Aerodynamics ~ Yaw and Twin Rotors

[Dave_Jackson]

It is stated that a disadvantage of twin rotor helicopters is the lack of authoritative yaw control. In addition, this problem intensifies during autorotation.

Coaxial helicopters rely on differential collective to provide all of the rotor induced yaw control and intermeshing helicopters rely on differential collective to provide some of the rotor induced yaw control. This differential collective necessitates pedal reversal during autorotation and this, in turn, probably entails some concern during flare.
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There is a future solution, which would appear to be totally effective and without cost.

At some point in the near future rotors will be produced that have Active Blade Twist (http://www.unicopter.com/1101.html) . This in-flight 'twistability' will result in greater payload/GW ratios during hover and forward flight. Soon afterwards, these blades will be further improved to provide Reverse Velocity Utilization (http://www.unicopter.com/1108.html) , and this will increase the helicopter's maximum forward speed.

The blades on the above rotors have the ability to vary their root pitch and their tip pitch, independently (http://www.unicopter.com/1096.html ) . In addition, both of the cyclic pitch changes need not be greater than once per revolution.

Yaw Control for Twin Rotor Configurations (http://www.unicopter.com/1184.html) can be effected by simply leaving one rotor in the conventional hover profile. (i.e. large positive root pitch and average positive tip pitch) and then placing the counterrotating rotor in a high drag profile (i.e. large negative root pitch and reasonably high positive tip pitch). This difference in rotational drag will yaw the craft. In addition, the direction of yaw will stay the same during autorotation. :D

Any arguments?

[Shawn Coyle]

Dave:
While the Chinook and H-46 don't have eye watering yaw control, they certainly aren't all that bad. Interesting idea, though. Interesting failure cases, too, I'd imagine.

[Dave_Jackson]

Hi Shawn;

Thanks for the critique.

Yes, the tandem helicopters are able to use opposed lateral cyclic for yaw. This is functionally identical to the earlier side-by-side configured helicopters, which used opposed longitudinal cyclic.
Unfortunately, on configurations with closely spaced rotors, such as the intermeshing and the coaxial, this opposed cyclic is of little to no use.

[Tandemrotor]

As my name implies, I feel I should contribute!

Whilst 'rate' of yaw in the CH47 may not be 'eye watering', it is certainly high enough to achieve impressive translational lift, normally on the aft head, demanding large amounts of aft cyclic!!

Not sure I could cope with any more!!!

[SASless]

Being a Chinook Pilot as well.....I can most assuredly promise you the absence of yaw control in the "A" model version could become "eyewatering" when you forgot yourself as an instructor and turned the SAS system off on a transition student in a turn without advance notice and confirmed and verified response from the guy at the controls acknowledged what was about to happen.

You ever hear droop stops pound on both heads as the aircraft slewed completely sideways at 100 knots despite full pedal being stuffed as hard as you could push it? Eyes watered....bladders voided.....bowels leaked......and Instructors went home much wiser ! No wonder I drink too much!

[Hilico]

Tandem, I'm fascinated. If you get TL "especially" on the rear disk, that implies it has a higher airspeed than the front; which in turn implies that the centre of rotation is not the middle of the aircraft, but somewhere closer to the front. I'm not saying you're wrong - I just wonder how that happens.

If this was in the hover then, and you wanted to spot turn quickly, your first control input would be pedal, followed shortly by aft cyclic. To stop - opposite pedal, cyclic forward. Yes?

Signed, a very inexperienced and occasional (but keen to know) R22-driver.

[Flight Safety]

Dave, I'm not sure how useful this idea might be.

You could use a small clamshell type Gurney flap on the trailing edge of each rotor blade. The "clamshell" could have a capacity for infinitely variable deployment from 0 to 90 degrees, and when fully extended, the 2 Gurney flaps (making up the "clamshell") would be deployed 90 degrees equally above and below the trailing edge.

This would have the effect of increasing or decreasing the drag of the rotor blade, with the variable deployment providing direct control over the amount of increase in drag. If the flaps (and mechanism) were small and light enough, they could even be cycled opened and closed during each rotorblade revolution to apply drag (and torque) where desired.

I can think of two possible means of inducing yaw with this idea. In one means, if we use the K-Max as an example aircraft, you could simply open the clamshells on both blades of a single rotorhead (without any rotational variance) to increase the drag on that rotorhead, to induce yaw. I think this would work well under power, but perhaps not too well during autorotation.

Another means would be to vary the opening of the clamshell so that only an advancing rotor blade on one rotorhead experienced increased drag. Say the clamshell started to open at 45 degrees after the advancing rotorblade passed over the rear of the fuselage, and closed again 45 degrees before passing over the nose of the fuselage. This would have the effect of increasing the drag on one side of the rotor disk, thus causing the aircraft to yaw towards the dragging side. I noticed on the K-Max that the advancing blade of each rotorhead passes over the fuselage, which would reduce the moment arm of an arrangment like this, but it might still work. I also think this arrangement might work better during autorotation.

Perhaps some cyclic control could also be built into this mechanism as well, since the K-Max already uses a flap type "tab" for cyclic control. This also might be a lot simpler than blade twisting.

I have no idea of the possible failure modes of an arrangement like this, but maybe the idea is useful.

PS - I also think this idea might work well with rigid rotorblades. :ok:

(edited to correct typo)

[Tandemrotor]

Hilico

I would have to confess to not entirely understanding it myself ( Not being a TP like Shawn) All I can suggest is either the fact that the aft head produces more lift than the forward head (approx 55% - 45%)

Or more likely, because as pilots we all like the yaw turn to be done about the cockpit, are possible factors.

[Dave_Jackson]

Flight Safety,

Thanks for your ideas and concerns about yaw and autorotation.

As an overview;
Tandem and side-by-side helicopters, with their widely spaced rotors, use opposed cyclic.
Coaxial helicopters, with their concentric rotors, use differential collective.
Intermeshing helicopters, with their closely spaced rotors, use opposed cyclic in conjunction with differential collective.

Differential collective is not an attractive solution, since it causes reversed yaw control during autorotation. Therefore, your idea of employing a Gurney flap is an attractive alternative. :ok:

It appears that your concern about the use of Gurney flaps during autorotation may not a problem. The following is from the Gyrodyne coaxial web page (http://www.gyrodynehelicopters.com/xron_history.htm). It has some neat pictures of the tip brakes.

"The Model 2C used Movable vertical surfaces (rudders) and differential collective in the rotors for yaw control. The results of the instrumented flight test indicated that the coaxial rotor configuration possessed excellent flying qualities in all regimes of flight except for the low speed autorotation where the yaw control means proved inadequate. In order to overcome this difficulty, Gyrodyne continued its research work toward improving the directional control characteristics. In March of 1953, the idea of using tip brakes on the tips of the rotor blades was conceived. Flight tests of this concept proved that the problem of effective yaw control in autorotation for a coaxial helicopter had been solved. This was a major breakthrough for the coaxial configuration[. The Company applied for and was granted Patent No. 2,835,331 on October 24, 1954."

Shawn likes the idea of an offset teetering rotor ;), so this is the current area of development. The use of Gurney flaps, or some type of air brake could be ideal for this rotor.

If interested, here's two pictures of the mockup (http://www.unicopter.com/temporary/Dragonfly_Rotor_Pics.html) and a web page (http://www.unicopter.com/1181.html) on the offset tri-teetering rotor. Assuming you don't object, the information in your post will be added to my site.

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An interesting thread might be that of trying to think out what happens at each of the two disks of a Kamov coaxial helicopter when in autorotation, and why the pedals must be reversed.

[NickLappos]

Dave is again solving problems that aren't! The typical twin rotor helo has gobs of yaw control power, down to low speed autorotation, where they tend to be very soft, but adequate (note that no twin rotor has failed to be certified, indicating adequate, if not spectacular yaw control. Dr. Maheyev of the Kamov bureau spent 1/2 hour with me defending autorotational yaw control for his coax helos, so he believes it!

Sasless, the fact that Chinooks swap ends at high speed is not a lack of yaw control, it is a lack of yaw stability, because Boeing builds no tails on their helos. Lack of tail fin (that rear pylon hardly qualifies) is the problem. Tail rotors are very powerful stabilizing devices, their disk area behaves like a fin of several times the area, because the disk produces lots of opposite thrust when it is cocked due to a yaw disturbence. Coax helos dont build opposing yaw through the rotors so they need the fin. This is similar to how the fenestrons behave, lots of control power but not lots of stability - thus the end plates.

[Dave_Jackson]

Nick,

The lead post in this tread was about a future means of yaw control, which could be applicable to all twin rotor configurations.

Re: the tandem configuration; [edited], I previously erred by generalizing, and thereby implied that yaw authority is inadequate in this configuration.

Re: the intermeshing configuration; other pilots have said that autorotation is not ideal.

Re: the coaxial configuration; you say "note that no twin rotor has failed to be certified, indicating adequate, if not spectacular yaw control." I believe that Kamov has been unable to get US certification for the coaxial helicopter. Perhaps you can advise if the reason is technical or political.

[MaxNg]

Nick

What ever happened to the modified S76 that you had with the nose extention to develope Fly by Wire technology.

Dave

If these helicopters were fitted with AFCS ( Automatic Flight Control Sytems) or just the basic yaw control then surely you would just keep your feet of the pedals during the flare :(

Nick

Would it be practical/feasable for the fly by wire A/C to recocnise a engine failure and automatically enter Autorotation into wind, Flare, level (on Radalt) and land with no pilot intevention.

And finaly

Would you trust IT

:\

[NickLappos]

Dave,
The certification of ANY Russian product in the Western world is problematic, since their entire set of standards is quite different, down to the way the metals are qualified. This does not infer that they are sub-standard, just that it is hard to compare them to the Western rules. Since Certification means that the government assures its quality and safety, the process seems so expensive that it has just stopped. Third world countries accept the Russian standards, so sales go through in those places. For most Western countries, Bi-lateral agreements tie the standards to a common acceptance, so the certification process is defined and economically viable. JAR is an even tighter unification of the rules.

I do not believe the coaxial's yaw properties in low speed autorotation are an impediment to certification, I'd be surprised if it was a problem. The rudders seem to provide acceptable handling down to low speeds, which should allow proper flight envelope.

MaxNg, The SHADOW is in our boneyard now, cacooned for long term storage. It did well, teaching us all kinds of stuff about Comanche technologies.

Regarding FBW controls, the automation of tasks is what they do best, it would be possible to automate almost any task. An auto might be the hardest thing, since picking out a place and getting to it would be best left to the human. The entry down-collective move is a good candidate, however. FBW is really good at helping the pilot do pieces of tasks, like holding limits and such, so it could be a real helper in the auto entry (suppose the rules you programmed into the FBW said always obey the pilot's collective unless the rotor rpm was at a low level, and plunging down at a high rate - in that case put the collective down to stop the decay, then wait for the pilot to say "Thanks")

It is this kind of thinking that will go on to improve helicopter handling and safety, not necessarily highly complex mechanical configurations that attempt to build the handling in through geometrics. Such systems as anti-lock brakes and electronic fuel injection (both FBW systems now common in cars) show us the type of things we should be insisting on, I think.

A simple set of autopilot functions for robbie types could be cheap as dirt, if we opened our eyes to the fantastic work being done on RC helos, and applied their free thinking to our problems. For US experimental helos, flying such systems should be fairly easy.

[SASless]

Ah but Nick....autopilots on helicopters....get real....the cheapass operator mentality would still suggest they are already paying the pilot so why spend the dollar and a half more. The manufacturers will have to find a way around that if they ever hope to provide the "seatweights" with truely modern kit.

One man's opinion here......just like a very large Arabian oil company that continues to operate Bell 212's at night over water with no visible horizion or surface lights and tells the pilots it is not IMC/IFR thus both legal and safe without SAS/SCAS/Sperry kit.

[NickLappos]

SASless,
What you need is a nice coaxial syncropter that flies hands-off at 200 knots. Can't you see that?

[Dave_Jackson]

Nick

You say, "The certification of ANY Russian product in the Western world is problematic, ......... Third world countries accept the Russian standards .."

I believe that Canada has certified one Kamov model; but perhaps some consider Canada as a third world country. Make sure you get payment from the Canadian government before delivering the S-92's. :D
Enough humor.

Your avid support of the tail rotor configuration seems to be softening a little, at least in respect to the coaxial configuration. Is this a harbinger of a future relationship between Kamov and Sikorsky?

If so, then perhaps United Technologies / Sikorsky can put aside the old conflict with its former employee, Charles Kaman. Then the intermeshing configuration can continue to make the significant advancements in Rotorcraft that it formerly did under the Germans.

[Another KOS]

Dave:

Wasn't it rumoured that a Kamov type was imported into Canada and certificated without dual hydraulics against a safety case (risk assessment) - for aerial work only?

Weren't the FAA a little disturbed about this due to the cross boder operational arrangements?

[Dave_Jackson]

Another KOS

You may very well be correct.

The story I heard was that Canada was more inclined to certify it then the Americans because Canada did not have a significant rotorcraft manufacturing industry to protect. I think that this story came from Lu. :O

Hey Lu, where are you???

[MaxNg]

NickLappos

Yer dam right it would be possible to install simple autopilot systems to even the small helo,s such as the R22, it,s even more feasable to install such aids to the new Hydrolically assisted R44.

It's seems crazy that you can by minature servos for R/C helicopters that are able to control yaw for under $200 but we still are unable to develope a simple AFCS for full size A/C.

Some years ago I saw a single seat light helicopter, I think it was called the "Angel" fly in the UK and at the time (1994 ish) it,s instrument panel consisted of one flat LCD screen that monitered Rrpm, Erpm, MAP, Barometric pressure, C/H temp, Oil temp, OAT and now doudt a few other things to boot, and as such it was able to calculate your density alt and MAP limits on screen with the added bonus that if any exceedances were recorded then the a/c flagged these up on screen, and required maintainance actions before it would let you start up again.

Computing technology has expanded exponentially since then and the cost of manufacturing such item would be insignificant compared to say a Rotor tach or altimiter, yet certification costs and reluctance to advance at a more realistic pace is actaully costing lives.

I fly the Mk 11 Puma and at £13 M with 4 screen EFIS and it gets its knickers in a twist if you try to display more than ten waypoints on its screen. and each one of these screens costs more than the "Angel"

:*

[NickLappos]

Dave,
The KA 32 is approved in Canada as a restricted catagory no passengers, which is not a real TC. This is granted when it can be shown that it is safe enough for crew use only. I think the motives have more to do with commercial loggers needing a lifter than softer criteria, as I know TC to be a tough and fair bunch, mostly.

[Lu Zuckerman]

Actually Transport Canada is a lot tougher than the FAA where aircraft certification is concerned. The FAA will certify a foreign built aircraft based on the certification criteria of the country of manufacture. (Mostly JAR countries). In the case of the A-310 the FAA performed some route proving flights and checked the aircraft for general handling qualities and approved the A-310 for use in American airspace. The FAA for a long time would approve a derivative aircraft by only testing the differences between the new design and the original design no matter how many derivative aircraft designs here were. The A-310 was originally designed and certified to FAA standards, which required an FMECA for all systems. This was at the system level but to get Transport Canada certification the FMECA had to go to the smallest piece part level of every component in a given system, which was much more stringent than the original JAR requirements.

Bombardier tried to certify the CL-604 design based on previously approved versions of the same aircraft. This is based on the FAA and their method of certification. Transport Canada said no and made Bombardier perform the product assurance analyses down to the piece part level just as was done on the A-310.

I assume this also applies to helicopters as well.

[Flight Safety]

Dave, no problem.

[Another KOS]

Lu:

Can you explain the FMECA review for a simpleton.

[Lu Zuckerman]

To: Another KOS

The certification authorities never see the FMECA (Failure Mode Effects and Criticality Analysis) and if they want to see it they must request permission and look at it at the manufacturers' facilities. The FMECA is used to define the failures and their modes and effects of those failures with this information feeding into the Safety Hazards Analysis (SHA). The SHA defines the probabilities of occurrence for different types of failures from minor failures to catastrophic failures. It stands to reason that if you go down to the piece part failures as opposed to trying to define the failures at the component level the information feeding into the SHA will be more detailed and the SHA will be more accurate in predicting the probabilities of the failures and their effects at the airplane level.

Even with the detailed figures as opposed to the less accurate figure the end result is defined by the manipulation of the numbers to show a probability of catastrophic failure at 10 9 or better and it is pure crap.

:mad:

[Another KOS]

Lu:

Thanks Lu; with regard to the difference between 'piece part' and 'component', how does one regard complicated components such as main rotor gearboxes (and even engines).

Is it possible to do 'piece part' analyses of main gearboxes and if so is damage tolerence considered? Is degradation due to wear considered and is this reflected in TBOs?

If the whole main gearbox (in effect a train of parts) has to meet the 1309 requirement of 10 -9, isn't it difficult considering the number of elements in the single failure path (have I expressed that well enough?).

What is more effective (practical), piece part analysis or component monitoring with HUMS?

It must be nice to be a clever bloke!

[Lu Zuckerman]

To: Another KOS

The piece part analysis would take into consideration every component in the engine or the gearbox including the nuts and bolts that hold the thing together. Damage tolerance is not considered in the analysis. The person doing the analysis will use accepted generic failure rates for each element in the system. If he does not have access to proven failure rates from his factories service records he can select failure rates from approved data sources. The failure rates for a gear might be manifold so in most cases the analyst will select the best number to make sure he can achieve the specified goal for the engine or the gearbox. In some databases the analyst can select the failure rate say for an electric clutch that was used in an atomic submarine. He can manipulate that failure rate by an approved K factor multiplying the failure rate for the atomic submarine clutch in order to get the failure rate for an electric clutch used in a non pressurized area of an aircraft.

You can see by this that the numbers start to skew from reality at a very rapid rate.

In a gearbox the most critical parts relative to catastrophic failure are the “Jesus nut” and the rotor shaft and most likely the analyst will select a failure rate of .01 10 6 which is realistic. The gears and the nuts and bolts will have similar failure rates where bearings would be significantly lower. Gear cases would also be shown as .1 or .01 10 6 depending on the stress loading however in the SHA the gearbox would be shown as a single entity having a failure rate that is an agglomeration of all of the piecepart failure rates. An internal failure of the gearbox would result in an autorotation, which provides the safety net.

Regarding the piece part analysis and the HUMS I would say that the FMECA and the SHA provide the necessary material for the FAA or CAA to approve an aircraft for certification but the HUMS will if it is designed properly and maintained properly will save your ass.


:hmm:

[Shawn Coyle]

REgarding the KA-32.
It was initially approved for use in Canada with the single hydraulic system under Russian registration, pending the design and approval of the dual hydraulic system and Canadian certification.
The certification is a bit strange "Transport Category with restrictions", if I remember correctly, which doesn't really exist as a recognized category. It was restricted to crew only, but I know they are working on getting a full Transport Category approval. This is going to take some changes of the instrument panel and other things, but nothing that can't be sorted out.
Nick- it also appears that the US has accepted Russian certification methods for materials, if my memory serves me right. But there are still a lot of problems getting Russian made machines accepted, let alone certified, in the West.

[Dave_Jackson]

Flight Safety,

Thanks again for your earlier ideas.

You said; ~ "the flaps .... could even be cycled opened and closed during each rotorblade revolution to apply drag (and torque) where desired."

This idea is particularly attractive to the intermeshing configuration. It's masts are at an angle from the vertical. Therefore, the activation of a rotor air-brake will result in a yaw-pitch cross-coupling, which will cause the nose of the craft to rise.

By having a tip air-brake provide thrust as well as drag, your cyclic idea can provide sufficient lift at the back of the disk to offset the nose up, caused by the rotor's yawing of the craft.

Mechanically, the linkage for this fits perfectly into Shawn's Offset Teetering Hinge. :D

[Wunper]

Heliswiss have been operating a KA32 for at least 10 years out of Belpe I am not sure what type of certification flag it flies under, it is a magnificent beast and much quieter than their lusty 214B, which in the land of cuckoo clocks, alpine horns and cowbells is a very good thing...

Wunper:ok: