A Little Technical Marketing

[Dave_Jackson]

Tilt-rotor http://www.Unicopter.com/Boxing.gifInterleaving

Two serious disadvantages of the Tilt-rotor are its small lifting capability, and its large rate of descent during autorotation. Both of these shortcomings are the result of the configuration's small disk area, which in turn necessitates a high disk loading.

These two disadvantages would not exist on the Interleaving configuration. For an equal stagger, the Interleaving configuration has 4 times the disk area of the comperable Tilt-rotor configuration.

http://www.Unicopter.com/1505.gif


Supporting info; http://www.unicopter.com/1505.html

[gadgetguru]

the smaller disk area necessitated by the capacity to tilt the discs to forward flight/fixed wing mode & miss the fuselage & maintain some obvious CoG limitations in the process

different application to your interleave design

curious what yours is intended for - large tapered blade roots benefit much in terms of efficiency vs weight

[Oogle]

Very interesting concept.

I would assume you may need bigger powerplants to drive the obviously bigger discs.

I suppose the designers have to decide if they want better autorotational capabilities offset against shaft horsepower requirements.

[Deemar]

Actually if I recall my aerodynamics correctly, a lower disc loading is usually more efficient at producing a given amount of lift/thrust, so you need less power to drive it. Remember, the smaller disks need to be driven faster and/or at a higher Cl and Cd than the large disks.

Almost all human powered helicopter attempts have had a phenomenally low disk loading.

[ShyTorque]

I'm not convinced by practicality the tilt rotor concept, except for a few specialised roles and to my mind, Dave's idea looks more likely to have universal appeal.

However, as far as day to day practicality goes, any "side by side" rotorcraft comparable to the "main rotor/tail rotor" I fly (by that I mean fuselage size) probably wouldn't fit in a number of certain important landing sites we use. Unfortunately I can't see myself ever getting to operate one!

[Graviman]

Nice to see e-see you again Dave. Your not really saying anything new or controversial here. Sikorsky will end up taking the market with coaxials anyway, after the X2 proves itself in November (?). Remember the important features of any engineering project are Cost, Quality and Delivery.

What have i been up to since i joined this forum?

http://www.terex.com/main.php?obj=prod&action=VIEW&id=14&nav=prod&cid=182b2104d7a1ce2c68b57b49f8c1436c

Not a heli, but believe me we still had all the fun and games trying to satisfy our customers... :ouch:

Mart

[Dave_Jackson]

gadgetguru
curious what yours is intended for The intention is to present a medium/large rotorcraft configuration that should be able to outperform other configurations, such as the tilt-rotor and coaxial X2, in almost all respects.

For example ~ forward speed.

The Interleaving would have slowed-rotors with wide chords,

combined with,
two ultrahigh-bypass turbofans,
http://www.unicopter.com/Ultrahigh-bypass%20Engine.gif

with the F-35 Joint Strike Fighter's shaft driven lift-fans replaced by shaft driven lift-rotors.
http://www.air-attack.com/MIL/jsf/jsf_vstolengine.jpg


Additional stuff; http://www.unicopter.com/1121.html;

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Hi Mart,

Interesting project.
It is certainly at the other end of the 'transportation spectrum' from helicopters. :)

[Robbo Jock]

Dave,

Wouldn't you be better off with your interleaving rotors' highest point being over the fuselage, lowest outboard ?

[gadgetguru]

somewhat curious in regards to coaxial disc separation; how close is too close

if a control mechanism was not restricting the proximity of hubs, how does one calculate the minimum seperation of (flexing/flapping) counter rotating blades (without sacrificing test pilots?) :confused:

are control designs the limiting factor that this is a non-issue?

[Dave_Jackson]

Robbo Jock,
Wouldn't you be better off with your interleaving rotors' highest point being over the fuselage, lowest outboard?The advancing blades are on the outside and they will be producing most of the lift. This is because the intent is to incorporate features such as; 'Absolutely' Rigid Rotors, Advancing Blade Concept, Active Blade Twist and Reverse Velocity Utilization.

One reason for having higher rotors outboard is to give the lifting surface (rotor disks) a dihedral and, hopefully, this dihedral will contribute to lateral stability.

Your suggestion might be correct and it could certainly be done.
______________________

Gadgetguru,

The gap between coaxial rotors depends upon a number of things. The primary one is the rigidity of the rotors and blades.

An example is Sikorsky's XH-59A ABC. The rotor diameter was 36 feet and the rotor separation (gap) was 30 inches. The minimum allowable tip clearance was 10 inches. As of May 1976, the minimum clearance recorded during flight tests was 14 inches.

That's Nick on the left. :ok:

http://www.unicopter.com/0891.jpg

[Robbo Jock]

Dave,
I can see you might like some dihedral in there, but just look where those rotor tips are. I know it's just a back-of-a-fag-packet drawing, but it's indicative of rotor/wing/engine clearance. I suppose you would roll one way by tilting the opposite disc towards the horizontal (making its lift vector more vertical) but the clearance still seems awfully small.

[Graviman]

One reason for having higher rotors outboard is to give the lifting surface (rotor disks) a dihedral and, hopefully, this dihedral will contribute to lateral stability.

Hmmm, don't forget this is a rotorcraft not a fixed wing. The cyclic is controlling the pitch/roll torque (ie acceleration) not velocity. The ONLY way to make cyclic control pitch/roll velocity, like a fixed wing, is to use gyro stabilisation. You favour electrohydraulic, i favour mechanical (partly because i have direct experience of how much of a headache hydraulics can be). Either way you need a gyro.

What you are proposing makes the machine more laterally stable if the pilot keeps the cyclic central. It does absolutely nothing for longitudinal stability, so you need the gyro anyway - assuming your objective was to reduce pilot workload. If you are going to fit a gyro, why compromise the packaging and aerodynamics? This is why i still favour outboard advancing intermeshing, with gyro stab. Even this is academic, since Sikorsky will soon set the counterrotating trend.

I have had fixed wing phugoid and spiral divergence demonstrated to me many times, and each time i have laughed at the design futility. If even a fixed wing cannot be absolutely stable, why bother compromising the aerodynamics? Next time you find yourself freighted in an Airbus or Boeing, watch the aileron/spoiler control surfaces fighting dutch roll. You will still get pilot induced oscillation anyway, since you can't damp the system. Northrop had to finally accept gyro stabilisation for the flying wing in the B2, and the helicopter is an even stronger case for not relying on aerodynamic stabilisation.

Why not just accept that rotorcraft suffer inherent instability, and design the control system to suit?

Mart

[Dave_Jackson]

Robo Jock,

The intent is that the disks do not tilt. The rotors do not have flapping or teetering hinges and the blades are to have absolutely minimal flex.
______________________

Mart,

I'm not against augmenting inefficient aerodynamics.

I'm against inefficient aerodynamics.

The following is from an excellent 20 page article in the latest AHS's Journal. This excerpt is discussing the side-by-side Platt-LePage XR-1. http://www.nasm.si.edu/research/aero/aircraft/platt-le_page_xr-1.htm

"The twin lateral configuration had inherent handling difficulties that the Germans had hidden well -- specifically that in cross-winds, or while maneuvering, one rotor would move through effective transnational lift while the other one had not, causing low-speed lateral instability."

First optimize the aerodynamics ~ Then add the Band-Aid's.

__________

Talking about fixed wing characteristics, I once owned a Grumman Tiger that was placarded against doing stalls; since the stall might put the craft into a flat spin. This might have been a way to evaluate reverse velocity flow over an airfoil. :)

The suggested way to exit a flat spin was to get out and stand on the wing.

[hotzenplotz]

How about the center of gravity?
When the discs are intermeshing then the fuselage length is limited.
http://img56.imageshack.us/img56/3840/v22an5mk.gif

[i4iq]

If the discs aren't tilting, why would it limit the length of the fuselage?

[Graviman]

I'm not against augmenting inefficient aerodynamics.I'm against inefficient aerodynamics.

The gyro would not be there as a band-aid to poor aerodynamics.

A light helicopter suffers inherent DYNAMIC control instability, because the pilot only has direct control over the pitch/roll torque moment - ie acceleration. That is why it is so hard to hover at first, since the tiniest cyclic input sends the machine careering off. The pilot is actually learning to differentiate his input. Adding lateral and longitudinal dihedral (you would need additional rotors for this) will improve STATIC stability, but the machine does not become easier to fly.

A fixed wing is stable because the pilot controls pitch/roll velocity, since the elevator and aileron are forcing a steady-state change in flow. The machine has DYNAMIC stability because the machine responds linearly to a given stick deflection. Lateral and longitudinal dihedral also give the machine STATIC stability, so that in addition to being easy to fly it will return to a wings level ground state with no input.

I am not saying that there is anything inherently wrong with the interleaving config. I am saying that it does not overcome the basic source of instability in a helicopter. For efficient hover you need the lowest disc loading, and for any given landing box a single disk will always win. If the need is to unload the retreating blade then coaxial or intermeshing are the next best options. Intermeshing anhedral will reduce stability, but if there is already a gyro then so what.

General thoughts on this config in roll yaw coupling: If the machine is in a sideslip, and the tail vertical stabiliser converts this to yaw which way will it roll? The rotor gyro precession favours correct sense roll, but considering which rotor effectively speeds up and which slows down the machine also suffers adverse roll. So you see that outboard advancing, which you need to make the machine fly efficiently at high speed, may reintroduce all the problems that Flettner rotation solved. Dihedral will help by sideslip correct sense roll before yaw, but the sideslip roll out then produces adverse yaw by gyro forces and correct sence yaw by aero forces. Which wins out sounds more like fixed wing the spiral divergence over dutch roll scenario.

The practical upshot is that ANY aerodynamic machine is a dynamic and static stability compromise. The only way forward is to accept this, and devise the simplest/cheapest practical control system - i favour the original Lockheed mechanical approach in a light heli. This then frees up the design for the best package compromise.

At any rate this is all academic. Sikorsky are already paving the way with the X2...

Mart

BTW: I enjoy our discussions, since it helps me formulate my ideas! ;)

[slowrotor]

Graviman is quite correct about dihedral. It is commonly thought that dihedral is needed because most of us start out with free flight balsa models that have lots of dihedral. But a piloted craft needs no dihedral. I finally understood that after flying an Extra 300 radio control model when it flew straight as a bullet. All aircraft have "roll damping" that slows the reaction and allows the pilot to maintain control. Dihedral is not needed.
slowrotor

[Graviman]

Dihedral is not needed.

A peculiar quirk of history is that the Wright brothers succeeded because they abandoned longitudinal dihedral. You can see the elevator working very hard in the first flight. They figured this because the bicycles they made had no static stability (stop steering and you fall off), yet the "pilot" was able to add dynamic stability as part of a feedback system. I suspect first flight duration was limited to pilot fatigue!

Dave's objective (if i understand it correctly) is to reduce pilot workload through machine design. This is a principle that makes implicit sense to me (see http://webpages.charter.net/nlappos/TechnologyofSafety.pdf from thread http://www.pprune.org/forums/showthread.php?t=215944 )
What i question is the method used to achieve this objective...

Mart

[Dave_Jackson]

Mart,

I think it is fair to say that all air vehicles experience some forms of static and dynamic instability. The helicopter has, perhaps, more instability than most. If we agree that aerodynamics is the major cause of instability, then it makes sense to look, initially, to aerodynamics for the solution.

As previously said, I am not against the implementation of mechanical or electrical means of control. It is just that they are not the first choice. Page 5 of Nick's presentation shows 'control malfunction' as being tied for third at 9%. Assuming that you were to double the complexity of the controls, you would theoretically move 'control malfunction' to second place at 18%.

You mention that " Intermeshing anhedral will reduce [static] stability". This may or may not be true. The 4-degree precone of the rigid rotors is intended to provide speed stability and lateral static stability.

The vertical stabilizer of the Intermeshing configuration is located above the plane of the disks. This has been done so that any sideslip resulting from roll will be 'partially' offset. Any yaw created by sideslip will be in the same direction.


______________________
Slowrotor,

I believe that 'dihedral' has to do with 'static stability', where as 'roll damping' has to do with 'dynamic stability'. Two related but separate activities.


Dave

[Graviman]

If we agree that aerodynamics is the major cause of instability, then it makes sense to look, initially, to aerodynamics for the solution.

But that's my point, Dave - it isn't. The fault really does lie in the control system. Many helicopters have sprouted aerodynamic devices to control attitude or directional stability, all have the same fundamentally high pilot workload to fly.

The problem really does come down to small cyclic inputs controlling pitch and roll acceleration, and not velocity. This means that the pilot sits there for hour after hour keeping the cyclic centred, with only tiny inputs to make the machine go in the direction required. The skill required never ceases to amaze me. You can't take your hand off the cyclic to scratch your nose like you can in a fixed wing. I'm way out of my discussion depth here, but i believe the same is true of larger rigid rotor machines.

You are proposing a method to improve the ability of the machine to return to a "wings level" state after any given input. What you really have not considered is how to improve the fundamental flying characteristics. This is the fallacy in the symmetrical rotorcraft arguement.

My arguement is get the control system right, then optimise the aerodynamics. The reason i suggest a mechanical gyro system is to avoid unecessary control system complexity.

Mart

[Dave_Jackson]

Mart, The fault really does lie in the control system.It may be more accurate to say that the fault lies in the rotor system. As rotors move from being highly flexible blades and teetering hinges toward the high rigidity of airplane wings, the response time between the pilots input and the helicopter's reaction will significantly reduce.

This will be to the advantage of the professional pilot's sub-conscious activity and the recreational pilot's conscious activity.

I flew to Mexico with a friend who had never been in a light airplane before. He learnt how to coordinate the wheel with the altimeter and compass in two minutes. Over the two-week adventure he did more piloting than I did.

[Graviman]

It may be more accurate to say that the fault lies in the rotor system.

Maybe so. It is worth pointing out that the Comanche has one of the most complicated FBW systems yet, despite having very rigid rotors. My guess is still that you need the gyro to provide system roll/pitch rate control. The rigidity would just reduce the mechanical gain required.

Like i say i'm not against the concept, but i am trying to offer critical feedback.

Mart