Rate of climb

[heliflyknow]

How do manufacturers get the climb rate of helicopters? Is it best rate of climb airspeed at 5 minute max power and minimum weight? Or is it a theoretical number derived from max horsepower and the lift equation? Rate of climb seems to be only a selling point since I haven't seen it in a POH, just wondering if there is a standard for its measurement or if it's just an estimate.

[Gomer Pylot]

There is no one number. In the performance section of the POH there should be a graph which lets you find the rate of climb under various conditions.

[heliflyknow]

I'm wondering where the number in the sales brochure comes from.

[Eric Hunt]

It will be the best possible combination of max power, min weight, min temperature.

 

Things that never happen together when you need them.

[Gomer Pylot]

Sales and reality are different areas, with little in common between them.

[iChris]

How do manufacturers get the climb rate of helicopters? Is it best rate of climb airspeed at 5 minute max power and minimum weight? Or is it a theoretical number derived from max horsepower and the lift equation? Rate of climb seems to be only a selling point since I haven't seen it in a POH, just wondering if there is a standard for its measurement or if it's just an estimate.

 

Under Part 27 airworthiness standards, normal category rotorcrafts, best rate of climb speed must be determined (§27.65). Two of the common procedures, in the standard given, to Determine Vy are the Sawtooth Climb and Level Flight Speed Power. Prior to flight-testing, calculation were completed as a baseline from the power required vs. airspeed results. Rate of climb is a function of excess power i.e. power available over power required.

 

(1) Sawtooth climbs may be used to determine V_y. If such a technique is used, climbs should be flown in pairs on opposite headings 90° to the wind at the test altitude. This procedure will minimize any windshear effects. All testing must be done in smooth air. Windshear is usually an indication of unstable air or a temperature inversion and must also be avoided.

 

The climbs are flown on reciprocal headings for approximately 5 minutes or through an altitude band using maximum continuous power at a constant airspeed. Periodic power adjustments may be necessary. Additional reciprocal heading climbs must also be conducted at different airspeeds above and below the airspeed at the lowest point of the power required versus airspeed curve. This technique can be repeated at different altitudes to obtain V_y throughout the altitude range.

 

(2) Level flight performance (speed power) may also be used to determine V_y. The testing should be done in smooth air. The advantage of this method is that less time is required, and the accuracy is equivalent to the sawtooth climb method.

 

The test can be repeated at various altitudes to determine the V_y throughout the altitude range desired for the rotorcraft. The test at each altitude should be conducted at a constant weight over sigma (W/σ).

 

The test is normally started at the desired W/σ with maximum continuous power, or at VNE, in level flight. A series of points should be taken, reducing airspeed 10 to 15 knots between points, with the lowest speed point around 20 to 30 knots. Weight should be computed for each point and the test altitude adjusted to maintain a constant W/σ.

 

After the data are reduced to standard day conditions, the minimum power required airspeed will be the V_y speed.

 

(3) Prior to the flight test, the rotorcraft should be ballasted to the desired gross weight and the critical center of gravity. The airspeed should be stabilized prior to data acquisition. Data to be recorded includes time, altitude, airspeed, ambient temperature, engine parameters, torque(s), rotor RPM, fuel reading, aircraft heading, external configuration, etc. Power setting, weight, and climb airspeed should be planned prior to flight.

 

For some turboshaft engines, temperature and/or engine speed limits may be reached prior to a limiting torque. The test team should verify that the resulting power utilized in these tests closely approximates the power producing capabilities of a minimum installed specification engine.

Edited September 26, 2014 by iChris

[Gomer Pylot]

This gives Vy. It does not give one rate of climb, because it can't. The rate of climb always varies, because the gross weight, temperature, power applied, etc are never the same. Thus you have to use the performance charts to get the rate of climb under given conditions. The airspeed for best rate of climb will be very close to the same, but the actual rate of climb will vary. The rate used in sales brochures is usually for low gross weights at ideal conditions, the best that would ever be available, like every other performance component. Believe sales pitches at your peril, but the performance charts will be relatively accurate.

[iChris]

This gives Vy. It does not give one rate of climb, because it can't. The rate of climb always varies, because the gross weight, temperature, power applied, etc are never the same.

 

Thus you have to use the performance charts to get the rate of climb under given conditions.

 

The airspeed for best rate of climb will be very close to the same, but the actual rate of climb will vary.

 

The rate used in sales brochures is usually for low gross weights at ideal conditions, the best that would ever be available, like every other performance component. Believe sales pitches at your peril, but the performance charts will be relatively accurate.

 

That's correct, prior to flight-testing; calculations are made as a baseline. From those results, power required vs. airspeed assumptions are made and baseline data charts created. The calculated best rate of climb speed (V_y) should fall near the dip in the chart since the max rate of climb is a function of excess power i.e. power available over power required.

 

The extra power required to climb compared to that required to fly level may be estimated. For a rate of climb (RC) in ft./min:

 

Δh.p._climb = (RC) (GW) ⁄ 33,000

 

or to find (RC)

 

(RC) = Δh.p._climb x 33,000 ⁄ GW

 

The max RC will be achieved at the speed for which the power required for level flight is a minimum, since at that speed the Δ h.p. available from the engine will be maximum.

 

Bottom line, calculations and assumptions are made and then verified in flight test. So that relationship is correct, anything that reduces the h.p. available from the engine or an increase in gross weight (GW) will affect (reduce in this case) the max (RC).

 

V_y and (RC) are evaluated during the same flight-testing.

 

 

Edited September 26, 2014 by iChris