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OTHER ~ Flight Dynamics - Rotor - Blade - Aerodynamic Twist

This idea may or MAY NOT be workable.

Objectives:

The primary objective is to provide a low disk loading that will allow autorotation, while not requiring an excessive amount of force by the pilot when applying 'cyclic' control.

Another objective is to provide a greater control over the rotor than that which is available from a teetering hinge rotor.

A third objective is to reduce the power required, due to the larger disk area.

To provide a simple means of cyclic flight control.

Future consideration for electric craft: A pair (X-axis & Y-axis) of electromechanical actuators might be used to power the cyclic flight-control.

On craft with multi-rotors it will be possible to locate the rotors closer to each other because of the extreme resistance to out-of-plane flapping and in-plane lead-lag. This means a smaller gap between coaxial rotors and a reduced Vee angle between intermeshing rotors.

Hopefully, the cyclic control will be more responsive than a teetering hinge due to its rigidity, while not producing an excessive vibration.

Hopefully, it will reduce the vibration that results from rotor to rotor aerodynamic interaction between the blades.

Hopefully, it will reduce the results of a perturbation.

Overview:

A weight-shift flight-control rotor utilizing absolutely rigid rotors that have a larger diameter than the propeller used on previous backpack helicopters.

This Aerodynamic Twist idea is very similar to the OTHER: Rotor Concept - Independent Root & Tip - Torque Tube Method, except that the idea on this page does not incorporate the torque tube that mechanically controls the pitch of the tip.

A method of active blade twist similar to the 'Free-Tip rotor' idea. Then combine it with a Torque-Pitch Coupling at the root, for collective control.

Apply this flight-control idea as an alternative to the three ideas currently shown on the SloMoCo drive.

This flight-control idea may be an attractive alternative to the Hub Spring.

Apply this rotor to bilateral rotor configurations to reduce the disk loading and the required power.

See the sections below for preliminary considerations of this concept.

Blade Profiles for Aerodynamic Twist; (also illustrating torque-pitch at blade root):

Arraignments:

All of the following arraignments are intended to produce similar results.

A/ The spare is parallel to the leading and trailing edge of the blade. However, it is closer to the leading edge then the normal 0.25 of chord. This will cause the aerodynamic effect to act on the full span of the blade.

B/ The spar is at an angle in respect to the leading and trailing edge of the blade. Vertical aerodynamic loads at and near the tip end of the blade will effect a pitch change at and near the tip end of the blade. A downward gust will make the tip end of the blade pitch up.

C/ The spare is parallel to the leading and trailing edge of the blade. However, the increased surface area on the trailing side at the tip will cause the blade to act a B/.

D/ The spare is parallel to the leading and trailing edge of the blade. However, the reduced surface area on the leading side at the tip will cause the blade to act a B/.

Application to Gyrocopter:

Gyrocopter where the teetering hinge has been replaced by an ARR and a shorter mast. It may need twin-rotors, such as coaxial, so that the blades are not so long.

Consider adding partially powered rotors to the above.

Application to Helicopter:

Micro light helicopter with fully powered rotors.

Notes:

The root of the blade has a two-position pitch change for gyrocopters with partially powered rotors, and a variable position pitch change for helicopters with full rotor power.

This root pitch change is by Torque-Pitch Coupling.

The pitch linkage between rotor torque and blade root may contain a spring and damper. The intention of this spring is to cause changes at the tip (by the 'free-tip') to have an effect on the pitch at the root of the blade also.

Changes at the tip of the blade are by pilot Weight-Shifting and by perturbations. One of the positive features of the 'free-tip' should be that the blade pitch resulting from a perturbation attempts to minimize the perturbation at the fuselage.

This Aerodynamic Twist may very well be better than the Hub Spring because the Aerodynamic Twist results in a small amount of the pitch being pulled out of the blade at the azimuth where the pilot wishes to go, and this is in addition to the control by weight-shift alone.

Outside Information:

Patent ~ US 4,137,010, Constant lift rotor for a heavier than air craft, Stroub, Have hard copy.

NASA; Tip Aerodynamics From Wind Semi-Span Wing ~ Stroub, Saved on E-drive.

NASA; An Analytical Investigation of the Free-Tip Rotor for Helicopters ~ Stroub, Saved on E-drive.

NASA; The Results of a Wind Tunnel Investigation of a Model Rotor with a Free Tip (Oct 1985) ~ Stroub, Saved on E-drive.

I believe, but cannot now find, that Stroub considered a 'Free-Blade' with the activation being the result of a chordwise trailing edge at the tip of the blade.

Thread on Rotary Wing Forum ~ Merits (if any) of free-tip rotor blades?

Thoughts:

I might be interesting to see how a pair of counterrotating coaxial (and perhaps intermeshing and even interleaving) rotors would aerodynamically interact if they had aerodynamic blade twist. Ref SloMoCo

Because of the very stiff blades (in-plane & out-of-plane) the the gap between the rotors could be quit small.

The intermittent pulsed thrust from the blades on the upper rotor would cause segments of the blades on the lower rotor to temporarily increase their pitch, which might reduce the induced vibration.

The intermittent pulsed downdraft from the blades on the lower rotor might cause segments of the blades on the upper rotor to temporarily increase their pitch, which might reduce the induced vibration..

This might be advantageous.

Calculations:

Tip-Pitch Ratio:

To get an understanding as to whether Aerodynamic Twist would be more effective than the Hub Spring for close centered coaxial rotors that utilize Weight-Shifting for control.

CONCERN: Will the forced pitch change result in a flap 90˚ later??? See 'Concerns' section below.

For specifications use SynchroLite's for this evaluation even though it is intermeshing.

Weight:

2 only 2-blade teetering heads is 2 * 20 lb = 40 lbs
Motor, controller etc = 70 lbs (estimated)
Rotor frame, back-pack stuff and tripod = 25 lbs (estimated)
Batteries = 80 lbs (estimated)
Pilot = 200 lbs (estimated)
Misc. = 10 lbs (estimated)
Gross weight = 425 lbs (estimated)

Disk Loading: 1.8 lb/ft²

Blade Loading: 25.5 lb/ft²

Assumption of Disk Tilt Based on Pilot Exerting Enough Tipping Force to Change the Pitch at 75% of Chord by 0.5-degree.

75% of Radius: 8.667 * 0.75 = 6.5 ft

Circumference at 6.5 ft radius = 41 ft.

Vertical change in one revolution = sin(.5) * 41 ft = 0.35 ft

Angular tilt change of rotor disk in one revolution = 0.358 / 6.5 = 0.0.55 = 3.1º

Rotor speed = 450 RRPM = 7.5 RRPS. Therefore 10º of tip will require (10º / 3.1º) / 7.5 = 0.4 seconds.

Note that there should be some additional tilt due to the actual brute tilting force applied by the pilot.

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Note that there will still be the requirement for torque-pitch at the blade root end for collective and for autorotation.

Calculations:

Tip-Pitch Force:

Assuming that the overhead stick has an arm of 2'-6" then a pull of 10 lbs by the pilot will exert a downward force of 30"/78) * 10 = 3.8 lb at 180º azimuth of the rotor. The total area of the four blades is 15.17 sq-ft therefor one blade will be experiencing a force of roughly (15.17 / 4 blades / half of the blade's area) = 1.89 lbs which is around 0.5 lbs per foot of blade maybe. The disk loading is 1.8 lb/sq-ft therefore the upward force on the blades tip end is increased by 25%.

This additional for should be able to increase the tips pitch by 0.5-degree. I hope. It will depend a lot on the elasticity of the chordwise fiberglass thred and the size of the increased area behind the pitch axis at the blade's tip.

Blade - Spar and Skin Ply Lay-up Logic:

To give a very large resistance to out-of plane bending of the blade.

Reason: so that the gap between upper and lower coaxial rotors can be minimized.

To give a very large resistance to in-plane bending of the blade.

Reason: To minimize unwanted blade activity.

To allow the blade to twist about the spar along the full length of it's span.

Reason: to provide a 'free tip' proportionately along the tip end of the blade. Thereby;

Allowing the relatively large rotor's to be controlled by weight-shifting.
Reducing vibration from rotor-to-rotor aerodynamic interaction and from perturbations.

Reason: to provide a Torque-Pitch Coupling at the rotor hub. Thereby;

Allowing changes in the power to the rotor on a partially powered gyrocopter and on a fully powered helicopter to instantaneous change the pitch of the blades. This will allow both types of craft to operate in autorotation and in rotor-powered flight.

Consider using mainly carbon fiber cloth with the thread running parallel to the spar and at 90º to the spar.

Flight Control - Overview:

Collective by torque-pitch coupling.

Cyclic:

Primarily by gimbal rotorhead and Weight-Shift control:

Secondarily by Aerodynamically Active Blade Twist.

Proposed operation of Aerodynamic Twist:

The very root of each blade is firmly 'anchored' in the rotating portion of the rotorhub. This 'anchoring' stops the blade from flapping and led/lagging while at the same time stopping the very root from twisting.
A pitch-arm is located at 10% of the blade's radius. When this arm receives a torque from the motor/engine it will pitch this section of the blade upward by roughly 0.5º. Because of the very high resistance of the blade to bend, this twist of 0.5º at 10% of the blade span will cause an upward pitch at the blade's tip of up to 5º.
The air velocity at the outer segments of the blade's span will reduce the pitch at the outer portion of the blade's span. The tip, of course, will be subjected to the greatest reduction with the inner segments receiving an ever-diminishing reduction of pitch. During forward flight the advancing tip will be experiencing a greater velocity that the retreating tip will experience. The will cause the advancing tip region to experience a larger pitch reduction the retreating tip region experiences.

Miscellaneous:

The SynchroLite's rotor diameter is 17'-6". This will be the overall length of a double-blade. A half scale model will have 4 foot long blades.

Design:

The foam core for the double-blade spar will consist of a 'rod' that is round in the mid-length location and extremely flattened ovals at each end. The circumference of this 'rod' will be the same at all sections along its length.

The spar will consist of unidirectional carbon tape (or pultruded carbon tow) over the above foam core.

Two holding rings will be bonded onto this spar. Each of them will be located approximately one foot away from the center of the spar. The rings will hold the double blade from moving in a spanwise direction and stop it from flapping and lead/lagging. It will not resist rotation about the pitch axis, in-fact they are the pitch rings that apply collective pitch.

Pitch Angles:

The ratio of 8.66:1 means that a pitch change of 1º at the pitch ring (root) will create a pitch change of 8.66º (approx.) at the tip. This may be too much.

Concerns:

Phase Lag; due to Aerodynamic Precession (gyroscopic precession)

Will the action of the floating tip result in the blade being at its lowest location 90º after the azimuth of greatest force? This is not what is wanted. Or will the lowest location be at an azimuth between the two (due to pitch change and physical force)? Which is also not wanted.

The phase lag would be 90º if the blade freely pitched along its full span. The phase lag would be 0º if the blade was absolutely rigid along its full span.

The floating tip will behave as a partially free pitching airfoil. The greater the area of blade that is 'floating' means the easier it is for the pilot to exert pitch change. However it may mean that the phase lag will be proportionately greater than 0º.

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Would this adjustable pitch tip operate better if a torque tube was used in the blade and controlled by a spider mechanism in the hub?

If a spider is used then it would be logical to have a change in torque apply a change in collective pitch.

The next question then becomes; Why even use a gimbaled head? Why not an 'Absolutely' Rigid Rotor and forget this Aerodynamic Twist idea?

The next question then becomes; Due to the depth of a spider mechanism why not purchase an existing Megaflux inrunner motor?

Would the 2P vibration coming back to the pilot be objectionable? (600RRPM / 60)* 2 blades = 20cps of the SynchroLite is at the most objectionable frequency. Offsetting this vibration might work a one frequency but it will not work over the anticipated range for this idea if it uses torque pitch coupling.

The idea may have problems. The change in the root pitch between powered and autorotation is way too much for the double-blade idea. The idea might well work with the two blade being mounted independently in pitch bearings but this extra will probably eliminate any advantages that the Aerodynamic Twistmight offer.

The 'twistability' of the blade about the span axis may result in instability due to an self-exciting oscillation. Blade Flutter. ~ involves the coupling of blade flexing and twisting with air forces. The blades should not flex (out-of-plane bending) due to the spanwise carbon thread.

A 3-blade rotor could easily be implemented if the 2-blade rotor causes an unacceptable 2P vibration. This 3-blade rotor should not be susceptible to ground resonance. Consider a longitudinal trim device to remove some of the (pulsating?) force.

Idea for Potential 2-blade Solution:

The vibration will be present when the cyclic stick is off center during pitch and roll maneuvering. This is a short-term event and may not be too objectionable.

The vibration will also be present during cruise, due to the speed stability, which results in the cyclic stick being held forward of center. This is a long-term event. Consider having a two part longitudinal trim control. The first relieves the forward pressure that must be exerted by the pilot. The second allows the pilot to adjust the frequency of a canceling moment so that it is in synchronization with the 2P for the two rotors.

Related Pages etc.:

This is the basic profile for the Electrotor-SloMo - Rotorhead - Blade-S

This may be the blade profile; SynchroLite ~ Rotor - Blade - VR-7b - Composite

This may be similar to the spar construction; UniCopter ~ Rotor - Blade - VR-modified

"Overhead (and joystick) are not weight-shift, because we are changing the rotor's cyclic pitch, not shoving the airframe around. Control forces with cyclic pitch control are a tiny fraction of those in weight-shift. (Some weight-shift trikes have wing warping wires, however -- which form an aerodynamic servo mechanism. This reduces control pressure below the "brute force" method of pure Weight-Shift.)" ~ from Doug Rilry on Rotary Wing Forum

Weight-shift Trikes with Wing Warping Wires:

Not much luck in searching.

Patent; Weight-shift flight control transducer and computer controlled flight simulator, hang gliders and ultralight aircraft utilizing the same

Ideas for Growth:

Build the first prototype as backpack configuration.

Build the next prototype with a simple seat and skids. The additional weight might be eliminated by refinements to the first one.

Build the third prototype with a fuselage. Add a third identical rotor to give the extra lift (and reduce the vibration) or increase the size of the blades.

Potential Applications:

OTHER: Helicopter - Inside - Coaxial - Electric Motor Located Between Rotors

OTHER: Helicopter - Inside - Bilateral - Electric Motors Located In Rotors

Outside Information that may be Relevant:

See Information in black 3-ring binder entitled 'Free-Tip'.

Aeroelastic Stability for Straight and Swept-Tip Rotor Blades in Hover and Forward Flight
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA480628&Location=U2&doc=GetTRDoc.pdf

Initially displayed: August 9, 2009 ~ Posted to Rotary Wing Forum: August 12, 2009 ~ Last Revised: February 13, 2011