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Tail Rotor vs Twin Rotor (ie Chinook)

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Tail Rotor vs Twin Rotor (ie Chinook)

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Old 8th Nov 2018, 08:38
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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 itís 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 donít?

Why arenít 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 Iíve 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. Iím just curious with no axe to grind and no agenda save for a thirst for knowledge.

BV

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Old 8th Nov 2018, 09:01
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What dangers do Chinooks (and similar types) face that other helicopters don’t?
Well, they don't like it when the blades start intermeshing..... https://assets.publishing.service.go...988_G-BWFC.pdf
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Old 8th Nov 2018, 09:06
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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
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Old 8th Nov 2018, 09:08
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ďwobbly death bananaĒ, ďno agendaĒ.

Hmmm. 🤔
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Old 8th Nov 2018, 09:23
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No Agenda

I genuinely have no agenda and Iím 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 wonít 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
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Old 8th Nov 2018, 09:33
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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?!
Ok - now you've done it!
As a fixed wing pilot (Fast Jets)
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Old 8th Nov 2018, 09:37
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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. Iíll leave you to decide whether Iím talking about the songs or the aircraft.

BV
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Old 8th Nov 2018, 09:42
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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.
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Old 8th Nov 2018, 10:35
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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
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Old 8th Nov 2018, 10:48
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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.
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Old 8th Nov 2018, 11:45
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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!
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Old 8th Nov 2018, 12:59
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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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Old 8th Nov 2018, 13:02
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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.

Followed by the S-58T/Wessex, Huey, and MD-500.
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Old 8th Nov 2018, 13:20
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Thankyou

Iím learning. So, money aside, should all helicopters be twin rotor?

BV
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Old 8th Nov 2018, 13:20
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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...
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Old 8th Nov 2018, 13:46
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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.
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Old 8th Nov 2018, 13:57
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The design doesn't seem to have caught on...
Except in those radio controlled toys- one sitting here in my floor right now.
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Old 8th Nov 2018, 13:59
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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.
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Old 8th Nov 2018, 14:10
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Itís good to learn new stuff but I, for one, am glad I havenít 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
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Old 8th Nov 2018, 14:11
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Originally Posted by Bob Viking View Post
So, money aside, should all helicopters be twin rotor?
As someone has already pointed out, most* helicopters are twin rotor.

Itís just the size, and orientation, of the second rotor which differs.





* No prizes for pointing out which type of helicopter doesnít need a second rotor. But that type of design really hasnít caught on.
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