This detail of the P-51 was called the “Meredith Effect”, after a British Engineer who discovered the principle. It was one important factor for making the P-51 faster than comparable aircarft.
I am no good at physics @Peter but this is how Meredith explains it: http://naca.central.cranfield.ac.uk/reports/arc/rm/1683.pdf
The practical validation of the paper seems to have worked well enough.
This is how an engineer explains it (better than me): http://www.supercoolprops.com/articles/meredith_effect2.php
Thanks for those links. Clever stuff!
The claimed 20mph, at 300mph, is equivalent to a massive amount of HP. Probably 300HP on a 2000HP engine. Fairly obviously there is no way to do this for an air cooled engine, but one could do it for a watercooled diesel.
Some more reading: http://www.supercoolprops.com/articles/meredith_effect.php
Note that Meredith’s theoretical work assumes 125C mean coolant temp. The Mustang was running 95 to 105c coolant temperature and it still apparently worked. You could run 125C mean temps with more pressure in the cooling system possibly (but admittedly I don’t know much physics).
Also obviously the effect is speed-dependent so on a 80mph Cub it’s probably overkill. The question is have we kept to low speeds because we wanted to or because technology imposed it. I’m sure even you Peter wouldn’t mind cutting the 7h legs in half :-)
LeSving, thanks for the RV/Subaru link. It looks like he had fun and learned a lot, which is the purpose of experimental category aircraft, but in terms of building a better plane more generally he’d have been better off sticking with a Lycoming, installing Light Speed ignition with low MP advance, and minimizing airframe and engine weight.
Re Twin Comanche versus DA42, I bet if you put Twin Comanche turbo IO-320 engines and cowlings on the DA42 it would be a pretty good plane, and a heck of a lot more practical than the existing diesel setup in places where the airports bother to sell sell fuel appropriate for light aircraft. The DA42 uses a extremely high aspect ratio wing and composite fuselage to slip through the air, the water cooled diesel engine installation is a mess.
Peter, if you want to recover waste heat/power from any engine, air or liquid cooled, at the expense of complexity you can do it with a turbo-compound approach as per Lockheed Constellation and etc. The Meredith effect similarly builds in complexity and weight by routing coolant pipes all over the plane and adding air ducts. Pick your poison, I like simple and effective best for things I own.
Believe it or not Diamond already thought of that but apparently the Lycoming powered DA42 doesn’t have that many takers at 20+ gph versus 14gph… regardless of the supposedly cleaner-more-aerodynamic-aircooled installation.
One of the posters here worked on certifying the DA42 Lycoming installation, which uses bigger non-turbo IO-360s. He could say more, but my impression is that it’s a hot rod in terms of climb performance and etc. The engine installation does not appear to be particularly well executed in terms of streamlining, when compared with the Twin Comanche, Cessna 310 or similar clean sheet designs. A bit of a hack job.
Two turbo IO-320s can be flown in cruise at around 16 gph combined.
aart wrote:
The -VI model (certification-wise called an NG though) has more aerodynamically shaped engine nacelles and does (at 18.000 ft):60% power, FF 10.4 gal/hr, 160 KTAS
80% power, FF 14.4 gal/hr, 182 KTASMax available power at that altitude 85%, FF 15.5 gal/hr, 188 KTAS
That’s interesting, and compares to the turbo Twin Comanche like this The same source provides data for the NG here. I don’t how accurate the Plane and Pilot data may be, but for the Twin Comanche it indicates 65% power from the turbo IO-320s giving 180 kts TAS, and burning 15 gph total… doubtless at some very high altitude because the service ceiling is 30,000 ft. I think you’d have to be leaning very carefully to get the engines to 7.5 gph each at 65% power, but that aside the cruise numbers are almost the same as the NG. You can also climb at over 1400 fpm max and/or carry 1300 lbs. Not bad for 1966 
The TwinCom will burn 15 gph or go 180 kts TAS but maybe you could explain how it could do both at the same time. It’ll burn 7.5 per side at 65% and 2200 rpm at best economy… However, top speed for the NA version is 165 knots at 2700 rpm, 160 at 2400 and below 160 at 2200 rpm, and that is at best power settings. It’ll only ever reach 180 kts TAS if the Rajays can maintain 65% power only around FL250. Can they? And that again is going to be on best power settings. And if you believe the book, 65% best power at 2400 rpm is more like 9gph per side (so probably 10 in reality).
@Bookworm and @Alan_South fly a Twin Com and they might be able to fill in some real data.
This may also be of interest.
