Item 1001

OTHER: Aerodynamics - General - Autorotation

Under development - For the fun of it - As time allows

Shouldn't the outside sketch of driven and driver regions of disk be on this page or linked to this page, See [PHA, section 5.4] and web page http://www.copters.com/aero/autorotation.html

. Also consider the fact that the SynchroLite rotates inside forward whereas the Unicopter rotates outside forward.

Outside Helicopter

Rate of descent - Ultrasport:

Has an autorotation descent rate of 900 ft/min.

Rate of descent - Bell 206:

Around 1800 ft/min.

Flettner FL-282

FORM: Flight - Autorotation

For this form to be run the following forms must have been previously opened and run.

The following data is preliminary SynchroLite.

	[Helicopter] ~ Open at desired helicopter.
	[Momentum} ~ Open & Run
	[Flight - Hovering] ~ Open.
	[Test Conditions] ~ Run.
	[Flight - Hovering] ~ Run.
	[Element Data] ~ Open & Run.
	[Rotor - Hub] ~ Open & Run coning angle.

Notes:

Rate of descent - Parachute:

A rotor in vertical autorotation has the same resistance as a parachute of the same diameter. This rate of descent is also approximately twice the hover-induced velocity.

Rate of descent - Maximum:

2500 ft/min. is a reasonable upper limit for larger helicopters.

Rotor Speed Decay: [tKE] [t/k]

tKE = rotor_speed_decay() from Access FORM: Flight - Autorotation

Dim J As Single 'Polar moment of inertia.

Dim Omega0 As Single 'Rotor speed at time of power failure.

Dim Q0 As Single 'Rotor torque at time of engine failure.

rotor_speed_decay = ((J * Omega0) / 2) / Q0

Equivalent Hover Time: [t/k] is the time that the stored kinetic energy could supply the power required to hover before stalling. [Source ~ RWP p. 363]

There is an algorithm on Prouty's page but I cannot find out what the value for C_W is.

See; OTHER: Flight Dynamics - General - Rotor Coefficients ~ Weight Coefficient

t_equiv = (J * Ω² (1 - ((C_W / σ)/( .8C_W / σ_max))) / 1100 * hp_OGEJ [in seconds]

Twist:

A slight positive twist (+1º) reduces the size of the stall region and this decreases the rate of descent slightly. I think.

__________________________

E-mail from P.C. January 5, 2003

The t/k ratio, which is the time in seconds that a rotor can lift the chopper when the engine stops. It is the ratio of J*Omega^2 divided by 4 times the power required in hover ( the 4 comes from the fact that one can only use half of the kinetic energy stored in the rotor system). Prouty uses a more complex formula that takes in account Cl and Cd of rotor system, but if you use the equation [( Power OGE = (61*10E-3/Dia rot)* SQR(m^3/ro)) all in metric (with ro= 1.225 Kg/m^3 at SL)], and divide the value obtained par 0.84 ( for TR power, and Transmission losses), and plug this value into the t/k calculation, it works...

So t/k= (J*Omega^2)/(4*Power oge) in seconds.

The t/k of the Robinson R22 is 0.8 ( far too low I agree), and practically, you want t/k around 1.2 to 1.7 sec, so roughly twice the Robinson.

Autorotation Index: [AI]

Bell's Autorotation Index: AI = (I_R* Ω²) / (2 * W)

Sikorsky's Autorotation Index: AI = (I_R* Ω²) / (2 * W * DL)

[Source ~ PHA p.184]

The above FORM: Flight - Autorotation [autorotation_index()] uses Sikorsky's index. I_R is the polar moment of inertia J, W is the gross weight and DL is GW/A

Twin Rotor Helicopter:

May have an advantage over conventional helicopters during auto-rotation since a portion of the conventional helicopter's power train plus its tail rotor must take power from the auto-rotating main rotors.

This presents some of the problems: OTHER: Helicopter - Outside - Intermeshing - Kaman - H-43 Huskie - Yaw

_________________

To eventually, and maybe, provide;

Amount of rotor decay between power failure and pilot reaction.
What is the minimum steady rate of descent.
etc.

_______________

Entry into Autorotation:

The incorporation of a Rotor Governor will reduce the need for high inertia rotors, since the [t/k] factor is not as relivant.

Synchropter Directional Stability Information from 'Stability and Control of Rotary Wing Aircraft:

When the blades are set at autorotative angle of attack, downward flow through the rotor and, conversely, upward flow tends to accelerate the rotor. There fore, in autorotation of a breast stroke type synchropter, the yawing moments created by side flow through the rotors create an unstable yaw moment which must be corrected by the addition of more fin area.

Stagger to Radius Ratio:

At the sides of the intermeshing helicopter, the 'driver' portion of the upper blade is directly over the 'driven' portion of the lower blade. This appears to be the case, whichever way the rotors turn. It might be of interest to know what effect this has on autorotation. ~ My thought.

Note that the ratio (and thus the situation) is much higher on the Synchropter and UniCopter.

Also consider what effect the cutout might have.

Helicopter:	1/2 Stagger: [ds/2] ft	Radius: [R] ft	Ratio: [(ds/2)R]
Flettner:	1.0	19.67	0.051
Kaman:	1.83	23.5	0.078
SynchroLite (Dragonfly)	1.0	8.0	0.125
UniCopter:	1.25	9.25	0.135

Possible Concerns:

Re compound configuration; "A large wing may cause problems in autorotation. If the wing supports a large portion of the gross weight, the rotor will be starved for thrust and will not be able to maintain autorotation." ~ [Source ~ RWP5 p.73]. Could this be of concern to the intermeshing configuration because different 'reagons' are located above and below each other. In other words, a driving region may be located above a 'driven region' or visa versa.

Could the fact that the tiltrotor V22 has had problems, or concerns, with one rotor stalling before the other present problems for other laterally displaced twin rotor configurations?

Postings on Rotary Wing Forum:

This is a very naive question but here goes;

Is pre-rotation by hand or pre-rotor necessary for takeoff. In other words, would it be possible to start the takeoff roll with the rotor stationary and then obtain the desired rotor speed if the runway was long enough?

Thanks Dave

Mad Dog;

Without pre-rotation of at least 50 RPM, by hand or otherwise, the rotor will begin to rotate backwards.

Racer;

Why does the rotor rotate backwards? I have noticed this when my gyro has been parked and wind blows across the rotor, it always starts turning the wrong way. Just curious.

birdy;

It depends on the type of blades.
Iv seen glass blades usualy start to spin backwards in a gentle wind wether the stick is full back or centered forward.
The alu ozy blades i use always spin forwards.

On a day with strongish wind, wen i had plenty of time to waste, i managed to get them to a speed where they would have gon on to flyn speed, without touching them. Just alot of patient trimming with the stick.

Related Thought: By self

If a rotor consisted of symmetrical blades (say NACA 0012), the blades had 0-deg pitch and no twist, I suspects that an airflow parallel to the disk will rotate the rotor in the correct direction. This is because the profile drag of reverse flow on one blade will be greater than the profile drag of conventional flow on the blade on the other side. All other things being equal this suggest that the rotor might have a preference for the correct direction or rotation.

Aerodynamics: Autorotation, Rate of Descent & Gross Weight:

Ask Ray Prouty: Gross Weight's Effect on Autorotation

Rate of Descent ~to~ Disk Loading (Gross Weight / Disk Area)

Only a fool would question Prouty. However, for the fun of fooling around, the following is considered

Autorotation in a Vertical Descent:

It has been stated that; "A rotor in vertical autorotation has the same resistance as a parachute of the same diameter." This implies that a heavier load will descend at a faster rate by autorotation as it will by parachute.
From page 113 of Prouty's main book."... a rule of thumb is that the rate of descent in vertical autorotation is twice the hover-induced velocity." A higher disk loading will create a higher hover-induced velocity,. Therefore by implication; it will result in a faster descent.
IMHO, the above two points imply that a greater Disk loading results in a greater VERTICAL rate of descent.

Autorotation with Forward Velocity

- The UltraSport-254 helicopter has an extremely low disk loading and an autorotation descent rate of 900 ft/min. It is said that in autorotation it can land then liftoff and re-land using only the inertia in the rotors.
- The Osprey V-22 has an extremely high disk loading. The test data indicate that the aircraft would impacted the ground at a rate of descent of about 3700 ft/min.

For an airplane; --Your glide range is determined by the L/D ratio as flown. Increasing the weight increases the speed for the best L/D ratio and increases the power required to maintain altitude. This increases the rate of descent with power off. From About Aircraft Speeds. Why should a rotorcraft be different?

Prouty states in his Oct 1, 2007 article; "At a typical autorotational forward speed of 60-80 kt, the induced power, which is the only power increment affected by weight, is only a small part of the total." However, the chart on page 130 of his main book shows the Induced power exceeding the sum of the Profile and Parasitic powers, in this speed range.

The Oct 1, 2007 article also states that; "This paradox has been verified by flight test.". He is verification appears to be tests done on the Lockheed Model 286 helicopter, which are presented on page 140 of his main book. Interestingly, these tests start at the Maximum Weight of the craft and then the weight is incrementally increased from there. Perhaps this suggests that the slower descent rates could be due to the increasing of the angle of attack and thereby the improving Lift/Drag ratio, (as the rotor gets closer to a stall)

Leishman makes no mention of this phenomenon; that I can find.

IMHO, the heavier craft will have more potential energy, as stated by Prouty. However, I suggest that this greater potential energy may be consumed at a quicker rate because it is supporting a greater weight.

Terminal Velocity:

From Chuck Beaty;

The drag coefficient of a parachute is 1.4 and according to "Gessow & Myers," the drag coefficient of a rotor in vertical descent is ~1.3. That's the average obtained from many carefully conducted tests.

That being the case, the vertical descent of a gyro would be:

Vv = 26*(disc loading)^0.5 [Where Vv is in ft/sec, and the disc loading in lb/ft2.]

That means 26 x the square root of disc loading.

A typical small gyro with a disc loading of 1.3 lb/ft² would thus have a vertical descent velocity of 29.6 fps or 1778 fpm.

A Bell Jet Ranger with a disc loading of ~3.5 lb/ft² will come down at about 2900 fpm.

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Outside Information:

Last Revised: Sunday, August 10, 2008