tilt-rotor vs. tilt-fan

[gadgetguru]

I am curious what thoughts are there on the X22 (http://www.unrealaircraft.com/gravity/pages/bellx22.php) in comparison to the V22 (http://www.unrealaircraft.com/gravity/pages/osprey.php)

Do you think that the 4x ducted rotor holds any benefit to the 2x exposed rotors

Has anyone thought to try it with turbo fans or are the powerplants required too prohibitive in physical size to thrust req'd ratios.

There has to be a better way; eliminating the requirement of 'exposed' rotor blades & the inherant safety factors for crew & pax in ground proximity

Survivability at the point of a potential engine failure, no glide capacity & if you cannot auto, its a short/quick trip...

Obviously many things to consider, but surely something that has already been considered at length, particularly by the bell boys.

Does anyone have any knowledge in these areas?

Where do we see the future of rotorcraft going?

Will sikorsky's ABC make a comeback in the X2 (http://www.globalsecurity.org/military/systems/aircraft/x2.htm), will projects like the Trek Solo EFV (http://www.trekaero.com/Springtail/Bluey.jpg) make it?
Will experimental concepts like the Moller aircar (http://142.26.194.131/aerodynamics1/Appendix/Aircraft/moller_aircar.html) ever make into mainstream? or
Will they only be remembered in history as flights of fancy, or a novelty item like a segway?

Are the technologies that these endeavours use misplaced?
Should the designers be focusing on application rather than commercial market & gain?

For example again take the segway, other than making it onto a few talk shows & being utilised in some trivial 'look-at-me' marketing campaigns, it hasn't 'revolutionised' personal transportation as most of the marketing hype for the device proposed, it simply added another item to the list of big boys toys .

But what if that same technology was re-applied to paraplegics? (who are otherwise unimpaired)

Imagine the mobility that could then be afforded to individuals restricted to wheelchairs...interesting concept.

Are any of the technologies being pursued in some of the aviation projects misplaced?
(potentially a whole other thread, but slotted in here as well).

[Dave_Jackson]

Are any of the technologies being pursued in some of the aviation projects misplaced?IMO, the primary obstacle that is inhibiting the development of significantly advanced rotorcraft is the lack of Active Blade Twist (http://www.unicopter.com/B372.html).

Independent Root & Tip Control by Torque Tube Method (http://www.synchrolite.com/UniCopter_Blade_NACA00xx.html), and the CNC workstation to build it (http://www.unicopter.com/A114.html), is a presumptuous attempt at overcoming this obstacle.


Anyone want to participate?

Dave

[NickLappos]

Before Dave takes this thread down that dark and lonely path, let me comment on the actual question:

The entire concept of hovering flight hinges on the concept of disk loading, where the power of the engine is harnesses to move air using a rotor/prop/shrouded fan as the device that converts power into lift. These devices are all of a bunch, and rely on the inexerable physics of fluid flow. In a single statement, for a given weight vehicle the bigger the fan/rotor you use, the less the power you need to hover. There is NO magic, a small rotor sucks big time relative to a bigger one. I have posted a bit on this on my web site. see:

http://webpages.charter.net/nlappos/Disk-Loading.pdf

Tilt rotors need about 40 to 50% more power than helos, and quads or ducted fans need maybe twice the power. Moller is not a fool, his machine will fly, but it will not make any headlines since it needs so much power/fuel/purchase cost that it is impractical. The entire science of vertical flight has existed as a benifit of how light engines have become. As they get lighter and cheaper, all these screwball ideas become less screwball, and more practical. What we have to stop doing is looking at the rotor configuration, its like looking at the magician's hands. The rotors need power, smaller ones need more power. The key to VTOL success has always been driven by the cast/weight and fuel efficiency of the power plant. If you want a Moeller air car, find a better engine, or wait for one to be made. Helicopters were made when engines came along that produced 1 HP per pound. Tilt rotors came along when that ratio got to be 4 HP per pound.

Comments on configurations:

Tilt wings are a very funny way to try and eliminate the awsome vertical drag of the wing that sucks the lift from a tilt rotor. The wing costs about 5 to 7% of the gross weight as lost lift - that's about 3500 lbs in the V-22! Tilting the wing is a strange way to treat the most important structural joint in the aircraft, the wing root. It is a nightmare, and has never passed the sniff test for anyone who actually makes money building flying machines. It also is a heavy structure, probably costs you 1/3 to 1/2 of what you tried to save when you tilted the wing to begin with.

Shrouded fans are cool, they act like rocket nozzles that control the flow off the rotor/fan, so they make the rotor act as though the disk is bigger. This reduces the power needed for a given weight, so they have a place in the sun. Look at Comanche's Fantail or the EC155's fenestron. But they have to be deep to work, and this makes them high weight and high drag. Thus they rob just about what they gain you.

[maxtork]

I have a question regarding this as well. I understand, as Nick stated, that larer diameter rotors are more efficient as they have a lighter disc loading and the tilt rotor aircraft with high weight and smaller rotors are therfore not as efficient as a conventional helicopter ....however.

What if we took two aircraft with the same weight, one conventional helicopter and one quadrotor type aircraft. It would seem on the surface that you could acheive the same disc area with four smaller rotors as you can with the one large one and end up with the same disc loading. Would this be a correct assesment? or do we lose something by having the downwash split into four streams as opposed to one?

Sorry didnt mean to hijack the thread I just thought it may be pertinent.

Max

[Dave_Jackson]

Maxtork,

Your assessment is probably correct. A single rotor and a quad rotor should perform roughly the same; when the craft is acting as a helicopter.

Small nuances would be; the quad rotor negates the tail rotor loss, the single rotor may put less downwash on the fuselage, two adjacent rotors have been said to produce slightly greater thrust than 2 times the thrust of one identical rotor, the single rotor has a smaller total circumference and therefore perhaps less tip loss.

The real problem arises when these rotors are ask to do double duty as propellers. As rotors they should optimally generate a low velocity thrust, whereas as propellers they must generate a high velocity thrust, to move the craft forward.

To bring Nick's and my previous post onto the same path, :) the question then becomes;

1/ Do you provide a craft with wings and speed the rotors up when they are acting as propellers in forward flight? (ref. V-22), or,
2/ Do you provide a craft with separate propellers and slow the rotors down to act as wings in forward flight? (ref. ABC)

For many reasons, my money is on the latter. Bets are that Nick has his money on the same horse.

[NickLappos]

Dave and max,
No, really the problem is that the 2 or 4 smaller disks cannot be arranged to have anywhere near the area as a large, overhead rotor. Try to lay out the pattern and see what I mean. A tilt rotor cannot get more than about 45% of the disk area of a helicopter of the same overall size, no matter what they do. If they grow the dimensions of the wing to get the rotors larger, the weight increases dramatically, and negates the efficiency of the larger disk.

There is a diagram of a helo and a tilt rotor laid out in this presentation (slide 9) that describes disk loading and shows specific helo/tilt rotor pairs to imllustrate the differences:

http://webpages.charter.net/nlappos/Disk-Loading.pdf

A quad is in much worse shape, its disk loading must be about 3 times that of a helo of the same overall size, thus its power needs to be much higher yet, about 1.7 times the helo's power for the same total gross weight to hover (power required goes up with the square root of the disk loading, so 3 times the disk loading is 1.7 times the power.) Since the beast has two wings and four gearboxes, it is even worse at payload efficiency than a tilt rotor. Since a tilt rotor has only got a payload ratio of about 33% (V-22 hovers at 50,000 lbs, with empty weight of 33,000 lbs. ) we can expect a quad to do even worse, especiially since its power must be even higher than a tilt rotor. More power means more fuel, heavier transmissions, etc.

Also, since the purchase price buys the expensive items, like the engines, gearboxes and servoes, we can expect a quad to be even more expensive that a tilt rotor (which is already twice the price of a helo). Operating costs almost perfectly track installed horsepower, so we can expect the quad to cost more than a tilt rotor to operate, as well.

It is always interesting how these dreams are pushed by folks who do not ever state what the challenges are, they just state what the capabilities might be. The difference between dreams and realities are made by folks who admit the challenges and conquer them, not by those who hide the problems and sell the soap with pretty pictures. The day a quad with a paying customer lifts to a hover is the day a formation of flying pigs land at JFK.

[Dave_Jackson]

Nick,

Good discussion, however;
Nick sez ~ "No, really the problem is that the 2 or 4 smaller disks cannot be arranged to have anywhere near the area as a large [single], overhead rotor." :confused: Me sez ~ No. The real problem is that a single rotor cannot provide the higher forward velocities that VTOL customers are demanding. ;)
Recall Sikorsky's Reverse Velocity Rotorcraft Proposal (http://www.unicopter.com/1281.html) :ouch:

[maxtork]

Thanks Nick,

That was an informative presentation. I agree that we can never get as much disc area with a multi rotor system as we can with a single large rotor given the same size aircraft (ie same foot print) but I was just asking about the aerodynamics end of it. If I had an aircraft with a 10000 pound weight and a 1000sqft rotor then my disc loading would be 10lbs per sqft. If I had the same weight aircraft with 4 rotors at 250sqft each my disc loading would be the same. I will of course conceed that the 4 rotor machine would be much larger and more complex meaning it will be heavier and the payload would be much less. I personally agree that the tiltrotor isn't as great as it may first seem. I have always been a proponent of the "make a helicopter faster" school of thought as opposed to the "make an airplane hover" avenue. At any rate it is a good exercise for the brain to think about things and understand why they are the way they are.

Back to the original posters question though. I understood the older Bell ducted tilt fan aircraft used the duct itself as a lifting surface when in the airplane mode. This would cut down on the downwash against a wing like the V22 has and might offset the additional weight of the duct as you dont need as much wing. My question is, how would the duct affect the possible autorotation characteristics, if indeed an auto was possible at all?



Max

[Graviman]

Good thread...

"...the day a formation of flying pigs land at JFK"

Yeah, but is that fixed wing pigs or rotary wing pigs?

Mart

[NickLappos]

Dave,

The question is to provide the speeds they wish for at the price they can afford. The arguments I made above are (I clearly was not communicating well) centered around the fact that the empty weight and horsepower penalties of the vtols made them about two to three times more expensive to buy and operate, per pound of payload. If helos are too expensive, then these vtols are off the page.

When you toss in simple growth of the vehicle to make up for its inefficiencies, please don't forget that each pound of empty weight costs about $1,000 to 2,000 in purchase price, at typical aerospace product costs. So if you blythely toss in a 500 lb extra fuselage and transmission, you just raised the bill by about 1 million dollars, and someone has to pay. I did a study once, for each million dollars in purchase price, the operating cost rises by about $70 per hour (at 1,000 hrs per year). That means if you add 500 lbs empty weight, it costs you $70 per hour just for that weight, and $2 Million more to buy the machine.

Max,
Again, I was not clear enough. Your wish to just make the machine larger so that you could get a 10 psf disk loading simply doesn't work. The machine would weigh about twice what a helo does, and would cost about three times as much.

Once again, to discuss the possible configurations without understanding the total implications is just conjecture. Like Moeller, if you think the pictures look impressive, you miss the point.

PS by the time the disk loading gets up to 15 psf or so, the aircraft will not autorotate, no matter what you try. But don't worry, the machines you are discussing need too much power for one engine, anyway.

[Dave_Jackson]

Nick, I agree that the power/weight ratio is a very important cosideration with rotorcraft. However, if one wants to build a faster car, a bigger engine will probably be one of the necessities. This necessity also applies to the single disk rotorcraft. The previously mentioned 'Sikorsky's Reverse Velocity Rotorcraft Proposal' shows 330 horsepower per passenger seat.
___________________________

Playing with a Loaded Disk

The simple concept of disk loading has served as a quick tool to evaluate aerodynamically independent rotors. However, this concept has two major flaws. One is that it assumes that the disk loading is equitably distributed about the disk. The other is that it does not take into account aerodynamic interactions with other items such as another rotor or the fuselage etc.

The up-and-coming rotorcraft will have multiple main-rotors. These rotors will aerodynamically interact. In addition, they will incorporate features such as; Slowed Rotors, Advancing Blade Concept, Reverse Velocity Utilization and Higher Harmonic Control. When all of the above is considered during hover flight, and then totally reconsidered during cruise flight, the concept of disk loading is no longer 'simple'.

[NickLappos]

Dave said, "The previously mentioned 'Sikorsky's Reverse Velocity Rotorcraft Proposal' shows 330 horsepower per passenger seat."

Dave, you are right. I had to duck three of those reverse velocity machines, and six of those multi-main rotor, aerodynamically interacting helos just to get home. They sell like hot cakes, because nobody cares about the cost, they want more blades and heads.

Those who spin dreams about complex mechanical solutions are like those kids who studied how to be car designers when I was in high school, they learned how to shape the fenders while REAL designers were busy with the REAL things. If you look to rotors and such, you missed it, Dave.

The future rotorcraft needs to have an engine at half the weight and a third the cost of today's best turbines, if your rotor-wonders are to be sold. Look to the engine first, Dave and stop adding blades and heads in your dreams. Engines, Dave, engines.

[Dave_Jackson]

Hi Nick,

I really appreciate your dissemination of knowledge and your provocation of thought, however, this time you've really caused some head scratching.

Your adversity to multiple main rotors appears to be without foundation. The essence of rotorcraft is the rotor-blade. The function of the hub, or the hubs, is only to control the activity of these blades in an aerodynamically optimal way.

Bell is trying to develop a series of twin rotor tiltrotors. Sikorsky is trying to regurgitate a series of twin rotor coaxials. In fact, your former company has patented variable length blades for tiltrotors, and on-blade controls for conventional rotors. The functionality of Sikorsky's hundreds of patents may be questionable, but I would hate to think that they were filed solely for the purpose of restricting the advancement of others?

If the policy of the US rotorcraft industry is one of sitting back and waiting for other industries to hand them a light weight engine or a piezoelectric composite, then the current lack of funding for rotorcraft research is not without cause.

[Graviman]

"The future rotorcraft needs to have an engine at half the weight and a third the cost of today's best turbines..."

I genuinely believe this to be multistage turbodiesel, including fuel weight. The turbo does a lot of the work, while the pistons handle the high temps - the large air mass per cylinder greatly reduces heat rejection. The limiting factors are combustion initiation technique and friction in the crank mech. Blower scavenge 2-stroke diesel looked very promising for a while, particularly Mercedes and AVL research into uniflow scavenging (The deltahawk is loop scavenged). There are still emission problems with ring oil loss, that may dictate a 4-stroke soln.

----

Regarding the speed/complexity discussion (i always enjoy this). I completely agree with Nick that unecessary complexity in any system is not cost effective. Tiltrotor has proven that throwing more technology at the problem does not always cure it - it does not perform that well in either pure rotorwing or fixed wing territory. Tilt fan suffers a weight penalty for the theoretical thrust gain. Odd that V22 has not considered tip rings though - these are very effective by bounding the tip vortices in the rotor plane, to force wake contraction above rotor/prop (ie larger effective diam).

The conventional heli problem really does come down to what to do with the retreating portion of the rotor. Do you use it or lose it? The real answer has to lie somewhere in between, which means lift imbalance - so you need counterrotation. So then, coaxial or tandem? Again, my contention is that the ideal aerodynamic compromise lies somewhere in the middle. Coaxial will likely suffer some lift variation in say foward flight, as the lower rotor partially feeds on the wake of the upper rotor. SBS tandem just ends up too big for the same disk area.

This then raises the stepniewski rotation intermesher/interleaver as a contender. Again, large rotor seperation small angle, or small seperation large angle? Well, for design and reliablity reasons, i would argue that a single gearbox betters a drivetrain. But, there are still a whole host of considerations. Blade twist/pitch control, actuation mechanism. Stability augmentation, required for this rotation direction - mechanical gyro being simplest. Packaging of all this stuff? The biggest single headache is that this config is almost guaranteed to introduce a whole lot of additional vibration problems - these all need solutions...

The practical upshot is that Dave, I'm still with you but you need to do a "wright brothers" to get a LOT of wind tunnel time...

Mart

Edit: Not much, but jees what rant! :}

[maxtork]

"The future rotorcraft needs to have an engine at half the weight and a third the cost of today's best turbines..."

This may be true but better engines does not defeat retreating blade stall. If we just added great engines we would have very efficient helicopters that still went 150 kts. Don't get me wrong I definately agree that better engine technology is needed. But I also think there will have to be some changes to rotor systems at some point. The tilt rotor does address this but we all agree that there is too many sacrifices by taking this approach. My questions is (and I'm sure only the future holds the whole answer) how much more complex will helicopters become before we get the speed of a tiltrotor? Even if we used the old approach of adding wings and jet engines or pusher props to a conventional helicopter it adds considerable complexity.

I don't know what the future holds but I'll sure have fun watching it arrive!

Max

[Ian Corrigible]

Details on a British tilt-fan project here: Kestrel Aerospace (http://www.kestrelaerospace.com/home.htm).

I/C

[NickLappos]

Maxtorque and Graviman-

When I say "The future rotorcraft needs to have an engine at half the weight and a third the cost of today's best turbines..." that means that without them, all the fun ideas cannot make money.

The Wright Brothers were able to fly, not because they had a better wing. They few because they built the first light gas engine, with almost 10 pounds of weight per horsepower. If their engine was a little heavier, they would not have flown.

The first helicopter was the produce of a deliberate wait by Igor Sikorsky until the engine had been developed that could produce 1 hp per pound, since Igor learned 35 years earlier that that was the engine weight that he needed to make the helo practical.

The tilt rotor waited until the engine could get 4 hp per pound, so it could get off the ground.

The wonderful ideas that come out on this thread rely on some way to make them practical, and I do not mean a better blade or a more cleaver head. The "engine at half the weight and a third the cost of today's best turbines..." is needed to feed the inefficiencies of these new configurations.

When you read my statement as negative to helo development, please do not, it is merely stating that any improvements must first have a better engine before they can compete with today's helicopter.

Physics do not have dreams, physics is physics. Dreamers make statements like "the public wants faster rotorcraft", but they bog down when they forget the REST of the statement "at the same price, with better safety".

[Dave_Jackson]

"The first helicopter was the produce of a deliberate wait by Igor Sikorsky until the engine had been developed that could produce 1 hp per pound, since Igor learned 35 years earlier that that was the engine weight that he needed to make the helo practical."Aw, come on Nick. Get serious.

The first helicopter to fly was not by Sikorsky. In fact, the first production helicopters were not by Sikorsky. Hanna Reitsch was flying 4 years before Igor, plus, the Fw 61 and the Fl 282 were both in production during WWII.

In addition, both the Fw 61 and the Fl 282 configurations consisted of twin main-rotors.

Igor may have been waiting for some outside company to develop an engine "to make the helo practical". However, Professor Focke and Anton Flettner were working with things like figure-of-merit and lift/drag-ratio "to make the helo practical".
____________________

In the vein of James Burke's Connections, I wonder where rotorcraft would be today if Kaman had remained at United Technologies and Sikorsky had departed?

[gadgetguru]

IC
Kestrels project has strikling similarities to the Trek Aerospaces' Dragonfly (http://www.trekaero.com/Trek_Aerospace_Dragonfly_Applications.htm)
I wonder who will get to jump on the other, the springtail has made some untethered flights already, dragonfly is still under development, but won't be far behind the springtail.
But if that amount of effort was put into a project you'd expect more than a single seater result. At US$1.25m each for a springtail, I can't see them replacing the concept of parartoopers or air-mobile infantry.

[Graviman]

"...without them (engine 1/2 weight, 1/3 cost of turbine), all the fun ideas cannot make money."

More power to weight for less cost is an engine mans bread and butter. I'm not sure that this application needs such extreme numbers though. The whole thinking behind the Stepniewski rotation interleaver is to achieve the similar disk loading for only marginally more hanger space. It's just that at forward speeds the effective disk loading remains constant. This increases in a single rotor by retreating tip stall, reverse velocity region, and the need for no lateral moment. The machine could be argued to be more efficient, which goes some way to offsetting the additional mass of twin rotor systems. The power could then be directed away from the main rotor system to twin pusher/yaw propellers - intended to maximise the prop area for a given thrust.

----

"When you read my statement as negative to helo development, please do not, it is merely stating that any improvements must first have a better engine before they can compete with today's helicopter."

I read them as the necessary criticism that any idea must withstand before going further. There are many obstacles to overcome, particularly aerodynamic and vibration related, which would be further complicated by engine development. Initially the machine simply can not be competative with a single rotor in terms of cost per payload. It would have to rely on the niche, in say business helis, where the customer wants to go point to point quickly. It would likely be far more cost effective than a tilt rotor/fan. As the design/concept progresses improved efficiency and powertrain will benefit it and single rotor machines.

This is why i feel that Dave would be better off building some wooden models and rigging up his own "wind" tunnel. I put that in quotes since a water tunnel would be a more cost effective way to keep the same reynolds number (since compressibility isn't an initial factor). I notice he lives by a large body of water, so a towing rig would work just as well. He could then present the results for discussion in this open forum, gaining public acclaim for his expertise and necessary criticsm to improve on the design. Any tech papers could be presented at (say) AHS, gaining further public approval, and business interest...

Anyone say "consultant"?

Mart

[NickLappos]

"This is why I feel that Dave would be better off building some wooden models and rigging up his own "wind" tunnel."

Well said, Graviman. Dave has great ideas, and there are powerful tools now to test them.
RC models at scale, and cartop carriers that are wind tunnels work very well. The instrumentation that is available for a PC is better than the stuff I had when flying the initial flights on the S-76!

Go get 'em, Dave!

[Dave_Jackson]

Borrowing the thread ~ under the assumption that twin fixed rotors will be better than twin tilt rotors.

Graviman,

A friend has been making very small simple R/Cs with different rotor configurations. Unfortunately, the small scale, the lack of accurate blade construction and the ability to only hover, has made the project to be little more than a fun one.

Your idea for towing the rotor(s) behind a boat and Nick's suggestion about mounting the rotor(s) on a moving vehicle are probably the most cost effective means of getting reasonably good comparative information.

Few would disagree with the statement that the rotor aerodynamics of hover are very different from the aerodynamics of cruise. In addition, the aerodynamics of very fast forward flight will be very different from these two.

This is where, IMHO, Active Blade Twist is so important. Large negative twist is optimal for hover, minimal negative twist is optimal for cruise, a slight positive twist is optimal for autorotation, and a large positive twist should be optimal for very fast forward flight (re. the area of reverse velocity).

The development and acquisition of equipment for the production of blades is already underway. The objective is to produce blades of a size where 6 or 8 of them can support a 600 KG craft (European JAR - VLR ~~ American FAA - SP/LSA ~~ British BCAR-S (http://www.unicopter.com/A030.html) ). The objective is also to produce a blade that uses a mechanical means of providing a large range of active blade twist. If ABT is not achievable, the equipment can still be used to produce strong lightweight blades.

The small size of this craft and the existing commitment to the blade production equipment means that these 'full size' blades should be be the optimal means of then testing and evaluating different rotor configurations; by boat, truck and eventually remote control.

Dave