Is that with the 2l Austro engines?
I think Aviathor and Shorrick are not talking about the same DA42 variants. 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 KTAS
Max available power at that altitude 85%, FF 15.5 gal/hr, 188 KTAS
aart wrote:
I think Aviathor and Shorrick are not talking about the same DA42 variants
I was referring to the NG with the 168 hp AE300 prior to the aerodynamic mods that resulted in the VI.
While it clearly worked, the explanation given in the text is nonsense.
The speed of the air through a cooling system has a relatively small effect on the efficiency of the cooling – and slowing coolant makes transfer of heat LESS efficient, so the “radiator” actually has to be a bit LARGER to achieve the same heat transfer.
Think about it – when did you last slow down your aircraft to improve engine cooling?
However, but by slowing the air down, the drag created by the radiator is MUCH lower than if it were exposed to the full airspeed of the aircraft, so this is a great way to deal with cooling drag.
As far as this “being like a jet engine” is concerned – I leave the calculation how much thrust you get from making air slightly warmer by passing it across a 80 degree radiator to somebody else. I dare say a bit less than burning fuel, adding 600 degrees and a resulting pressure ratio of 40-50 in a modern jet engine…
The extra thrust is not due to warming the air but the difference between inlet and outlet flow velocities. This of course if you choose to believe North American’s wind tunnel data that was used to refine the initial version of the duct on an actual aeroplane.
Maybe the data is in fact bogus and it flew faster just because it was prettier (like the Comanche).
Shorrick_Mk2 wrote:
Is that with the 2l Austro engines?
No, it’s a Subaru engine.
Shorrick_Mk2 wrote:
The extra thrust is not due to warming the air but the difference between inlet and outlet flow velocities. This of course if you choose to believe North American’s wind tunnel data that was used to refine the initial version of the duct on an actual aeroplane.Maybe the data is in fact bogus and it flew faster just because it was prettier (like the Comanche).
Everything helps when speed is of concern. The aim for North American was to reduce the cooling drag as much as possible. They sliced away 90% of it, which is an excellent result in my opinion.
I don’t doubt they used wind tunnel data to refine the duct and that this led to a narrow nozzle with a high outflow velocity.
I just doubt that (thrust – drag) from the whole thing is positive (it delivers “extra thrust”). It is simply impossible without adding energy somewhere in the duct, and the radiator just makes it borderline plausible.
As far as engine nacelle drag is concerned – it should be obvious that it is only one small factor for an aircraft. The frontal area of the engine is small, compared to the rest of the aircraft, so adding, let’s say, 20% to it won’t change the overall drag characteristics. The cross section of the fuselage is MUCH larger.
Not “extra” thrust but enough thrust to almost cancel out cooling drag.
The extra thrust is not due to warming the air but the difference between inlet and outlet flow velocities. This of course if you choose to believe North American’s wind tunnel data that was used to refine the initial version of the duct on an actual aeroplane.
Is this physically possible?
I mean, is it possible to construct a “duct” which generates net thrust simply by containing a hot object?
I know about ramjets but that seems a different principle. In a ramjet, you compress the air first and then you heat it.
