Item 1079

DESIGN: UniCopter ~ Pusher Prop - General - Pusher Prop Assist

 Description:

Alternatives:

Reasons for Including a Pusher Prop:

Re Sikorsky ABC ~ A nose down attitude of about 12 degrees is required at 156 knots airspeed.

Reasons Against Including a Pusher Prop:

Concerns:

Restricted Airflow to Upper Quadrant of Propeller;

Power Consumption During Hover;

Torque about X-axis;

Posted on rec.aviation.rotorcraft & science. aeronautics

Does anyone know if a pusher propeller is more likely to cause an oscillation about the airplanes vertical axis than a tractor propeller will? A second and related question is ~ Should the pusher prop cause oscillation, will the alternating drag values near the wing tips act as a damper?

The reason for the questions is to get a better understanding of the compound configuration helicopter, where the pusher prop is very far aft and the wings are relatively insignificant.

A question that was not asked is ~ Would an oscillation of the fuselage self-excite due to it's effect on the airflow past the fuselage to the propeller?

Another Concern:

Will the propeller draw so much air from below the front of the disks that it reduces the lift at the front of the disks? This may add to an already existing desire to pitch down. See; DESIGN: UniCopter ~ Pusher Prop - General - Tractor vs. Pusher

Torque~Pitch Cross-Coupling:

In forward flight it may be possible to offset the pitch change caused by the rotor torque changes by the thrust of the propeller. However, it may only be possible to exactly balance this at one specific forward speed. In addition, the propeller will not help at all during hover and vertical flight.

Vibration resulting from the propeller acting on a disturbed airflow:

The direction of the airflow to the fuselage is fixed therefore the fuselage might be shaped (maybe with the inclusion of mini-fins) to deliver a more even and less turbulent airflow to the propeller. However the passage of the rotorblade will still have an affect on this problem.

 Notes:

"In the auxiliary propulsion mode, the ABC rotor requirements are reduced with increasing air speed with maximum L/D being achieved at or near autorotation. The power required for the auxiliary propulsion devices is, of course, highest at high speed. " ~ Advancing Blade Concept ABC High Speed Development, page 3.

Does this mean that all or just about all of the power will be going to the propeller? If this is the case then engine failure will not result in the loss of rotor torque pitching the nose down.

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Thrust Line of Propulsor:

The design intent (at present) is for the craft to have a slight downward pitch (-3 or -4 degrees) during cruise. The rotor will contribute a little to the forward velocity and the propeller will contribute a little to the lift.

I suspect that the thrust line of the propeller should go through the CG &/or the Center of drag of the total craft, including rotor .
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Lycoming Engine and Two Props:

If the Lycoming engine is used in conjunction with two propellers, the drive train to the propellers will require a speed up. Maybe not if the blades have wide chords. Look into CarterCopter propeller.

If the propellers are behind the engine, inline with the engines output shaft then the reduction could be by belt and sheave. The belt could pass inside two airfoil shaped tubes. These tubes could support the two propellers to the engine.

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Drive from rotors, so that there is no sudden pitching down if power lost during fast forward flight. Maybe not. There is no need for fast forward speed during autorotation and the propeller will draw power from the rotors. Drive the propeller from the engine, which will also simplify the reduction. As well, locate an overrunning clutch between the engine and the prop, so that it can free-wheel when no power is being applied. Can the single propeller be driven from the back end of the crankshaft? Link the propeller's pitch to the forward cyclic so that the horizontal thrust increases as the cyclic is advanced. Could consider a foldable propeller similar to gliders. The nose drop may only take place when the prop is engaged, therefor can it be handled by some other means?

The propeller(s) axis should have a 5-10 degree upward tilt at the front. This is because the helicopter will be flying with a nose down attitude and we don't want the prop thrust having a downward component. This upward tilt should have the advantage of having the propeller axis and the crankshaft axis parallel.

Variable Pitch:

Since the RPM is probably going to be fixed, the propeller(s) will have to have a variable pitch. No thrust for hover, high thrust for fast forward and low drag for autorotation (unless the propeller was to be permanently linked to the rotors and used to contribute some of the torque to the rotors).

For instance:

Rear Landing Gear Idea:

The rim around the rear single prop could have fixed vanes on both sides of the propeller. This would be a safety feature but more important, it would brace the ring and allow it to be used for mounting the rear supports. If the propeller is to be foldable then the blades will not be near the rim during landing and takeoff therefore the rim could be a fiberglass construction and thereby off a 'spring' to the supports.

Mounting of Propeller:

Could the propeller be mounted to the engine or the engine's attachment points. In other words, do not attach the prop to the fuselage.

Prop Generated Torque (Roll):

A single operational propeller will impart a rotational moment to the craft about its for-aft (X) axis. This will probably be more noticeable than it would be in a plane because the plane has large wings.

Some potential solutions are;

    1. The fuselage might be given lateral dissymmetry to counter some of this torque during fast forward flight.
    2. Fins on the fuselage.
    3. Twist on the empennage's airfoils. This may necessitate an increase in boom and tail strength.
    4. Static blades in a duct, which surrounds the propeller, should achieve a similar result.
    5. Two counterrotating props.
    6. Have a percentage of the prop's pitch dependent upon the amount of forward cyclic stick. Have the remaining percentage dependent on the craft's forward airspeed. This way the prop's angle of attack may never get large enough to generate an excessive roll moment.
    7. September 8, 2004 ~ The intent is for the propeller's pitch is to be set by the air speed and forward cyclic. The pitch of the blade is the main contributor to torque If Independent Root & Tip Control is implemented then one or both of the above inputs can be used to adjust the rotors lateral blade root controls to compensate for torque of the propeller,

A concern with the above is that the torque from the propeller will vary depending on the amount of forward cyclic stick and if the engine should fail. Item 1/ appears to be associated with crafts forward velocity where as Item 4/ might be a little more closely associated with the propellers torque.

See also: DESIGN: UniCopter ~ Pusher Prop - Trim, Stability & Control - Overview and Cross-Coupling

Wishbone Boom:

A wishbone configuration for the boom, where the two arms originate at the two hubs may allow for a higher elevation propeller and a stronger boom. An alternative is twin booms. It may be aerodynamically advantageous (less drag from booms) to have the booms bend towards each other as they move aft. In addition, this will allow the two vertical stabilizers being closer to each other and this should mean that resistance to yaw is retained in forward flight but is reduced slightly during hover, which should be good. The closer VS's will allow them to be located slightly further forward, which should be good.

Ground Clearance:

The drawing currently shows the ground clearance below the prop at 11.5". For a tricycle airplane the minimum ground clearance is 7" and for a tail dragger the minimum ground clearance is 9" in takeoff attitude. The propeller has pitch distribution and this will probably result in some air disturbance at the tip, even when the mean pitch is zero degrees, unless the propeller is 'part-time rotation'.

Larger Diameter and Ground Clearance:

If a larger diameter prop was required, and if lowering the engine would not overly lower the center of gravity, and the center of thrust and if it was a four blade prop, and if the rotation of the prop could be stopped (note ~ prop could be on a separate axis from that of crankshaft or the prop was 'part-time rotation') then the prop could be stopped with the two lower blade pointing to the sides at 45 degrees from the vertical.

Stone Guard:

The propeller may have to be shrouded to minimize small stones hitting the propeller. Note that the propeller is fully feathered while there is no cyclic..

One Prop versus Two Props:

Using propeller thrust for yaw control: A single propeller will require collective pitch and lateral 'cyclic'. If two propellers are employed then they will only require independent collective.

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From Eng-Tips:

The Cessna push-pull twin, "Skymaster", is said to have noticeably better performance in cruise and climb when flying with the aft engine alone as compared to the forward engine only. First, note that this was an entry level twin and single-engine performance is only barely acceptable in either case; but, yes, it does a little better on the aft engine. Why? The blunt upsweep of the aft engine cowl has an impossibly high pressure recovery profile that the air cannot follow (i.e.,poor streamlining, which was done to avoid a long shaft). This causes considerable turbulence that causes high base drag when the aft prop is stopped; but with the aft prop actively pushing, the lower pressure ahead of the aft prop draws air smoothly around the cowl and eliminates most of the turbulence there

Pitch Change Requirement:

Geometric Pitch of propeller: The circumference at the 75% blade station of a 60" diameter propeller is = 0.75 * 60 * (22/7) = 141" = 12 ft. The engine speed is 2400 rpm. Therefor the distance traveled by the 75% blade section is 12 * 2400 = 28,800 feet in one minute. The forward speed of the craft is 150 knots = 15,190 ft/min.

The Geometric Pitch of propeller is therefore tan(15,190 / 28,800) = 28. The effective pitch will probably be 10 to 15% greater, to account for the higher parasitic drag of the helicopter less the assistance from the main rotors. This will result in the pitch of the prop being variable from 0 to 33

Note that constant-speed propellers only vary their pitch by only 15 to 20.

 Prop Thrust Requirements:

The following must be review and redone since the current idea is to have the torque split 50/50 between the rotors and the propeller. I.e. 12 hp /2 or perhaps 180 HP / 2

Notes:

Two, higher, props will impart less of a nose up moment than the one, lower, prop. Is this good or bad?

They might be used for yaw, as well.

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To Overcome Parasitic Drag of Fuselage:

See: DESIGN: UniCopter ~ Trim, Stability & Control - Stability - Static - Longitudinal (Speed Stability) ~ Calculations

See: Rough and Wild Notes in DESIGN: UniCopter ~ Dimension, Area & Drag - Drag - Parasitic, in Forward Flight ~ Calculations

F = 1/2 ρ CD A v2

F = 1/2 * (0.002377 * 1 * 1.57 * (175 * 1.4667)2)

F = 123 lbs.

Where;
F is the force [in pounds]
ρ (Greek letter "rho") is the density of air = 0.002377 slugs/foot3 (The paragraph below says to use pounds per cubic foot, which is actually = 0.0765 pounds/foot3, but it appears that the slugs/foot3 value is the correct one.)
CD is the coefficient of drag; which is 0.04 for a streamline strut (NACA 0025), 1.0 for a round wire, 2.0 for flat wire or 1.5 for a parachute. For values for helicopter component shapes see [Source ~ RWP1 p.280].
A is the area of the surface [in square feet]
v is the velocity through the air [feet per second]. (To convert from miles per hour to feet per second; multiply mph by 1.4667)

Propeller:

An efficient prop will develop around 5 pounds of thrust per horsepower. Therefore 123 / 5 = 25 hp must be delivered to the propeller(s).

Beaty's table shows around a 41" dia. prop.

Powerfin:

25 horsepower ~ 3125 rpm ~ 42" diameter ~ 3 blades = 132" circumference = 573 fps

See: OTHER: GyroHeli_V - Propeller

Tip speed maximum of 760ft/sec. for optimum efficiency. Should be run around 550 ft/sec for lowest noise.

A tip speed of 600 fps would be the most acoustically desirable, but I think that this is for a shrouded propeller.

Tennessee Propellers Inc.

4 BLADE PROPS 1.50 X 32" Dia. for Solo 210 (2.5:1) $380.00 US. ~ for twin side props. It looks like they are wooden and if they're for parasailing then the pitch may not be enough.

Note:

For Ultraprop charts it looks like going from 2 to 4 blades, while keeping the diameter and the rpm the same, will increase the thrust by approximately 70% and increase the power required by approximately 80%.

 Outside Web Sites:

Cal Vert HIGH-Speed Intermeshing. See section 4.5 Propeller Design http://www.enae.umd.edu/AGRC/Design99/calvert.pdf Have hard copy

JC Propeller Design

Here is something about Theodorsen's Theory of Propellers but not about ducted fans: http://naca.larc.nasa.gov/reports/1949/naca-report-924/

http://naca.larc.nasa.gov/reports/1944/naca-report-777/index.cgi?thumbnail14

JavaProp by Martin Hepperle:

JavaProp - Design and Analysis of Propellers

Other:

The program PROP OPTIMIZER is in the computer. It appears to be quite sophisticated, however, this version has been limited to 2 blades etc. There is an earlier attempt called 'DaveTest' in the 'Prop 20' folder that can be upgraded to suit.

Propeller Recommendations

Rotor System Research Aircraft

X-Wing

Lockheed AH-56 Cheyenne More

Compound Helicopters

Making a Composite Prop

Folding Propellers:

http://www.technoflug.de/english/carat/propeller.htm

Inherently Ducted Propfans and Bi-Props

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090027755_2009028349.pdf

Control of Pusher Propeller: This section to eventually be a separate page in Control - Flight

Preliminary Conceptualization:

Outside Information:

"Cockpit controls needed to operate the compound helicopter's systems will be integrated into the aircraft's existing mechanical controls to reduce pilot workload. The only noticeable difference in the cockpit will be the addition of a manual prop pitch override on the collective for the ring tail," Piasecki said.

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Last Revised: September 9, 2010