The return of Climb and acceleration

[niklas]

But outlet pressure isn´t constant of the supercharger. It runs with constant RPM. Today they change the AoA of the blades in the turbos, but that was not made 60years ago. You have to reduce afterwards pressure. In front of the boost limiter pressure can be quite high.
Because you have to "destroy" more energy afterwards your air gets a bit more hot, and this means you get less filling in the cylinder.

niklas

[F4UDOA]

Niklas,

But doesn't that mean you get to acheive higher boost faster??

[hitech]

No F4UDOA, the pressure and work done are messured at different parts of the compresor system.

It would be the same as saying you should be able to run at higher pressures at lower alts by shifting the compresor gear.

The easyest way to view ram air is just to think of it as lowering your alt a few 1000 feet. The compresor is producing more MP then you can use.

[niklas]

F4UDOA, "faster", why faster?

RAM effects helps you when your supercharger can´t build up desired MAP anymore.
There exists an altitude where the compressed pressurer is equal to desired boost. Below that altitude, the supercharged air has a higher pressure - it will be limited to MAP. Above that altitude, supercharged pressure is below your desired MAP. The higher you fly over that alitutde, the lower your MAP will become, because atmospheric pressure drops quickly.
This altitude is called critical altiude afaik (Volldruckhöhe).

Now you can do simple math again to understand whats going on:
Image you have  for slow flights without RAM a critical altitude of 6km for 2bar MAP. In 6km atmospheric pressure has dropped to approx. 0.5bar. This means your supercharger must have a compression ratio of 4:1.

In 2km atmospheric pressure is approx. 0.8bar, so after your supercharger you have approx. 3.2bar. This has to be limited down to 2bar.
In 7km atmospheric pressure is approx. 0.45, so you get maybe only 1.8bar. Less than full desired MAP, so your engine power is already less than full power.

Now if u assume that in a fast flight you have a RAM effect equal to, let´s say, 0.2bar, then u get 0.8*4 +0.2 = 3.4bar in 2km.
That doesn´t help you, because your boost limited won´t allow more than 2bar in any case.
But in 7km, you get now  0.45*4 +0.2 = 2.0bar. This is equal to full MAP, and because it´s equal it is also your new critical altitude. 1km higher than for climb power in 7km (1.8bar)

Now it did some serious mistakes here, because a supercharger won´t work like a compression "ratio", so the differences in critical altitudes maybe not 100% correct. Nevertheless you can see the most visible effects of RAM pressure: It shifts your critical altitude upwards, to higher altitudes.

Just compare the climb and speed charts in AH, the critical altitudes for speed are always higher than for climbs. And actually you can take a P51 in AH, climb until you see in ~10k feet the MAP pressure drop. Then level out and watch the MAP gauge. With more speed it will raise, now u get more MAP due to RAM effects.

Now i try to make visible the "negative" influence of RAM below critical altitude:
if u look at engine performance curves, you´ll see for engines with fixed gear ratios usually a power gain with more altitude, until u reach the critical altitude. For example the ouput at sealevel of a V-1710 without RAM was afaik 1650PS, in 10k feet 1710PS. Imagine the power for climb at -1km (just imagine u can fly in hole). It would be less than at sealevel logically, following the trend downwards,  let´s say, 1620PS. Now shift the performance curve upwards for a gain in critical altiude of 1km, and u get 1620PS for the engine with RAM effects at sealevel. LESS than without RAM!!!(ok, ok, very simple assumption, but i hope you understand what i mean...oh well, probably i confused you now completly:) )

niklas