| Looks like you are perfect or hot weather ops, and as you said, could close down the exit flow more for low temp or low power cruise. I'd like to do the same by reducing the base drag at the open aft side of the cowl bottom ... thinking about a pair of longer bottom cowl controllable ramps, so the external flow can go gently back to and along the fuselage bottom. I haven't pulled the bottom cowl off yet to make measurements... would have to add some side walls to the inner cowl to keep the seal alongside the ramps. There's a lot of stuff in there. On my last design, the Magnum Pickup, I used an auto power window w/ rack and pinion to actuate the big flaps, and that worked great... a ramp on each side, not the bottom.
I tried to reduce the Lancair's inlet airflow with inch-thick short airfoils just inside each inlet, but the air just slips right past them... little change and CHTs still running around 300F at 24/24, so I'll probably pull them back out, and work on the cowl flap idea. Yours did the job.
Thanks, Terrence O'Neill
On Jun 1, 2010, at 10:33 AM, Frederick Moreno wrote: Update on cowl flaps. I closed off the exit area about 20% by changing the cowl flap exhaust ramp shape (added some curved sheet metal). Temperatures at 65% cruise 50F LOP rose modestly to above 300F during flight conditions that were 9F above standard at 8500F. Speed up about four knots. Looks like I can reduce exit area even further! This would be consistent with the observations of big radials with closed cowl flaps operating in economy cruise. Full power climb with best power mixture (worst heating conditions), cowl flaps open at Vy produced maximum temperatures of about 340F starting with 65F ambient at sea level take off. I would expect about 380-390F maximum with a 100F sea level departure. Further cooling can be obtained by accelerating from Vy (135 knots) to 160-170 IAS. Cooling with cowl flaps open is thus more than adequate even with abusive conditions. Larger inlets are GOOD (assuming you use cowl flaps) because they permit pressure recovery in front of the inlet as the flow slows and then spreads around the inlet. Small inlets require flow deceleration inside the inlet which is much harder to do without flow separations. The rule of thumb that seems to work and is fairly widely used is to figure your cruise air flow requirements (actual cubic feet per minute) and size the inlets so that the velocity at the throat of the inlet is about 50% of the free stream velocity. This gets about 75% of the available pressure recovery in front of the inlet, and with some good internal design should get up to 90+% total pressure recovery. This gives plenty of surplus pressure to accelerate the hot flow out the cowl flap nozzles and recover most of the lost momentum thus reducing cooling drag. Fred Moreno
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