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I spent a bunch of time studying cowlings on 7 Sep 02 at the EAA 486 RV Fly-in. I noted one thing that is worth thinking about if you want to reduce drag - there was quite a bit of variation in how high the air inlets were. Van used to specify the height of the air inlets as 2 5/8 to 2 3/4 inches - the plans were later amended to remove this dimension. I didn't have a ruler with me, but it was quite clear that many RVs have air inlets that are quite a bit larger than what used to be called for on the RV-8 plans. These RVs also had quite a large flat area extending above and/or below the spinner, which also supports my theory that their cowl halves weren't trimmed as much at the front as Van intended.

So what, you ask? Well, if the height of the cooling air inlets is increased, the amount of air that goes into the cowling increases. If we look at the aircraft as our frame of reference, all this air has to slow down to pretty close to zero speed. Or, looking at the earth as our frame of reference, all this air has to be accelerated from a stop to the speed of the RV. This acceleration requires a force to be exerted on the air by the aircraft, and this force is felt by the aircraft as drag.

I did a very rough back of the envelope calculation, and if the air inlet height on both sides is increased by 1/2 inch, that gives about a 4% drag increase, and would cause a speed decrease of about 2 kt for a 160 hp RV, and about 2.25 kt for a 200 hp RV. If the inlet height is increased one inch, you can about double those numbers.

This calculation ignores the drag of the flat part of the cowl that may be exposed above and below the spinner if the front part of the cowl has more height than Van intended. So, the real effect could be greater than this back of the envelope calculation would suggest.

Caveat - this calculation used quite a few simplifications. Total drag of a nominal aircraft was calculated based on Van's 75% cruise numbers, and an assumed 79% prop efficiency (obtained from a Hartzell prop efficiency map). Drag increase calculation assumed that the air entering the cowling was at free stream ambient conditions, and that the air was slowed to 10% of its initial velocity (relative to the aircraft). So, if you want to get Van's performance numbers, be sure to pay attention to the height of the air inlets on the cowl.

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  • Well after posting this item I discovered another important facet to this story.

    I had trimmed my cowling to obtain the 2.75" inlet height that was specified on the original plans, as I had missed a plans update. I mentally congratulated myself on the prospect of have a lower cooling drag, but was a bit worried about whether I would have enough cooling. The jury is still out on the cooling side, as I haven't flown yet.

    But, I later found that I didn't have much clearance between the upper cowling and the left cylinder heads. I don't have enough clearance to put baffle material on that side. I will need to make a plenum chamber with a closed top instead of using baffles. I had planned on doing that anyway, so this isn't a real big deal for me. But it could be important for someone who was planning to use baffles.

    I would be cautious about trimming the cowling too much. You can always modify the inlets later to decrease the area if you find that you have more than enough cooling air.

    Kevin Horton

    Comment last edited on about 4 years ago by Kevin Horton
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