# Power needed to spin rotors

#### Oskar

##### Member
I've always been curious as to how much power is actually needed to spin rotors. Finally got the chance to measure it acurately, and was surprised by the results.

The test bed is a Mosquito helicopter where the 2 stroke engine has been replaced with an electric motor. Power is measured on the DC input, the measured power thus includes losses in the inverter, motor and drive system where applicable. Unless specified all measurements are at rated speed, motor and tail rotor speed are 2500rpm and main rotor speed is 550rpm.

Measured power was:

Motor only: 900W
Motor plus TR: 2500W
Motor plus TR plus MR (pitch set for minimum power): 9000W

To hover with AUW of 190kg requires between 16 and 17kW.

The most surprising result was that to spin the rotors while generating zero lift required 9kW, but to generate 190kg of lift required only an additional 7 to 8kW.

And here's what the setup looks like:

#### XXavier

##### Member
I don't understand the last statement:

'The most surprising result was that to spin the rotors while generating zero lift required 9kW, but to generate 190kg of lift required only an additional 7 to 8kW.'

With the rotor turning, there is always some lift. Of course, if the lift is less than the weight, the machine won't lift off, but, with the rotor turning, the weight (the force exerted on the ground) of the machine is always less than that corresponding to a mass of 190 kg, because there's some lift...

#### bryancobb

##### Junior Member
I don't understand the last statement:

'The most surprising result was that to spin the rotors while generating zero lift required 9kW, but to generate 190kg of lift required only an additional 7 to 8kW.'

With the rotor turning, there is always some lift. Of course, if the lift is less than the weight, the machine won't lift off, but, with the rotor turning, the weight (the force exerted on the ground) of the machine is always less than that corresponding to a mass of 190 kg, because there's some lift...

My guess is Oskar was surprised that full down collective and making little or no lift consumes so much horsepower because induced drag is zero or near zero.

XXavier, helicopters have to have enough negative pitch "rigged-in" to be able to keep rotor RPM constant and in the green during autorotations. When on the ground, with collective full down and RPM in the green, and with the wind dead-calm...the inner portion of the blades is stalled and makes zero lift. The middle portion of the disc is creating a small amount of lift. The outer portion of the blades is at negative pitch and are blowing air upward. It's very possible that the sum of the three is ZERO or even less.

#### XXavier

##### Member
My guess is Oskar was surprised that full down collective and making little or no lift consumes so much horsepower because induced drag is zero or near zero.

XXavier, helicopters have to have enough negative pitch "rigged-in" to be able to keep rotor RPM constant and in the green during autorotations. When on the ground, with collective full down and RPM in the green, and with the wind dead-calm...the inner portion of the blades is stalled and makes zero lift. The middle portion of the disc is creating a small amount of lift. The outer portion of the blades is at negative pitch and are blowing air upward. It's very possible that the sum of the three is ZERO or even less.

I understand the rationale of having negative pitch on the ground, so to 'press' the machine downwards, but ¿negative pitch for autorotation...?
The usual, fixed positive pitch for gyros, that are in permanent autorotation, is about 3º. Can't see the reason why helicopters may be different in that..

#### WaspAir

##### Supreme Allied Gyro CFI
Gyros don't have the blade twist that is common on helicopters. When one speaks of negative or positive, one might also need to specify the span location.

The Westland Wasp had significant negative pitch available to provide a downward force to keep it stuck to the deck of a pitching and rolling ship (its four castering wheels might lead it to go for a swim otherwise). Most common civil helicopters aren't rigged to squish into the ground that way.

For those not accustomed to measuring mechanical power in Watts (that is, Americans), the conversion is about 746 Watts to one HP. That means 9kW is about 12hp.

Last edited:

#### Doug Riley

##### Platinum Member
Since helos vary in their "full down" collective pitch setup, I suppose the experiment would be more meaningful with scales or strain guages between the landing gear and the earth. These should be arranged to detect either up-lift or down-lift. Either one will add induced drag and therefore a load on the powerplant.

Negative lift will cause the blades to cone DOWN, perhaps adding to the risk of decapitating bystanders.

BTW, gyro rotor blades can be set at either small positive pitch or small negative pitch, and still create up-lift. They're generally more efficient at a small positive pitch, but in the era of hand-started rotors on small gyros, a slight negative pitch made blade-starting easier. Bensen specifically approves this technique in his manual.

The mechanical pitch setting on cambered blades can be deceptive. They generate lift at an angle of attack of zero, and don't hit the zero-lift point until AOA is -2 deg. or more.

#### Jean Claude

##### Junior Member
Oskar, what diameter and chord?
What does MR and TR mean, please?
Thank you.

The ground effect is great at Z/R =.6 Therefore the collective pitch required to lift 190 kg is much smaller than at Z > R; as the induced power.
This fig. from Naca TN D-234

#### Smack

##### Re-member?
MR = "Main Rotor"
TR = "Tail Rotor"

#### All_In

##### Gold Supporter
Greetings my friends.
PRA is sponsoring 12 educational kits with 4 colleges. One of them is a jump takeoff gyroplane.
They have the most excellent engineering results that concur with Dick De Graw, PRA expert working with the Jump man team on this kit.

Our test rig is a single place KB2 and it takes 35 horsepower to spin 25' blades set at zero pitch to 550 rotor RPMs needed make the jump.
All the engineering is included in the kits as well as software, CNC code, etc. We would be testing it right now but Corvil 19 shutdown the machine shop so 1/2 is 3D printed plastic parts. We were lucky only one part locked in the CNC machine and will have to be scraped. We will have the next semesters class finish the machining but have to wait until next June graduating class of seniors.

#### C. Beaty

##### Gold Supporter
The Mosquito helicopter uses non-twisted Dragon Wings rotor blades with tip weights; the nearest NACA equivalent would be NACA 2312 with reflexed trailing edge for zero pitching moment. The angle of zero lift is ~ -3 degrees relative to the meanline. Maximum camber of the DW blades is a bit nearer the leading edge than NACA 2312. Not to be confused with NACA 23012.

#### All_In

##### Gold Supporter
Sorry for the off-topic question. But need advice from the master.
Chuck, what blades should we use for the Jump-take off project? We are looking for tip weighed 25' dragon wings.
But sourcing those in the future for others to follow may/ will not be possible as things are today?

#### Oskar

##### Member
Jean Claude, Main rotor diameter is 18' (5.5m) and chord is 7" (177mm).

Minimum power was at a collective setting about the same as it would be in a stable auto. Full down collective required about 1kW more power.
I didn't measure the lift, but suspect that at full down collective the rotor force will be downwards.

#### C. Beaty

##### Gold Supporter
Sorry for the off-topic question. But need advice from the master.
Chuck, what blades should we use for the Jump-take off project? We are looking for tip weighed 25' dragon wings.
But sourcing those in the future for others to follow may/ will not be possible as things are today?
That’s a question that I don’t have a good answer for.

You need light weight blades with tip weights to keep weight down but stored energy up and blades with zero pitching moments to prevent them from twisting and to prevent intolerable collective loads.

They ain’t none.
Wait a minute: Hughes helicopter blades (269-300) would do it, 7" chord and are easy to cut down to appropriate length.
And another oops: I just reread your post and realized you're looking for a 25' rotor; no cutting necessary!

Last edited:

#### Resasi

##### Gold Supporter
Sorry for the off-topic question. But need advice from the master.
Chuck, what blades should we use for the Jump-take off project? We are looking for tip weighed 25' dragon wings.
But sourcing those in the future for others to follow may/ will not be possible as things are today?

Denis. Razors?

#### C. Beaty

##### Gold Supporter
Denis. Razors?
Don’t know; rotor blades where collective pitch and helicopter type cyclic pitch control is used need to be suitable for use on helicopters. Meaning zero pitching moment coefficient and balance about the aerodynamic center. If not a standard NACA airfoil, an analysis is necessary.

Airfoil programs that run on PCs provide sufficiently accurate pitching moment coefficient.

#### okikuma

##### Member
Chuck, if the Hughes 268/300 blades were turned upside down as you and others did during those early days for effective autorotative quality, the de-pitching of the blades would have to be around negative eight degrees for the angle of incidence at the tips to be near zero.

Thinking out loud, the simplest way would be to use the full Hughes 269 rotor head and swash plate for cyclic and collective operation, modified for opposite rotation for the upside down blades.

Better yet, don't even modify the Hughes rotor head. Just get rid of the Hughes blades and replace with Dragon Wings for original rotation as designed.

A collateral thought. If it weren't for the certification, one could replaced the run out blades on the McCulluch J-2 with blades that have a VR-7 airfoil, wider cord of around eight inches, and about two feet in length to give the J-2 a rotor disk of 30' in diameter, and a higher solidity ratio. If not giving the J-2 true jump capability it certainly would give it better performance and lower rate of decent in a full power off "glide."

Wayne

#### okikuma

##### Member
Oskar,

With the absence of the weight of an internal combustion engine, the absence of the weight of fuel, with a light weight pilot, and the power for the electric motor provided by an umbilical "extension cord" (no weight of batteries) it's not surprising it would only take 17kW (22.8 hp) to hover.

BTW, I just don't "see" where that specific electric Mosquito airframe takeoff weight of 190kg (418 lb).

Wayne

#### Oskar

##### Member
BTW, I just don't "see" where that specific electric Mosquito airframe takeoff weight of 190kg (418 lb).

Wayne

190kg was what the scales said.
The next step is to strap about 50kg of weights to the frame and see how much power is required to hover then, hopefully less than 23kW.
We then have enough information to calculate how long it will be possible to stay airborne with a 50kg battery on board, should be somewhere between 6 and 10 minutes.

#### bryancobb

##### Junior Member
Gyros don't have the blade twist that is common on helicopters. When one speaks of negative or positive, one might also need to specify the span location.

The Westland Wasp had significant negative pitch available to provide a downward force to keep it stuck to the deck of a pitching and rolling ship (its four castering wheels might lead it to go for a swim otherwise). Most common civil helicopters aren't rigged to squish into the ground that way.

For those not accustomed to measuring mechanical power in Watts (that is, Americans), the conversion is about 746 Watts to one HP. That means 9kW is about 12hp.

You are right. Most times, the TIPS are where pitch is measured for rigging the head. A normal amount of twist for a helicopter blade is about 8 degrees over the span. Minus 1 at the tip would of course be +7 At the blade grips.

#### Jean Claude

##### Junior Member
Oskar, Here are my calculated hover powers for MR alone, 15°C, sea level.

On the ground, with no lift: 6,8 kW (Aerodynamic pitch 0 degree)

at 190 kg IGE : 12.5 kW (Pitch 4.25 degrees if no twisted)
at 190 kg OGE: 19.2 kW (Pitch 6.75 degrees)

at 230 kg IGE : 14.4 kW (Pitch 5.25 degrees)
at 230 kg OGE : 25.4 kW (Pitch 8.3 degrees)

at 230 kg +3 m/s vertical flight 28.6 kW (Pitch 9 degrees)

Last edited:

Replies
10
Views
437
Replies
12
Views
477
Replies
20
Views
1K
Replies
15
Views
663
Replies
11
Views
230