Comparing helicopter performance...

[rotorwash4944]

If I wanted to make a comparison of the "max performance takeoff" capabilities of various twin-engined helicopters, is there an easy way to 'normalize' the numbers for a quick comparison, utilizing a few parameters ?

In other words, its obviously not sufficient, nor accurate, to just compare max hover ceiling, or max engine SHP numbers off a manufacturer's spec sheet when trying to decide which aircraft will have the largest power margin suitable for your mission in a max performance takeoff scenario.

I figured if you knew certain parameters, you could roughly calculate a power required to hover, and a max power available, and the difference between the two showing up as available rate of climb.

Parameters such as the aircraft's transmission torque limits, rotor RPM, rotor diameter, your mission gross weight, and your atmospheric conditions would be involved... The answer is probably staring me in the face, I just cant find the equations I'm looking for. I didn't want to get into a full-blown "thrust coefficient / power coefficient" hover perf data reduction scenario, I just wanted rough numbers I could more easily compare aircraft in a given scenario.

Thanks.

[Shawn Coyle]

There is no such thing as a 'maximum performance takeoff' - no charts, no set procedure, no way to even begin to compare helicopters this way.
There might be other ways to do a comparison, but not with a 'maximum performance takeoff'.

[RugGun]

I read this as you wanting to compare max climb rates at takeoff? If you know the % of first limit in the OGE hover at a given config, then that gives you the comparison immediately. But if you want more indicative numbers....

As long as you stick in the hover/straight up there are quick (and fairly dirty) ways. I'm afraid I only know metric units.

in the hover, thrust = weight (obviously) - so calculate the hover induced downwash:

vi (downwash velocity (m/s)) = square root (mass (kg) x 9.81/(2x air density (kg/m3) x rotor disc area (m2))

Hover power (watts) is simply hover thrust (newtons) multiplied by vi - the downwash induced velocity. If you know what the aircraft actually uses in the hover at that weight, then the percent power or torque difference (actual against estimate) will remain roughly constant as long as the aircraft has zero groundspeed.

If you don't, then I think you need to add about 40% to the number calculated above to accommodate profile drag, transmission losses & the tail rotor. This accounts for the engine & transmission power that goes into things other than lifting the helo. I haven't done this calc with real numbers for about 4 years, so it'll be a bit rusty!

Anyway, once you've got an estimate of hover power, you need to establish how much more power is available (watts). Then the rough climb velocity in the hover will be: excess power (watts) / (mass (kg) * 9.81). You should reduce the excess power by whatever efficiency factor you worked out (like knock 40% off if that's what you assumed earlier).

That'll give you a rough vertical climb performance in m/s.

Don't know if I explained that well - it's based on 'foundations of helicopter flight' by Simon Newman. It'll be fine for a rough stab & basic comparisons. I used them in the opposite direction to work out torque change for a given rate of climb.:8 (came out within 5% of actual across the speed range, which was good enough for my application)


The excess power to climb rate bit works for all collective led climbs - for rough indications - but the induced velocity bit changes quickly with speed.

The problem is in the losses. 40% is a reasonable rule of thumb, but it can easily vary 10% each way. And bear in mind it's an efficiency compared to a perfect isolated actuator disc, not to any form of installed or practical system.

Hope that's useful?

[rotorwash4944]

Perhaps I wasn't being clear by calling it a "max performance takeoff," but I was just referring to a vertical takeoff from a hover.

I thought I could get a rough estimate of power req'd to hover at a certain weight vs. maximum power available (at the transmission torque limit) assuming sea level, and this would give me an indication of power margin.

Maybe I should restate the question... what's a simplistic way to make a baseline comparison of aircraft performance, knowing little more than a manufacturer's spec sheet numbers, and perhaps some limits in the flight manual?

I ask this way, because knowing the aircraft's max gross weight, hover ceiling, and engine SHP, etc. as shown on a spec sheet, does not give me any immediate indication of whether or not it will perform better or worse in a specific mission profile, when compared with another aircraft listing similar specs.

RugGun, your explanation was just what I was getting at, thank you. I can convert the units ;)