Intermeshing Helicopters

[Graviman]

This post has been prompted by my interest in the helicopter concepts that Dave Jackson has put forwards:

http://synchrolite.com/SynchroLite.html

http://synchrolite.com/UniCopter.html

I first came across these ideas on the Eng-Tips forum, and freely admit to not immediately being convinced. Since then i have debated with Dave, and (not being content) have also read up a number of books on aerodynamics and helicopter design (Including Prouty). Allow me to put in my own words the advantages i perceive with the intermeshing concept (in particular Unicopter):

1. Both rotor downwashes will feed nicely into wake contraction, whereas single rotors suffer tip flow distorsion.

2. Symmetry of main rotors will allow reduced pilot workload/training.

3. Careful design could lead to commonised parts between LH and RH rotors.

4. Removal of tail rotor will reduce power requirement by at least 10% (includinh high speed stabiliser trim drag) - downwash rotation wastes at most 2%.

5. By ultimately designing a rigid rotor, retreating blade may be unloaded. This avoids tip stall drag and reverse flow region. Outboard advancing, may allow optimum blade AOA in all conditions.

6. By ensuring that effective wingspan is always across both rotors at high speeds (conventionals end up lifting in front and rear quadrants), more efficient cruise may be obtained.

7. Twin pusher props will allow efficient cruise, as well as effective yaw control in reduced g manouvres.


OK, this is a very brief summary of the conclusions i have come to. I thought to start this thread to pick the minds of the best helicopter experts out there - the guys that fly 'em. To some extent i will sit back, and see what points come out of this thread.

Thanks in advance for your thoughts.

Mart

[Dave_Jackson]

WOW! Talk about Marketing.

"i will sit back, and see what points come out of this thread."

Hell Mart. You can sit back and see what 'pointy things' come your way. I'm going to do a Lu and duck for cover.

[slowrotor]

My opinion:
For a personal use helo the first consideration in configuration and rotor size is that it have good autorotation. Cruise is secondary.

Putting another rotor above the other only increases downflow and therefor is not as good for autorotation.

Coaxial has some advantages for easier to hover. But a large single rotor should be somewhat slow in reacting as well.

Just a friendly reminder, try searching this forum for extensive discussion on this topic.

regards
slowrotor

[Dave_Jackson]

re: Safety

Slowrotor,

The intermeshing Flettner FL 282's were the world's first helicopters to enter and exit autorotation. In addition, the FL 282 helicopters were equipped with rotor governors.

Dave

[Graviman]

Thanks SlowRotor,

"Putting another rotor above the other only increases downflow and therefor is not as good for autorotation."

Intermeshing can be thought of as a "bunched up" side by side config. Retreating blade goes above other side, and is unloaded by feathering. This means autorotation performance is improved since advancing blade runs at optimum AOA rel TPP. I have suggested tip trim to Dave - this forces tip AOA to 0 in all flight conditions, so swash plate actuates only root (like BERP to some extent).


"Just a friendly reminder, try searching this forum for extensive discussion on this topic"

Quite right, i should have done an extensive search first. I really just wanted a concurrent view on this config, since i have only just finished studying chopper design...

Mart

[headsethair]

We've been flying one of these around the office. You should get one - the autos are c**p.
Go here and look for Bladerunner....http://www.dot2shop.com/detail.asp?s=58&id=18931

[slowrotor]

Dave,
Early helicopters all had low disc loading and probably autorotated well.

In my mind, two rotors are less efficient than one. Just like two wings are certainly (biplane) less efficient on an airplane.

If I could, I would go with one blade. The working part of the blade is near the tip. The inner part is mostly just there to support the outer part. A twin rotor configuration has a higher percentage of useless inner rotor.
In other words, one large rotor will lift more pounds per horsepower than two smaller ones.

Headsethair,
That Bladerunner sure is stable in a hover! The blades have too much pitch to auto unless you get the rotor to reverse rotation. Take the rotor off and drop it from the ceiling and it will spin in reverse.

[superiorheli]

You might want to take a good look at the Kaman Kmax design, which is a production version of the intermeshing, side-by-side rotor concept. There are other factors to take into account including aircraft weight, profile, etc., but to my knowledge the Kmax is the most efficient fuel burned/weight lifted and horsepower-to-lifted weight rotorcraft out there, due in large part to its efficient large chord, slow moving blade system. It is extremely quiet for its size and and the rotorwash is somewhat self canceling and minimal for an aircraft that will lift 3 tons. In short, it has many of the features you are looking at (although not the fast forward airspeed - it was built for lifting).

[Dave_Jackson]

slowrotor,

As you know, the subject of autorotation encompasses numerous things, such as; rate of rotor deceleration, cleanliness of entry into autorotation, rate of descent, and usable rotational inertia at landing. However, I think you will find that there is no significant relationship between autorotational efficiency and rotor configuration.

Each configuration will have its nuances. There is a pitch-torque coupling on intermeshing helicopters that causes the craft to pitch upon loss of engine power. Twin-rotor craft do not have to consume some of their autorotative power driving the tail-rotor and its power-train. As mentioned by superiorheli, the coaxial and intermeshing helicopters take advantage of 'swirl-recovery'

There probably is a relationship between the number of blades and the rate of descent. However, this has to do with the fact that helicopters with more blades normally have greater disk loadings.

It's anybody's guess whether a 4-blade coaxial or a comparable 4-blade single will have the slower descent rate, but I will bet on the coaxial.


superiorheli,

Interestingly, the initial market for Charles Kaman's helicopters was crop dusting.
Unfortunately, this created a belief that the intermeshing configuration is low disk loading and slow speed .


Dave

[slowrotor]

Dave,
"swirl recovery" and self canceling rotorwash, as mentioned by superiorheli above.
Can you elaborate on that?

I would think the downwash speed of the Kmax would be similar to other helos of similar disc load (maybe higher), is that not correct? The Kmax is efficient because of the large slow rotor more than or in spite of the intermeshing configuration. Maybe the customers (loggers) just didn't want to deal with tail rotor strikes.

Two coaxial props on an airplane regain some from swirl recovery, but still not as efficient as one large prop, I think. Coaxial props are used on very fast or over powered airplanes. Not used for efficient personal planes. I have not ever seen a coaxial or intermesher of full size or model fly.

But I am still interested.

[Dave_Jackson]

slowrotor,

As you have mentioned, swirl is a small component of the velocity in the rotor wake, induced by the spinning rotor. More on swirl..

A slow rotor is only more efficient than an identical fast rotor because the collective angle of attack must be increased (toward the optimum of roughly 8º) to maintain the same thrust. At some high pitch angle, this becomes unsafe, particularly during forward flight and maneuvering. You may find US patent 6,007,298 ~ Optimum Speed Rotor of interest.

"I would think the downwash speed of the Kmax would be similar to other helos of similar disc load (maybe higher), is that not correct?"

Yes and no.
A 2-blade single rotor (1-blade rotor excluded) provides the best lift/power ratio, however, the necessary tail rotor consumes 5-15% of the engine's power. Therefore, the side-by-side configuration, with 2-blade rotors and a combined disk area equal to the previous single rotor, becomes the best. This configuration is a little 'unwieldy' so one can start bringing these two counterrotating rotors together. First comes the interleaving configuration, then the intermeshing, and finally the coaxial. Even thought the blade radius of all four rotor configurations is the same, the effective disk area gets smaller and smaller, and therefor the disk loading increases. What has happened is that the craft has morphed from a craft with two separate 2-blade rotors to what is basically a craft with one 4-blade rotor.
Each configuration has its pros and cons.

The following assumes what your desire is.
Your requirement for a slow forward speed and good thrust (to support stronger components) will be helped by blades that have a large (-12º) negative twist.
Your requirement for a slow autorotation speed will be helped by blades that have a small (+1º) positive twist.
The way to achieve both is by Active Blade Twist , and the big boys are still trying to figure this one out.

Dave

[Graviman]

Wowsers, i kindof feel like a lamb to the slaughter!
Search "Intermesh":

http://www.pprune.com/forums/showthr...ight=intermesh

http://www.pprune.com/forums/showthr...ight=intermesh

http://www.pprune.com/forums/showthr...ight=intermesh

http://www.pprune.com/forums/showthr...ight=intermesh

http://www.pprune.com/forums/showthr...ight=intermesh


I have just had my second go in an R22. Ever had that amusing occurence when you pull collective to flare then wonder why you have turned to the right? I imagine switching from clockwise to anti-clockwise main rotor provokes similar experience. Soon figured my goof, and naturally thought of the lateral symmetry of the intermesher. Less likelyhood of stinging yourself too (no i didn't) - no tail rotor...


Thinking about some of the points raised in all the above threads:

Since there is no transfer shaft, i see desynching mains as unlikely - as long as gearbox is well designed/tested. Interleavers far more vaulnerable.

Don't believe synchropters would be any worse or better than a similar disk loading conventional. A twin 3-rotor would have similar performance to 6-rotor - ie higher profile drag, but very low vibration.

Since, ideally higher, retreating blade is unloaded there will be no "bi-plane" drag. Agree that active blade twist is required to optimise AOA across blade in all conditions. Still suggest tip flaps to force elliptical lift washout at tip (lowest tip losses). Convinced this offers superior highspeed performance potential to ABC - Nick?

Regards ease of piloting, now at least i have some experience of the problems of conventionals - will have a couple of more goes, but don't tell my bank . Pedal symmetry a definate plus! Rigid rotor would allow fast response to input - ie good pilot feedback. Dave you really must think about hand-off hover stability. I don't see how synchropter will be improved over conventional rigid. I honestly suggest a lockheed CL-475 style spring input gyro system - being mechanically simple, and so reliable.

Any thoughts, or is everyone quietly hoping Dave will jus' built it an' see? My offer of FEA/CAD still stands BTW...

Mart

[Chiplight]

intermeshing

My own experience with intermeshing helicopters has not been especially rewarding.

[Dave_Jackson]

Mart,

"Dave you really must think about hand-off hover stability."

The Unicopter has Absolutely Rigid Rotors and they rotate in opposite directions. They will provide the stability, due to the opposing gyroscopic precession.

Dave

[Graviman]

Chiplight:

Wow, they'se helichoppers is good an' intermeshed! They are radio control choppers? Had me pretty worried at first look...

Dave:

Really don't understand this point. If (say) a gust caused undemanded roll, i don't see how this would resist input. If anything counter-rotating systems are normally done to avoid gyroscopic coupling, thus reducing effective rotational inertia.

The two counter-rotating rotors will avoid cross coupling, simplifying pilot workload. He/she/it still can't take his/her/it's hands off the cyclic the way he/she/it could the collective or pedal, given enough height and forward velocity (naturally).

I am still a big fan of this project, and wouldn't raise these concerns if i didn't feel they were valid. OK y'aint gonna get as good as full fly-by-wire with laser gyros and ground hugging radar, but i'm trying to suggest a practical means of reducing required hours to solo - hence cost...

Mart

[Chiplight]

Graviman,
you are correct on several points;
those are r/c choppers in the "intermeshing" photo.

Counter rotating disks(such as coaxial props on a single shaft) nullify gyroscopic precession effects. You might expect twice the resistance to movement out of plane, but in fact it is near zero.

I fondly recall a discussion on a gyro forum with Chuck Beaty about this. It is one of the few times he's had to eat crow. He finally resorted to doing a hands-on experiment and was stunned to discover that the forces do indeed cancel as I'd learned from searching on the net.

[Graviman]

I sincerely hope Dave takes the criticism in the constructive way in which it is intended.

The Lockheed system, as fitted to CL475 (and initially to Cheyenne, before DOD spec increased disk loading to give dynamic problems) was exceptionally clever. I can only begin to describe it's subtlety:

Underneath the rotor, as part of the swash plate was a large gyroscope. Being rigid, the lead angle to plate was less than 90 degrees (~30 I seem to recall, since even "rigids" have flex and a floaty helicopter inertia). The cyclic was connected to this gyro via springs, with an additional lead angle of 90 degrees. The gyro tried to follow the horizon, and effectively flew the helicopter. Pilot cyclic input caused the control gyro to precess (in same way as teetering swash plate), by being 90 degrees lead. The springs were really just there to provide a sensible movement for a given input force.

If you think about it, the chopper will always try to fly in trim with the gyro. The pilot is thus directly controlling the pitch and roll rate of the gyro with stick position, hence directly controls heli pitch and roll rate. I just can't think of a more linear system, in all flight conditions. I was hoping Nick Lappos (ABC man) might like to comment on his thoughts about this system, but respect if he prefers to steer clear of this thread.

It would be easy to take a lead off the gearbox to drive a much higher speed gyro, thereby reducing required mass. The cyclic would go to the gyro, and the gyro to the two swash plates. Some additional mechanical complexity (which i appreciate is bad), but with FMECAs and design calcs/simulations a very easy bird to fly.

Mart

[Dave_Jackson]

Mart,

You're correct. My reply was very short and doesn't properly address the subject of stability. The web page UniCopter ~ Trim, Stability & Control is an attempt to address this subject in respect to the UniCopter.

Your original statement was "Dave you really must think about hand-off hover stability. I don't see how synchropter will be improved over conventional rigid. " I suspect that you are correct. An intermeshing (synchropter) helicopter will probably not be any more stable than a single rotor helicopter; if they have identical rotor types. (Teetering v.s. teetering.) (Offset flapping v.s. offset flapping.)

The UniCopter should be a different story.
The two rotors are extremely rigid and they are rigidly mounted to the rest of the helicopter. Their rotational inertia will want to resist (hopefully not too much or not too little) the pitching and rolling of the helicopter. In addition, because their axii are close to being aligned and they are rotating in opposite directions, the gyroscopic precession of one will offset the gyroscopic precession of the other.A little elaboration. You are correct in that the above will only reduce the rate of pitch or roll. However, I believe that this will be an improvement over conventional rigid rotors. In addition, the UniCopter's absolutely rigid rotor will provide the pilot with a very fast cyclic response to correct the pitch or roll.


Concerns by you and others are desired, because they cause an assessment or reassessment of potential problems. If there is any disagreement with the above, please fight back.

Dave
______________

A not so little sales pitch.

slowrotor is very interested in safety.
Theoretically, the UniCopter should have trim, stability and control that is quite similar to that of an airplane. In fact, I think that the rotors could be fully stalled out, just like an airplane, and then the craft be put through a stall recovery.

[Chiplight]

Dave, from your website,

If we put two counterrotating gyroscopes on the same rigid axle and again apply an upward force on the West the opposing North and South force will cancel each other. The axle can freely yaw but there is resistance to pitch and to roll.

No, I think you are incorrect in the last sentence. See my previous post above.
The angular momentum of a spinning disk is represented by a vector pointing along the axis of spin.(right-hand rule?) Two disks spinning in opposite directions have vectors pointing in opposite directions, so they sum to zero. What remains is the inertial mass of the static masses, but no rotational momentum. Like I said, Chuck did the experiment and conceded this was correct.

If you put two gyroscopes on the same axle, spinning in opposed directions, you would have no way of knowing that you were holding a gyroscope by feel alone. There would be no resistance to movement, although intuitively we think there should be.

[Dave_Jackson]

What an interesting subject.

Chiplight,

In other words, it appears that you are saying that the two precessional forces will self-cancel, leaving only the requirement for enough force to rotate a static mass. The following excerpts from two web sites may support your position.

Damn it!
__________________

http://www.accs.net/users/cefpearson/gyro.htm

"Sometimes precession is unwanted so two counter rotating gyros on the same axis are used. Also a gimbal can be used. Gyroscopes, when gimbaled, only resist a tilting change in their axis. The axis does move a certain amount with a given force."
__________________

http://www.halfbakery.com/idea/Enhan...ce_20generator

"Of course, with two gyros in counter rotation, the center point of the shaft is rotated so that Laithwaite's demonstration is performed by two gyros together. However, this requires a steady place to turn from (Laithwaite had his feet on the ground, and used his body to twist the gyro around, with respect to the ground.). Laithwaite claimed a few percent decrease in the apparent weight of his two gyroscope system (not antigravity! - just force in the upwards direction). He also had big problems with the shafts breaking.
In space, this system could never work, since as the gyros were twisted one way, the spaceship would twist the other."

___________________________________

Mart,

Yes. A number of peolpe have given positive comments and information about the Lockheed system on this forum.