Hi Bada,
>now, i suppose (and only suppose) that if the BHP definition is right, in this case, we have to replace the wheels and gearbox with the reductor and the propeller.
I'm not sure of the exact details of the setup, but with aviation engines, it seems to have been standard operating procedure to use a specially designed "brake propeller" for power testing. It generated enough drag to dissipate the engine power and at the same time generated some cooling airflow.
As engines for most WW2 era combat aircraft had to use a reduction gear to harness their power to a propeller efficiently, the losses in this reduction gear were already figured into the brake losses. Reduction gear losses seem to be an important consideration for the designers of modern experimental aircraft because they have to compare the use of an ungeared engine without these losses, but a possibly less efficient propeller to the use of the same engine suffering some losses, but swinging efficient larger propeller more slowly for better efficiency. I don't think any main combat types of WW2 were equipped with engines for which a direct drive option existed - if they did, they certainly were obsolescent or at least low-powered.
Further considerations when discussing engine power:
- Are the figures given for the engine at rest, or for an engine as installed in an aircraft with a forward-facing air inlet under the assumption of a certain forward speed? The former case is considered for "static" pressure, the latter for "ram" pressure. The resulting curves for the same engine are similar, but the ram power curve will be shifted to higher altitudes compared to the static power curve.
- What kind of exhaust system is installed in the engine? The exhaust system determines the so-called backpressure that has an impact on the efficiency of the exhaust stroke. Jet exhaust nozzles tend to increase back pressure, decreasing brake power but increasing overall thrust from the engine.
- What is the effect of exhaust thrust on the actual thrust power in flight? In high-speed flight, as much as 25% of the available thrust might be provided by the exhaust system, which doesn't even register on the dynometer brake.
- According to which conventions is the power curve calculated? Difficult to find out - as a rule of thumb, early in WW2 the methods for calculating a power-over-altitude curve were imperfect, while later on they got better. Due to limitations of the test machinery, it was not possible to test engines on the test stand under realistic conditions, so some data points were measured and the complete power curve was calculated. Of course, there were also "flying test stands" - Jumo for example used a Ju 52 for this purpose, mounting the test engines in the nose while keeping the original outboard engines to sustain flight even if the test engine (often a prototype) failed.
- For which operating conditions (rpm, boost pressure) was the power curve valid? Most major engine types were re-rated now and then during WW2. You'll have to add the question for altitude if you're asking for a single power figure and not for the entire curve.
>So, my pb here is: if the BHP definition is right, was was the Final power of the merlin61? (in this case ,only as exemple)
Usually, you should be able to take the figure for rated power, check for the conditions supplied with it (if you're lucky enough to have them supplied), and take it as brake power figure. It's a good idea to check for the altitude though as rating systems could differ - the British engines were rated for optimum power, while the German engines are often quoted with their sea-level powers. Nothing wrong with either, but they're not directly comparable of course.
>How much power loss was due to the reductor and propeller?
I'm not sure about the reduction gear - maybe 1 or 2 %, I guess. The propeller of course was a major source of losses: even an ideal propeller inevitably has losses due to its operating principle. In the main operating range, it had maybe 10 to 20% losses, but at very low or very high speeds, it could be much more than that.
Regards,
Henning (HoHun)