Daniel, it's because a more powerful engine is already required for higher speeds. So there is less of a compromise in requiring more power for hover. If you take the power vs speed curve the higher disk loading has advantages at speed, due to less parasitic drag, so increasing disk loading just pushes the min power Vy to a higher speed. Ultimately retreating rotor stall limits how far you can push the effect though.

Like everything it's a compromise. It is probably why Nick commented about variable radius being all gravy, since you could increase the disk loading with speed and reduce it for hover. But mechanical complexity and poor transient performance limit its practicality.

Later thought: Increasing disc loading also reduces rotor mass (as you comment), but variable radius would still have you carrying around unecessary mass. This likely is another good reason why variable radius has never got beyond concept testing.

Dave, i'm amazed Leishman find's time to write all this stuff. Paper printed off for a quiet evening...

Deemar,

An interesting argument. We are probably talking about the same thing, but from slightly different perspectives

Let's forget the vectors since; the proprotor's thrust and the craft's motion will be vertical during vertical climb, and the proprotor's thrust and the craft's motion will be horizontal during forward flight.

As you say "It's the change in momentum that matters." Momentum is the quantity of motion of a moving object, equal to the product of its mass and its velocity. In other word; the mass can be large and the velocity low, or the mass be small and the velocity large.

As you have said, large rotor is more efficient for hover, but as Mart has said "a more powerful engine is already required for higher speeds".

The 'geometric pitch' of the propeller must be high enough to provide the desired velocity of the craft during forward flight, however as you say, the diameter of the disk should be large to provide efficient lift during hover.

The idea of this thread is to change the 'effective' disk area between hover and cruise so that it is closer to the optimum diameters for both requirements.

In addition and as you have previously said, the proprotor will have to have collective pitch, and the pitch will be much higher during cruise.


slowrotor,

You are probable correct if the intent of this tread was that of conceiving an economical, safe and simple VTOL craft.

Perhaps I should have mentioned at the beginning that the object of this thread's concept is to consider a craft that improves on the shortcomings of the V-22.

The Advancing Blade Concept is a beautiful 'Rotor & Propulsor concept', as a contrast to the tiltrotor's 'PropRotor & Wing concept'. However, it is appearing that the ABC concept may only be able to provide cruise speeds of 250 knots and higher if a reverse velocity blade can be conceived. At the present time, to my knowledge, any improvement to reverse flow through the blade will create a greater dis-improvement to the forward flow through the blade.

The idea on this thread is just another attempt to get a 'Free Lunch' out of Nick.


Mart, It is a very interesting article. Maybe his students do the 'grunt work'.

At the risk of winding Nick up ~ again :O, this report could have been done, and should have been done, many, many decades ago if America had not been redirected away from twin main rotors.

Dave

That's the problem with America. No free thinking, no innovation. Everything we do is just because somebody from some other country, like Russia or England or France told us to do it. :ugh:

-- IFMU

Dave, the problem here is the classic one for new concepts - they do not go into details until too late. To convince skeptics you need to take a concept, stick with it, and flesh out the details to the point where there is a counterarguement for every concern.

If that concept was symmetrical then no problem. But there would need to be detailed calculations showing the performance of the design. Everytime a concern is raised there needs to be another calculation to quantify the concern. Ignoring cost, that needs to be at least power vs speed performance, range, component mass (including structure), and handling.

Your spreadsheets are a good start. To provide a compelling arguement you have to show quantitatively that symmetrical will outperform conventional. If the numbers added up even i would be convinced. My strong suspicion is that the benefits do not make themselves present until high cruise speeds.

IFMU,

Don't get depressed.

Igor Beaver over-hyped the single rotor. A number of American gurus, in the 1940's, said that the single rotor was not the way to go.


Mart;This is the very point that I was trying to make in the previous posting; in respect to disk area.

The disk area is the cornerstone in building rotorcraft algorithms. Yet no rotorcraft Aerodynamicist has ever developed the calculations that mathematically and comparatively interrelate the disk areas of all the various rotor configurations. It was never done in Momentum Theory. It was never done in Blade Element Theory. It was tunnel vision on the single rotor.

Leishman's article is a step in right direction.


Dave

Dave, looking at your idea for this week:

http://www.unicopter.com/AeroVantage.html

I notice that the machine magically appears from hover to cruise state - perhaps with the same post edit "ping" that masked so many cheap effects on 60's TV Sci-Fi series... :)

I still think that if you want the best transition performance you need a low front rotor and high rear rotor, to avoid interference effects. Keeping the rotor/prop hubs at the same position in space will also simplify the drivetrain. Even so, the penalty of carrying those wings around in hover will be high.

But i see what you are trying to do, take fixed wing performance and give it limited VTOL capability for short duration hover. If a seperate engine drives each diagonal rotor pair you also maintain some failsafe capability.

But the verdict still has to be: Too complicated...

Mart,
The concept definitely has pros and cons, but so far - so good.

Some quick replies.
Yes, transition is currently the primary concern. Here are Seven preliminary ideas for effecting the transition.

The V-22 looses 10% of its thrust in hover due to its downwash on the wings (Leishman and one other). This 2x4 PropRotor only places a downwash on the wings during transition and the downwash will not be normal to the flat-plate surface of the wings.

The drive-train consists of four independant PropRotor+Motor, with a liberal sprinkling of electronic SAS.


Theoretically the 2x4 Effective Aerodynamic Disk Area should give a better hover AND a faster cruise than the current V-22.

A testimonial; "This is actually a very good idea. It is one of the most cleaver solutions I have seen yet. You have actually had me looking at your idea for about 20 minutes now." ~ Dennis Fetters.

Mart, Any ideas for a good transition mechanism?

Dave

Dave,

I understand that in the laminar flow environment of a university wind tunnel critical spacing of the propellers ensures perfect reversal of the induced swirl.
(unless i found a way to break the tunnel by not securing the turbulence generator boards :uhoh:) In the real world of turbulence, gusts, manouvres, tired pilots, etc without ducting to force this you will need to use a large amount of tip washout to at least minimise BVI.

In practice, i just don't see the advantage. Leishman comments that it is only really applicable for high disk loading, which we aim to avoid in helicopters - right? Even so similar diameter disks have the advantage of the rear rotor drawing in clean (laminar) air from outside of the front rotor. For your concept I just don't get what the benefit is.

If we are agreed that critical spacing is of interest only to PhD students trying to expand the capabilities of CFD software modelling, then forget trying to cram in the prop-rotors ahead and astern the wing. Space them out nicely so that you can have a nice simple vectoring system - consisting of sprockets and chains (like Harrier). Trust me simple solutions work - every time! (I'm used to having to justify why i want to redesign castings and repackage layouts to the money men).

What i mean by "transition" is that at some point your concept must go from its very power hungry, hence time limited, hover to its real comfort zone of high speed cruise. It is designed for >300mph high speed, right? Otherwise, why not just put down an order for "son of X2"? Your nice pictures of laminar flow stream tubes will become a jumble of lines where that wing is partly stalled and our PhD students have a field day seeing whose CFD code gets the closest to right answer. You could eat even more payload with a wing swivel mechanism. You could provide a self stowing helium balloon...

The point is that the Chinook development team faced similar problems and opted to put the rear rotor up out of the way of the messy front rotor outwash. If you want to see a 21st century bi-plane, look up the Quickie Q2 then see whether the rear wing is the low or high wing.

Perhaps i have laboured my points too strongly for your tastes. But my honest opinion just has to be that this concept is just too complex to be competetive... :)

Mart,
Cruise is the 'comfort zone'?
Look at some Russian aircraft with counter-rotating propellers. Then look at their speeds; which are up to 570 mph.

'When the going gets tough, the tough get going.'

http://news.xinhuanet.com/english/20...nt_7693921.htm

Maybe Dave in action again? :)

Cyclic Hotline,

Notice the decal. :)

[Sikorsky X2-1/2]

Dave

Dave,

Do you have any thoughts about this concept? It is perhaps an overly complicated approach, for a cargo machine. The basic principle of combining the tilt rotors to one large low disk loaded assembly is an interesting idea though. The down-side is still how the machine can be made to respond in the event of Cat A double engine failure during transition.

http://www.baldwintechnology.com/

http://www.baldwintechnology.com/Airplane_Mode_r02.png

Graviman,

It's an unusual, but interesting, concept.

I believe that it is being considered as an entrant for the US 'Joint Heavy Lift' (JHL), along with a large Boeing tandem and a Bell V-44. Sikorsky had submitted two coaxial rotorcraft, but it appears that one or both were recently rejected.


Dave

Now, if they are able to separate the rotors for hover and combine them for cruise ......:)

Graviman,

Baldwin's Mono Tiltrotor is not a contender for the JHL. They only mention the JHL program from an evaluative perspective.

These were the original five contenders.
--Sikorsky X2C, X2 Technology Crane - coaxial rotor (165 knots);
--Boeing ATRH, Advanced Tandem Rotor Helicopter (165 knots);
--Sikorsky X2HSL, X2 Technology High Speed Lifter - advancing blade compound (245 knots);
--Bell Boeing QTR, Quad Tilt Rotor (275 knots); and
--Frontier Aircraft OSTR, Optimum Speed Tilt Rotor (310 knots).

Recent information;
Lockheed Martin and Karem Aircraft Team to Support Department of Defense Joint Heavy Lift Program

Dave

A couple of Transition Methods for the interested
 

DESIGN: AeroVantage - Transition Mechanism - Proposed Methods - Forward and Aft Moving Elbows

DESIGN: AeroVantage - Transition Mechanism - Proposed Methods - Upper and Lower Arms

Very complex transition methods. Why not pivot the cabin/compartment in the centre of a frame with engines left & right or fore & aft, and have the transition of rotor alignment to cruise achieved aerodynamically by the changes in rotor cyclic & collective, rather than physically? This would mean a much simpler hanging/bearing arrangement for suspending the cabin.

The transition of the cabin position could be partly achieved by a controlled/damped/assisted use of its own CofG, in hand with small wings/canards for returning from cruise at speed.

This would of course mean siting the whole power and transmission system remotely from the cabin area, maybe in a separate enclosed aerodynamic compartment.

Overdrive

You are correct. All of the proposed mechanisms under consideration may be complex, and therefore relatively heavy.

Mechanically it is unattractive, but it appears to be very attractive aerodynamically. Hopefully, some method may provide the 'free lunch'. Your idea of using cyclic control to transition the PropRotors will be considered. Thanks.


Dave

Dave, no comprendo. What is so aerodynamically attractive about operating with high disk loading in hover, then making one rotor sit in the turbulent air of another during cruise?

Mart Attractiveness is a relative thing. In this case it relates to the tasks that are being requested of future VTOL aircraft and their ability satisfy these tasks.

Current helicopters satisfy a very small portion of the continuum between the balloon (100% hovering) and the supersonic airplane (100% fast-forward). The objective is to have an aircraft that can expand its portion of the aircraft continuum by offering more than current VTOL craft. One market that has been mention is that of commercial traffic over distances of approximately 250 miles or less.



cattletruck.

You maybe referring to this craft - Igor's VS-300-C
The three tail rotors were intended as substitutes for the rudder and two elevators that are used on airplanes. This was necessary for the control of the helicopter during hover.

http://avia.russian.ee/foto/gallery/...k_vs-300_8.jpg

Sikorsky is working on this, except that they are slowing the main rotors. ;)


Dave

Dave, what i'm confused by is that you were championing seperate lift and thrust rotors. Why the change of view to what to me appears less ideal? :confused:

CattleTruck, the multi tail rotor was a short term compromise introduced only while the dynamics of cyclic control were being investigated. The ~90 degrees lead angle that we all take for granted was not then understood - and why would it be, since it is not an issue for smaller diam stiffer propellers.