Concept for New (maybe) VTOL Craft

[Dave_Jackson]

The linked web page AeroVantage is the preliminary evaluation of a configuration that could outperform the tilt-rotor and the tilt-wing.

It is submitted for consideration, critique, and to place it in the public domain.

Dave

[slowrotor]

Well, tilt props is not a new invention.
The Curtis-Wright X-19 had four tilt props.
Focke had a tilt prop STOL.

I think prop lift is the way to go, but I prefer the method of Focke and Dornier (DO29). That is, extreme STOL first, to be followed with VTOL.

slowrotor


Dornier DO 29 http://www.airliners.net/open.file?id=1179047&size=L

[Dave_Jackson]

slowrotor,

True, your slower approach may be the safest.
__________

The concept on the linked web page is that of providing a change in the disk area between hover and cruise. The result being a better Figure of Merit and a higher forward velocity.

It is the same objective as Sikorsky's extended blade patents are attempting to achieve.

The existing and currently proposed quad-rotors still put a downwash on the non-rotation wings during hover. However, more importantly IMO, it appears that their fore and aft rotors are not intending to take advantage of a higher induced velocity that is obtainable from coaxial rotors, which share the same stream tube.

In addition, all flight control, in both modes, might be obtainable from only; collective blade pitch and PropRotor tilt control.

Just an idea.

Dave
_______________

Addition:

slowrotor you may find this web page interesting; http://www.vstol.org/GermanVSTOLTransports.pdf

[maxtork]

Dave,

As I understand it you are trying to gain some effeciency by putting the rear rotors in the slipstream of the forward ones as a make shift coaxial set up. I think that when the prop rotors are rotated down into forward flight mode they will be a good distance apart. From what I have read the coaxial loses the performance gain as the rotors are set further apart. I have thought about this arrangement myself and one possible solution would be to use the forward rotors as pushers pointing down mounted on a high wing and then rotating aft which would put the prop closer to the rear rotor. The rear set could be tractors pointing up on a low wing and then rotating forward. I'm not a CAD whiz so I can't draw it up but I have played with the idea on paper a bit and it seems to work out.

I think one of the reasons the tiltrotors are not as effecient is due to the load they are trying to carry with it. For a military machine or one that is being used for revenue this is important but for a personal air vehicle maybe not so much. Take a V22 and cut down the gross weight to just empty plus fuel and crew and the required structure to carry the lesser load and the disc loading goes down a bunch. With lower disc loading you need less power and therefore smaller engines etc. Things might start to look a bit more promising at this point.

Just some random thoughts about your idea for what they are worth (not much at todays exchange rate!!!).

Max

[Dave_Jackson]

Max,

Yes, you are correct.

A very large gap between coaxial rotors is most efficient for vertical lift due to the large effective disk area and low induced velocity per sq-ft. Conversely, the same rotors with a very small gap will have a smaller effective disk area and a higher induced velocity per sq-ft., and this would be better for forward thrust.

As you probably already know, this is the main problem with the tiltrotor (and the tiltwing) configuration, such as the V-22. It is using the same disk area for vertical lift as it is for forward propulsion. It must therefore compromise between the two requirements.

Sikorsky has a number of patents for a Variable Diameter Proprotors (larger diameter for hover and smaller diameter for propulsion) and this is aerodynamically attractive. However, the mechanism for obtaining variable blade span creates its own set of problems.

Your desire to place the counter-rotating proprotors in close proximity to each other, for propulsion during forward flight, is ideal since this will halve the disk area from when the are two aerodynamically separate proprotors and are providing lift.

It may (or may not) be impractical to get the coaxial proprotors in close proximity to each other for propulsion, as you are suggesting. My thinking is to allow a reasonable distance between the fore and aft proprotors, but have the diameter of the aft proprotor equal the diameter of the front proprotor's streamtube at the location of the rear proprotor.

This also should/may result in an 'effective disk area' that is the average of the actual front and rear disk areas. And just as with your idea, the effective disk area of the proprotors in forward flight will be half the disk area of the proprotors in hover.


The essence of this idea is probably the application of separate electric motors at each proprotor. Your idea will probably involve four identical motors, whereas my idea will require smaller motors on the aft proprotors and larger motors on the forward proprotors.

So far, so good.


Dave

[Graviman]

Dave, I've actually seen the Dornier Do-31 at the Munich Deutsches Museum

http://www.deutsches-museum.de/en/fl...vtol-aircraft/

The problem with all of these is high disk loading and complexity. High disk loading is bad because it reduces all up weight for a given power, complexity is bad because it reduces the percentage of the all up weight that is payload. There is also the cost of all those turbines, props, driveshafts, gearboxes etc. I notice you have not sketched in any structure for the rotors.

Separating lift and thrust is good, if only because it allows a streamlined cruise attitude, but for good hover capability you need as large an uninterrupted rotor area as possible. This needs to be as compact a configuration as possible or you limit where the machine can demonstrate its flexibility.

Later thought:

Hadn't we all agreed that, despite the assumption of incompressibility, having a puller prop ahead and below a rotor did in practice produce a measurable increase in pressure hence reduction in pitch?

Alternately put a pusher prop above and behind a rotor. Besides the rear rotor is best off being out of the streamtube of the front rotor. Like Chinook this reduces BVI vibration and increases the intake area. So you want that design to fly backwards. Still monstrously inefficient though...

[Dave_Jackson]

Mart,

The following are a bunch of obvious points, but they are being presented as logically comprehensible steps toward the essence of this concept.

The concept being; An Aerodynamically Variable-diameter PropRotor.

1. Helicopter rotors are efficient for lift because they have a large disk area and a low induced velocity per sq-ft.
2. Airplane propeller are efficient for propulsion because they have a small disk area and a high induced velocity per sq-ft.
3. The V-22 and the proposed V-44 have fixed diameter PropRotors. They are a compromise between a rotor and a propeller.
4. Sikorsky has considered a variable diameter rotor, however the mechanisms to vary the length of the blade's span are problematic.
5. The intention of the idea on this thread is that of using four proprotors that have physically fixed diameters. However, the pairing them allows for two different aerodynamically 'effective disk areas'. The larger area to satisfy the above #1 and the smaller area to satisfy #2.

For an elaboration on #5 consider;

• Two rotors with given characteristics and arraigned in a side-by-side configuration will provide 'a large disk area and a low induced velocity per sq-ft.'. As wanted for #1.
• Two rotors with the same characteristics and arraigned in a coaxial configuration will provide 'a smaller disk area and a higher induced velocity per sq-ft.'. As wanted for #2
• In other words, if these two rotors are arraigned as side-by-side for hover, and then rotated 90-degrees and arraigned coaxially for propulsion, they will give a better figure of merit in hover AND a faster forward velocity in cruise, than current tilt-rotors do.

This is a large view of the initial workup of the craft. It shows the PropRotors in their Hover configuration and in their Cruise configuration.

Dave

[slowrotor]

What is the point of designing a bit more efficient high speed VTOL..... there are no civilian VTOL's. The small civilian proprotor has not been invented yet.
I would focus on just getting off the ground and consider high speed efficiency in a future dream.

In addition, I would question your claim that streamtube contraction is significant at high forward speed.

[Dave_Jackson]

slowrotor;

Halftime at the Super bowl?

It appears that this could be significantly more efficient than the current design of tiltrotor; assuming that major problems don't crop up. Theoretically, an increase in the diameter of the rotors will improve the FoM at the expense of high forward speed.

Leishman (2006) says that the current V-22 looses 10% in Figure of Merit due to the downwash on the wing alone. He also says that the streamtube only contracts to 0.8, not the originally held belief that it contracted to 0.707. However he says that it is fully contracted within .25R below the rotor.

AERODYNAMIC OPTIMIZATION OF A COAXIAL PROPROTOR

Dave

[Deemar]

Dave Jackson said:

Airplane propeller are efficient for propulsion because they have a small disk area and a high induced velocity per sq-ft.

Actually, I thought that most airplane propellers were as large as practical given the size and weight of landing gear. I don't believe that there is an advantage to having a high disk loading, particularly when you've got a variable pitch prop that can cater for the differences between high speed cruise and low take-off speeds.

Daniel

[Heli-phile]

Electric motors

Hi,
had a look at the plans.
My instant concern would be there appears to be no reduntantcy for a engine failure (electric motor failure)
If each motor is independant with no shared drive the machine would very quckly loose control whilst in hover/transition (maybe even cruise)
What is the OEI proceedures??

[Dave_Jackson]

Danial;

Actually, I thought that most airplane propellers were as large as practical given the size and weight of landing gear.

Thanks for mentioning your concern. I suspect that there is an optimal propeller diameter. However, what you say is correct because in most cases the diameter is physically limited before it can reach this optimal size.

IMHO, the very large helicopter rotor will probably have to be reduced to reach the optimal propeller diameter.

There are people on this forum that are much smarter than me on this subject. It would be appreciated if one of them would add to the discussion.


Heli-phile;

Another valid concern.

I hope that this is a viable answer.
So far, at least, I prefer the theoretical advantages of a single motor at each proprotor. However, it might have to be a custom motor. The reasons for this preference are;

• Electric motors are very reliable.
• A slow RPM motor will not require gearing.
• The motor is brushless.
• The only moving parts on an Outrunner style motor will be the permanent magnets and their outer encasement, plus bearings and stub shaft.
• The coils, which are stationary, can be divided up among 2 or 3 independent power and control circuits. The idea is to include this redundancy, but put it within a single frame.

Just thoughts for consideration.


Dave

[maxtork]

Dave,

Your idea on the redundant electric motor might work but could you also get all the props turning in synch with each other like a mechanical drivetrain would? I would suspect that having one or more get out of phase might cause some kind of vibration issues. I'm sure it could probably be done but how much complexity would it add? I guess if the props are balanced individually they shouldn't care what the other props are doing but I suspect that aerodynamically the interaction between props and airframe and prop to prop might cause some issues. Just a thought.

Max

[Dave_Jackson]

Max,

The following is presented as potential ideas to overcome valid concerns.

... but could you also get all the props turning in synch with each other like a mechanical drivetrain would?

Each motor would be equipped with one, or more, encoders. These encoders, plus gyros and pilot inputs, will vary the power to each motor, as desired. If a motor starts getting too far ahead of the demanded requirement it can be temporarily made to act as brake by electronically turning it into a generator.

I suspect that aerodynamically the interaction between props and .... prop might cause some issues.

This appears to be the biggest concern, to date. As the craft maneuvers during cruise, the streamtube from the forward proprotor may 'wander' about the plane of the disk of the aft proprotor. An idea is to try and have the fore & aft proprotors in close proximity during cruise and spaced apart during hover.


Please destroy any of the potential solutions if they are not valid.


Thanks,

Dave

[Deemar]

Dave said:

I suspect that there is an optimal propeller diameter. However, what you say is correct because in most cases the diameter is physically limited before it can reach this optimal size.

I think you'll find that momentum theory will tell you that larger is always better.

The limits that determine "optimal" are likely to be related to:

• Tip speed and compressibility effects - however this is more of a limitation for helicopters as the advancing blade has a higher tip velocity in translational flight
• Structural limitations - how heavy is the structure of the rotor/propellor.

Daniel

[Dave_Jackson]

Daniel:

"I think you'll find that momentum theory will tell you that larger is always better."

I totally agree, if you are talking about a hovering rotor.

If the helicopter only needs to hover, then the induced velocity can be 500 ft per second (through a minuscule rotor) or 0.5 ft per second (through a monstrous rotor), since the craft is not going anywhere.

If the craft is required to climb at, say, 50 ft per second, then the induced velocity must be at least 50 ft per second.

The same thing applies for forward flight and this is probably one reason why faster helicopters have a higher disk loading and a smaller rotors.

I speculate that this will also apply to propellers. If the intent is to have a fixed wing craft that will fly at 250 kts (422 fps) then the induced velocity through the propeller must be greater than 422 ft per sec. The designer will pick an optimal tip speed and an optimal pitch for this forward velocity. Then he will increase the size of the propeller until it is large enough to overcome the drag of the craft.


In the case of this thread's proprotor craft, the proprotors should be large for an efficient Figure of Merit in hover, but they will probably be quite a bit smaller when providing the desired forward velocity in cruise.

Dave

[maxtork]

Dave,

While thinking about your idea I came upon a variation on the same theme that you might like. Your initial idea comes from the basic fact that bigger rotors are better at a hover and smaller are better in foward flight (turned to act like a prop). Folowing those same lines what about using an intermeshing design? If you take a intermeshing rotor system with the rotors set at an angle to each other and then rotate them both forward for fast flight then each rotor would only "see" a portion of the oncoming airstream (think Kmax with short stiff rotors flying nose straight down). I guess it is a bit like a full size rotor flying edgewise through the air like a traditional helicopter just without the disymetry of lift or a need for a pusher prop. You would have a disymetry of thrust as the advancing blades would be either on the top or the bottom of the discs now but that may be useful depending on the design. I don't know how much of an angle to each other the rotor would have to be in order to get any bennefit in forward flight (or if the theory would work at all for that matter). Getting the whole ball of wax to rotate around a set of wings would be a bit of a trick as well but I originally thought of this as a tail sitter arrangement but I'm sure it could be made to fit another set up.

Yeah yeah I know...I should quit thinking and drink more right!

Max

[Dave_Jackson]

Max,

The basic idea is to have two independent streamtubes for lift get combined into one streamtube for propulsion.
Your second sentence is correct.

However. it appears that your idea is not changing the 'effective' disk area from when the rotors are facing up (lift) to when they are facing forward (propulsion).

Is this a one rotor version of your idea?
http://patft.uspto.gov/netacgi/nph-P...S=PN/6,382,556

or ~ if the above does not work

This patent number 6,382,556 to go in this page http://patft.uspto.gov/netahtml/PTO/srchnum.htm


Dave

[Deemar]

Dave said:

If the helicopter only needs to hover, then the induced velocity can be 500 ft per second (through a minuscule rotor) or 0.5 ft per second (through a monstrous rotor), since the craft is not going anywhere.

If the craft is required to climb at, say, 50 ft per second, then the induced velocity must be at least 50 ft per second.

The same thing applies for forward flight and this is probably one reason why faster helicopters have a higher disk loading and a smaller rotors.

Not quite right.

The induced velocity is the vector difference between the inflow velocity and the outflow velocity. It can be small compared to these velocities. For example, a helicopter that has a 10 ft/sec downwash hovering will have something like (but not exactly) a 50 ft/sec inflow and a 60 ft/sec outflow while climbing at 50 ft/sec. It's the change in momentum that matters. I believe you have a copy of Stepniewski's "Rotary-Wing Aerodynamics", it is explained in the early chapters of that book.

I'm not quite sure why higher disk loadings are favoured for faster helicopters. I'm sure Nick Lappos has posted about it in the past. Maybe I'll check Stepniewski tonight.

[slowrotor]

Dave,
The prop size for hover and forward flight are so different that I would consider using a small prop for forward flight and just shut down the lift props after wing borne lift is achieved.
The electric lift fans cannot operate for long anyway with limited battery.

1) large electric lift fans for hover and slow flight.(1% of flight)
2) small prop on heat engine for forward flight (99% of flight)

slowrotor