R22 Manifold Pressure

[Mitch00]

I was ask a question recently that really I couldn't answer:

How much does 1 inch of manifold pressure lift?

I later found out that for the R22 beta 2 (helicopter taken for the example) the answer is 50lb (roughly), but why, and how do you come up with that number?
I know it's something you have to estrapolate from the OGE or IGE chart.

bigger more complex helicopter have this kind of charts already (so I was told) but not small helicopters like robinsons

Another question was also, how much temperature is 1 inch of manifold pressure equivalent to?
The answer is 20°C and I think, but I'm not sure, the explanation is, if you look at the OGE chart and see how high you can hover at 20°C then drop to 0°C you'll see that you gain 1000 ft which is around 1 inch of MP. "I THINK" this is how it works but if not, and someone knows the right answer, please jump ahead.


Thank you all!!

[michael.scheller]

Aerodynamics is cool. So cool in fact that many people try to understand it with simple equations such as: 1PSI (or 1%TQ or 1%HP) = XXXX pounds of weight. This, in mathematics terms is a linear equation and looks like image 1 and it graphs out like a straight line (hence the term "linear"). Look at your performance charts and you'll find little resemblance to a line for very long on any of your charts. Every basic aerodynamics course includes a version of this formula: LIFT = 1/2 Rho(density of the fluid) x Coefficient of Lift x Surface Area x Velocity^2. Since Velocity is squared, this is no longer a linear equation and thus almost all charts about aerodynamics and performance look more like image 2.

 

You might find areas of chart two that resemble chart one and thus be able to simplify the understanding to 1psi=50pound grossweight, but that does not do you much justice in understanding that at lighter grossweights your 1psi might lift an additional 75 pounds while at very heavy grossweights it might only lift 25 pounds (if that).

 

Hope this helps.

Linear.bmp

[heliflyknow]

I think the best answer to "how much does 1" of manifold pressure lift" is it depends. Your DA will determine that, Robinson derates its engines so that they last longer, and so that their power rating is applicable to higher altitudes. An r22 will make just as much power at 9,000DA as it does at sea level if you obey the MAP limits. But you should use the HIGE chart to make sure you have the power available to land at an unfamiliar area.