certification would matter for basic physics

You can do less optimization. A certified plane has to work throughout the envelope with no adverse effects. An experimental can be tuned to max cruise, even though the cooling is inadequate in hot weather, climb, it could overheat on idle and so on. Another thing is that efficient cooling of an air cooled engine is very difficult, and requires lots of tests and fiddling and precise shape and fit of plenum chamber etc that will never pay off by increased sales or larger price. Never mind the fact that it is practically impossible to design the optimal cooling of flat four mounted in the nose of an aircraft, in terms of aerodynamic efficiency. A liquid cooled engine is much easier, but an optimum design (0 drag) is so practically challenging (expensive) that no commercial aircraft has it. I took a picture of one of the pioneers (or the pioneer) for cooling in experimental aircraft. He has a Subaru engine (liquid cooled) and has designed a P-51 style cooling, blended into the fuselage. The plane has flown for ages with this setup, but only a few has copied him, just too much work and too little gain. It is something for those with special interest or need (racing).

ENVA ENOP ENMO, Norway

From here

…and, we were told at the excellent APS seminar, that most of the cooling air in a horizontally opposed cowling is flowing forward, counterintuitively. They have tests to prove it.

That is is why baffling problems are often so, er, baffling.

EGKB Biggin Hill

Timothy wrote:

They have tests to prove it

Do you have these tests, or have seen them?

ENVA ENOP ENMO, Norway

Google for e.g.

airflow under a cowling

and you get loads of hits e.g. this one which (search for “Cooling: Then and Now”) has what looks like good info from no less than Mike Busch.

Administrator
Shoreham EGKA, United Kingdom

I have written to John Deakin for a reference. All I know at the moment is what was said on the course, in answer to a question, about some experimental work they did which demonstrates that the diagram above is misleading.

But then, you could produce hundreds of POH references to demonstrate why their whole LOP thing is wrong. It ain’t necessarily so.

EGKB Biggin Hill
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