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Tail Rotor vs Twin Rotor (ie Chinook)
Please excuse the intrusion into the Rotary forum by a fixed wing pilot. As a fixed wing pilot (Fast Jets) with 19 years experience I have perhaps more knowledge of helicopters than the layperson. However, I cannot claim to have anything even approaching comprehensive knowledge of the magic involved in keeping them airborne. Clearly my thoughts are prompted by the Leicester incident and it’s possible cause. So, my question is, which is inherently safer? A conventional tail rotor set up or the twin rotor format of the Chinook. What dangers do Chinooks (and similar types) face that other helicopters don’t? Why aren’t there more aircraft with twin rotors? Is it pure cost or a size and role issue? Which would you rather fly in? Ironically in my 19 years in the RAF I’ve never flown in a Chinook. I have flown in Puma, Merlin, Gazelle, Blackhawk, Jet Ranger and Sea King though. Apologies for the the long winded post. I’m just curious with no axe to grind and no agenda save for a thirst for knowledge. BV |
What dangers do Chinooks (and similar types) face that other helicopters don’t? |
100 near misses a second...
I realise this is the obvious danger. How likely is it? Which is a more reliable set up? The Chinook gear box or a tail rotor? Is the wobbly death banana as bad as it seems in my head or is it actually less risky? BV |
“wobbly death banana”, “no agenda”. Hmmm. 🤔 |
No Agenda
I genuinely have no agenda and I’m not hoping to prove anything. I think all helicopters are evil. I believe aircraft should land and then stop not vice versa. I believe the Harrier was ludicrous for the same reason. I realise I won’t endear myself to helicopter pilots by stating their beloved machines are the spawn of the devil. However, surely we all share the same sense of humour?! This thread is all about genuine, idle curiosity. I defer to your superior knowledge. BV |
I realise I won’t endear myself to helicopter pilots by stating their beloved machines are the spawn of the devil. However, surely we all share the same sense of humour?! As a fixed wing pilot (Fast Jets) |
As if to prove I do have a sense of humour I have seen that many times and I agree that it is funny and well made. I was also a Jaguar pilot. There were many songs about that too. I also laughed at them. There are songs about helicopters too, but I find them a bit slow. I’ll leave you to decide whether I’m talking about the songs or the aircraft. BV |
Single rotor and twin rotor helicopters work on the same principle: The engines drive a gearbox which then drives two rotor systems. On the so-called single rotor system a big one holds it up and a small vertical one stops the fuselage spinning with the torque and on the twin rotor two middle sized rotors hold it up and each stops the fuselage spinning because they go in opposite directions.
Modern helicopters have a long shaft that connects one of the rotors to the gearbox. Should that break then you are in big trouble single or twin. Singles have been known to get away with it but with twin the sudden dissymmetry between the two rotors is normally fatal. Fareastdriver: Five years on fast jets and then forty three years on rotary. The best move I ever made. |
Fareastdriver
Thankyou for the explanation. Are there any sort of accident rate figures to support which has proven to be the more reliable? 43 years rotary?! That must be some sort of record. Any TR failures in that time? BV |
45 years here, and none.
The extra bonus for a Chinook is the bigger spread of cg allowable, by having 2 points of suspension from the rotors instead of one. The drawback is the complicated mixing lever system using differential cyclic for yaw control, and the resultant drift from having the airflow across the middle always going in one direction. |
Some years ago we were on a joint Puma & Chinook exercise - as usual there was more than a little "banter". A certain Chinook senior officer (an antipodean) and I (flying a Puma) were in a minibus being driven out to our respective aircraft at a major airport in Germany. He was (predictably) asking me what it felt like to be flying a "plastic" helicopter (he was ex Wessex).
I replied that at least if the rear rotor failed I had a chance of getting it on the ground safely, rather than the twin rotor systems clashing, all the blades coming off and subsequently being a passenger in something that looked and flew like a ballistic railway carriage. As we got to his aircraft I noticed the hull letters of his Chinook..."BR" (British Rail) and we both laughed out loud! :E |
With a conventional helicopter, the tail rotor stops the helicopter spinning around the mast as explained by Fareastdriver. So the tail rotor keeps the helicopter pointed in the right direction without contributing to the overall lifting capability of the helicopter. However it does require engine torque to be keep turning and this takes away power from the main rotor. In certain conditions of wind etc, the tail rotor can be absorbing up to 18% engine torque - without producing vertical lift. A Chinook-type rotor system does not need a tail rotor therefore all engine torque is directed to the main rotors producing lift, a far more efficient result. The downside of a Chinook is that it has two main transmissions, two main rotor heads and two sets of main rotor blades and this, in turn, means that although the efficiency is increased they're bloody expensive to buy, maintain and operate - which is why, generally speaking, it is mainly military operators who have them.
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BV asked us which one we would rather fly.....and there is exactly one answer to that.....the Chinook!
Once a Helicopter Pilot does a Max Power Vertical Climb with full left pedal applied in an empty Chinook ending at 3,000 feet AGL then descending vertically doing right pedal turns back to the landing site...he is "Hooked" for the rest of his time in this World.:ok: Followed by the S-58T/Wessex, Huey, and MD-500. |
Thankyou
I’m learning. So, money aside, should all helicopters be twin rotor? BV |
Hi Bob,
How would you like to go for a ride in one of these machines?:} It has 2 rotors but is sort of in between a Chinook and a conventional helicopter. The design doesn't seem to have caught on... |
Bob.....do you wear Dancing Pumps on a Mountain Hike?
Different designs for different purposes for very good reasons. The Tandem Rotor design has advantages that single rotor helicopters do not....and the reverse is true. The Chinook design with two aft landing gear that are steerable....long cabin length to accommodate the Rotor system that accommodates large numbers of passengers and wheeled vehicles or pallets of cargo makes for a nice set up. For the longest time it is and in a lot of cases remains the fastest helicopter in the fleet and capable of operating at high altitudes with some reduction in All Up Weight. |
The design doesn't seem to have caught on... |
Seriously and of course sadly as a fixed wing PPL I have learnt a lot about helicopters in the last couple of weeks. I hadn't thought that the tail rotor would have variable pitch. I imagined it was somehow variable speed though the gearbox.
A pity we didn't do a bit about them in our aircraft technical exams. |
It’s good to learn new stuff but I, for one, am glad I haven’t needed to stray into rotary theory. Besides, you need to be a member of the Magic Circle to truly understand them. Voodoo, Witchcraft and sorcery I tell you. I definitely feel a little better informed now though. BV |
Originally Posted by Bob Viking
(Post 10305325)
So, money aside, should all helicopters be twin rotor? It’s just the size, and orientation, of the second rotor which differs. * No prizes for pointing out which type of helicopter doesn’t need a second rotor. But that type of design really hasn’t caught on. |
Originally Posted by Octane
(Post 10305327)
The design doesn't seem to have caught on...
Some of the most modern designs use co-axial rotor systems. https://www.pprune.org/rotorheads/52...97-raider.html |
CG Range: Single vs Tandem Rotor A popular myth which has survived for years in spite of facts to the contrary. One example I was able to access online just now: CH-47D Manual. Max Weight: 50,000 lbs CG Range at 50,000 lbs: 322-331 inches= 9 inches MH-53E Natops Max Weight: 70,000 lbs CG Range at 70,000 lbs: 343.9-365.0 inches= 21.1 inches |
BV, in the late `50s Westland built the Belvedere,tandem rotor helo,twin engine,originally for the RN,who decided they didn`t want it ,so it ended up in the RAF.The engines ,however were mounted vertically,one behind the cockpit,the other down at the aft end.The rotors were conventional 4bladed metal,with power controls ,but both rotors/g`boxes were joined be a `syncho-shaft,.The difference between this system and the Chinook is that each engine effectively powered each rotor,but the synchro-shaft kept the two power units balanced.If you had an engine failure,either engine could drive the lot,so only in that case was the s-s taking full torque..Also the aircraft did not have such a blade` overlap` as the Chinny,and had more height difference between front and rear rotors..There were a couple of s-s failures,but aircraft landed cautiously,as the rotor RPM could be balanced on the twin throttles..
There were 3 main problems for flying the Bevelgear,;getting in,climbing a 6ft ladder,starting the engine behind the cockpit ,cartridge -Avpin-light-up,and getting out in a hurry if the Avpin starter blew up...!! By the time it left service in the late 60`s,most of it`s tech. problems had been sorted,but it did great work in the Far East,was able to lift home 3 Whirlwinds home that I had scattered around Borneo( one was a tail-rotor and gearbox departure) ,all returned to service later... As an aside I did get a trip in a JagT-Bird with Ron Burrows,laden with `dumb test.bombs`,took 99.95 % of B-Ds runway to get airborne(over 1minute rolling),and the noting control deflections ,G,yaw reponse,for an hour.....I was a bit of a `sucker` for those trips...and didn`t puke.... |
Sycamore
Thanks for the information. Boscombe has a very long runway as well! BV |
I think all helicopters are evil. I believe aircraft should land and then stop not vice versa. |
Hmmmmm.
I think we can agree that the Swan is an incredibly graceful bird. A fine example of the genus. Have you ever watched one land on water? They don’t stop first. Helicopters. Wrong, wrong, wrong. I refuse to be persuaded otherwise. BV |
Piasecki produced several models of tandem-rotor helicopters. The Piasecki YH-16A Transporter was a beast of a tandem-rotor helicopter:
https://en.wikipedia.org/wiki/Piasecki_H-16_Transporter |
Bob,
To master the art of 3D flying (up/down/fwd/back/yaw) one has to work miracles. In nature this has been developed to allow creatures like the humming bird or the bee to get to places and food, that normal animals would struggle with. The same goes for us humans. We have come up with a solution to resolve 3D flying - helicopters. This allows us helicopter pilots to stop at any time, anywhere, to go backwards, rotate, slide, yaw - at rates unavailable to FW. This gives us access to dimensions that people like you only dream of. Falling vertically off a glacier. Flying through and around buildings. Landing on mountain pinacles. Landing on moving platforms. It is the nearest to natural flying that anyone can get. Only in a helicopter.........your loss. |
Have you ever watched one land on water? |
Dropping a Winch line to a drowning person has its own small reward or using the unique capability of the helicopter to save a person from a life threatening situation adds a dimension to our kind of flying that Fixed Wing Pilots do not get to enjoy.
When someone gives you a long tight hug and thanks you from the very bottom of their Heart for what you did....that feeling is hard to beat. Handing a Teddy Bear to a small child in the hospital room after they rode in your helicopter to the Emergency Room after being very seriously injured and seeing the smile of their face....yes....helicopter flying is different.....very different. |
Don't forget the "other" twin rotor configuration: synchropter (twin intermeshing rotors like Kaman's HH-43 and K-Max.)
Somebody once told me that the tandem rotor configuration is more resistant to vortex ring state (which is what downed the stealth Blackhawk in the Bin Laden compound.) On the other hand someone else said the tandem rotor was more sensititve to vortex ring state. Don't know which, if either, is true, but I'd guess they are equally vulnerable. |
BV, many years ago when CFS(H) was at Ternhill,all prospective helicopter pilots( volunteers or pressedmen) had to undergo a selection process in the `roofless` hangar;before the days of `simulators and x-box stuff that you guys play about with now,.There would be a table with 3 judges,usually the Staish,CFI,and OC Ops,all venerable helicopter pilots,of many years experience.Each candidate was issued with a pair of roller skates which you put on,stood in the middle of the arena,and then had to rub your stomach in a circular manner,whilst `patting` your head with your other hand.At the same time you were expected to `flow around the arena ` ,pirouetting gracefully,and alternately changing the direction of patting and circular movement of your arms. In the winter,as it was a roofless hangar,you were given ice skates,as rollers wouldn`t work; nowadays a skateboard would be used...
You were allowed 2falls and 1 submission,with the `judges marking for originality,artistic content,and freestyle..Only the best were of course selected to continue the helicopter course.... So ,next time you get offered a trip in a helo,just remember the skill and daring that helo pilots used to undertake and how priviledged you should feel.....!!!;););) |
Here's a link to an image for another advantage of the tandem rotor design: https://www.hoax-slayer.net/wp-conte...er-rooftop.jpg |
Don't particularly care for those ugly chinookesque contraptions, and don't get me started on the (could they be any uglier) stacked rotor monstrosities! Not a fan of that douchie looking engine fan style thing either.,...and NOTAR, you ain't winning any style contests either.
Just give me a plain old "traditional" tail rotor, thank you. |
BV, many years ago when CFS(H) was at Ternhill,all prospective helicopter pilots( volunteers or pressedmen) had to undergo a selection process And thanks to John Dixon, I now have another "urban myth" to add to Nick Lappos' list. |
Ken V: re the Bin Laden Hawk and the wall. First thing I did upon seeing that picture was find out the altitude and temperature just using the data in the media. Then I figured out the HOGE weight for what I thought was a reasonable load of troops ( I asssumed the 75th Rangers travel heavy for that sort of action ) . They should have had no trouble at all. Looked at the photo again and noticed that the engine intakes and exhaust had been modified, pretty extensively, as had the tail rotor. Could not come to any conclusion re the main rotor. Designers of intakes and exhaust can easily affect the engine power available, and an aerodynamically inefficient tail rotor can require more horsepower than the original. ( It was pretty obvious that the modifications to the machine were major, and if it was done by anyone at SA, it was a mystery to me. I did have one pilot go on travel with a flight test engineer once, on a program we wern’t running, that he couldn’t talk about ( and remains so to this day, far as I know ). Bottom line about all this is that I’d opine that they had a hover performance deficit. People who haven’t looked into doing vortex ring testing throw the term around with ease, but if you’ve really looked for the classic case, you’d know that you have to be in a perfect vertical column to obtain the vortex circulation. The vortex ring conditions put out there by some, that roughness at 20-40 kias with a rate of descent of 1000fpm or so is a quite different aerodynamic story and has no performance implication ( although it can most certainly have vibration related consequences, a simple example of which is all the sheet metal beef ups required once Ericsson put the Skycranes into the logging business and they were in that environment continually ). Correction, I assumed 75th Rangers at the time-later contradicted by the real unit in this case.Same basic idea, though as to weight. |
John - I'll have the same as you're having :rolleyes:
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The Bin Laden raid VRS theory holds some water since the downwash circulation was likely exacerbated by the tall courtyard walls (I believe I read that the teams trained on exact replicas of the compound save for a fence in lieu of a wall, for some reason).
And to go on with the dual rotor argument, I have yet to seen a mention of these particular contraptions: |
A-C, this was of course true as well for those of us out of basic FTS,the others were generally `old`(over 30)...I ,however,as a Sgt,had for years had those words `S%%^&**&`,GET YER F$$$$$NG skates on`, shouted at me ,so it was a bit of a no-brainer really...;)
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Originally Posted by 212man
(Post 10305150)
Well, they don't like it when the blades start intermeshing..... https://assets.publishing.service.go...988_G-BWFC.pdf
And two people survived that! |
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