Tail Rotor vs Twin Rotor (ie Chinook)

[Bob Viking]

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

[212man]

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.gov.uk/media/5422fdb840f0b61342000861/2-1988_G-BWFC.pdf

[Bob Viking]

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

[Bravo73]

“wobbly death banana”, “no agenda”.

Hmmm. 🤔

[Bob Viking]

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

[212man]

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)

https://www.youtube.com/watch?v=ewzwvAnFFRE

[Bob Viking]

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

[Fareastdriver]

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.

[Bob Viking]

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

[Ascend Charlie]

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.

[ShyTorque]

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

[Saint Jack]

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.

[SASless]

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.

[Bob Viking]

I’m learning. So, money aside, should all helicopters be twin rotor?

BV

[Octane]

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...
https://www.youtube.com/watch?v=Xy0GMyfGu6s

[SASless]

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.

[22/04]

The design doesn't seem to have caught on...


Except in those radio controlled toys- one sitting here in my floor right now.

[22/04]

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.

[Bob Viking]

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

[Bravo73]

So, money aside, should all helicopters be twin rotor?

As someone has already pointed out, most* helicopters are 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.

[Bravo73]

The design doesn't seem to have caught on...

That’s not really true. There are plenty of co-axial helicopters out there (the Russians love them).

Some of the most modern designs use co-axial rotor systems. https://www.pprune.org/rotorheads/524130-s97-raider.html

[JohnDixson]

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

[sycamore]

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....

[Bob Viking]

Thanks for the information.

Boscombe has a very long runway as well!

BV

[Fareastdriver]

I think all helicopters are evil. I believe aircraft should land and then stop not vice versa.

Birds have been flying on this planet for millions of years. They know far more about flying than human beings.

[Bob Viking]

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

[evansb]

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

[Thomas coupling]

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.

[Fareastdriver]

Have you ever watched one land on water?

They don't do it very often. 90% of their life is defending a few yards of riverbank or sticking their necks underwater to chomp.

[SASless]

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.

[KenV]

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.

[sycamore]

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.....!!!;););)

[KenV]

Here's a link to an image for another advantage of the tandem rotor design:

https://www.hoax-slayer.net/wp-content/uploads/2017/02/chopper-rooftop.jpg

[Robbiee]

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.

[Ascend Charlie]

BV, many years ago when CFS(H) was at Ternhill,all prospective helicopter pilots( volunteers or pressedmen) had to undergo a selection process

Our selection process was to choose the smartest, best-looking highest-scoring pilots off the jet course and put us onto helicopters.

And thanks to John Dixon, I now have another "urban myth" to add to Nick Lappos' list.

[JohnDixson]

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.

[Thomas coupling]

John - I'll have the same as you're having :rolleyes:

[SansAnhedral]

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:

https://www.youtube.com/watch?v=jHvq--A1vww

[sycamore]

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...;)

[tartare]

Well, they don't like it when the blades start intermeshing..... https://assets.publishing.service.gov.uk/media/5422fdb840f0b61342000861/2-1988_G-BWFC.pdf

Jaysus.
And two people survived that!

[SASless]

To be fair....two people survived.

Airbus aircraft that shuck Rotorheads take everyone with them.

[megan]

I think all helicopters are evil. I believe aircraft should land and then stop not vice versaBob, there is a firm belief in sections of the helo world that anything that needs to accelerate down a runway to reach XXX knots in order to fly is inherently unsafe, despite what the following has to say.

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1204x1600/helicopter_pilots_are_different_e0a22b8fcab33bc1180f150c8f0a 0aae5a9ed7b2.jpg

Just to round off, most helo pilots learn to fly on fixed wing then graduate to helos. :E

[tartare]

That is brilliant.

[jimjim1]

The rotor is driven only by tip jets. Air from a gas turbine is piped to the rotor tips where it is burned with fuel. Much whizzing round, no torque reaction. Oh, and NO GEARBOX in the rotor system. It seems that there will be far fewer plumet-out-the-sky-right-now single points of failure when compared to a helicopter with mechanically driven rotors.

A commercial failure in the 1950's apparently due to very high noise levels from the tip-jets.

https://en.wikipedia.org/wiki/Tip_jet

The Rotodyne was "A helicopter at take-off and an autogyro during forward flight".

A singe rotor variant was developed first. The best known model confusingly had three rotors (as per this thread's terminology) however only one of them was used for vertical takeoff, hovering and landing, the two conventional propellers being just that and used for high speed forward flight only. True single rotor variants were built.

Single rotor variants


https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1024x737/fairey_ul_helicopter_g_apjj_bag_18_06_15r_edited_2_aace9bd47 0c2bea7d4e15e21d25e65d3b50c2b2a.jpg


Fairey UL Helicopter


https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/800x600/hiller_yh_32_hornet_790460a5509d29c370500f1033f9b3388f5f1516 .jpg

Hiller Hornet


https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/1024x661/1024px_sff_002_1055526_fairey_rotodyne_c823f44147a53b473827a aef8a187ffe619439fb.jpg

Fairey Rotodyne


https://en.wikipedia.org/wiki/Fairey_Rotodyne

https://www.theengineer.co.uk/issues/28-june-2010/fairey-rotodyne-was-the-future-of-aviation/

https://www.youtube.com/watch?v=SJqcVVnk3DM

https://www.britishpathe.com/video/rotodyne-sets-new-world-record-for-rotocraft/query/US+AIRLINER
8mins, no sound

https://www.bbc.co.uk/news/magazine-35521040
Why did the half-plane, half-helicopter not work?

[jimjim1]

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...
https://www.youtube.com/watch?v=Xy0GMyfGu6s

I was impressed - right up until the pilot didn't flip it over and mow the grass where it had been standing all year:-)

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/592x343/kamov_3222882b62b03ce81fbb1c6f8ddc21256883dfe0.png

Please don't forget the grass ...

[cattletruck]

You mustn't forget that with the popular setup of having a little ant-torque rotor at the back, Mr Sikorsky had originally planted it facing up much like a tandem setup but spinning a little faster. Theoretically it should have worked but flying it in that configuration proved to be more exciting than is necessary so they bolted it back to point to one side.

Another concept for Bob's fancy is the intermeshing rotors of the K-Max. It'll knock your head off if you don't approach it from the front.
https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/220x130/220px_usmc_17567_f31f6d78586c1f3ed197aa143699b8a525148032.jp g

[Ascend Charlie]

Funny that the soundtrack for the Rotodyne sounded more like a B47 2-blader than a 4-blader plus 2 propjets.

[India Four Two]

AC,

You have a point there but in real life, it actually sounded like a steam locomotive.

I lived in Maidenhead, about three miles from White Waltham in the early 60s. This was the last days for the Rotodyne and the last days of steam. The Western Region mainline ran right by the airfield. It was nearly impossible to distinguish the noise of the Rotodyne from that of a King or Castle at speed.

[Fareastdriver]

If you want something REALLY complicated; try this.

https://www.youtube.com/watch?v=OTw7ZagGzj8

[India Four Two]

... and by way of comparison, here's a minimalist approach - the Hiller YROE-1:

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1948x1280/hiller_yroe_1_f7150f3632872f254b98eb595acb4dda27eca598.jpg

[Thomas coupling]

https://c1.staticflickr.com/5/4033/4628449577_4f6a576129_b.jpgCare to tell me what this is then. Your starter for 10!

[PDR1]

It's an autogyro used as a towed aerial platform to provide longer-range surveillance to sufraced U-boats.

In the UK we tried the "Hafner rotabuggy" as a strap-on rotor system to turn a jeep into and autogyro for air-deploying the jeep. The tail and rotor system were to be removed after landing:

http://jeepclubs.com/wp-content/uploads/2014/06/Hafner-Rotabuggy-2.jpg

The same guy came up with the "Rotochute" as a system for getting a soldier plus full kit (more kit than a contemprary paratrooper) and ammo into hostile territory:

http://www.fiddlersgreen.net/aircraft/Flying-Jeep/IMAGES/Rotachute-Color.jpg

You name it, it's been tried...
PDR

[Thomas coupling]

PDR - you spotter!Focke Achgelis Fa 330

It was flown as a kite towed by the submarine on a 150 metre cable, and could be quickly assembled or dismantled and stowed through a U-boat hatch. It could be deployed on the deck by two people. The rotor blades were set in motion by a pull cord, or by hand if there was enough wind. The kite would then be deployed behind the U-boat with its observor pilot aboard, raising him 120 metres above the surface where he could see much further. To bring it back in the towing cable was pulled by a winch. When the aircraft landed the rotor was stopped with a brake. Two to three members of the crew were trained to use the Fa 330...................

except when there was an emergency "dive" required......:{

[Fareastdriver]

What that doesn't mention is the occasions when they saw an threatening Allied vessel or ship and the submarine cut off the 330 and dived without him.

[Tailspin Turtle]

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

Very interesting, John, and counter intuitive. All that from the canted tail rotor? Some from the height of the rotor head above the cg? Some peculiarity of the CH-47 control mixing? I've toyed with writing a paper on the Piasecki HRP development and want to be sure I don't believe something silly.

[Tailspin Turtle]

Another difference between tandem and single-rotor helicopters is the relative indifference of the former to the wind direction in a hover/departure versus the need to be aware of it when flying the latter (less so in a Sikorsky than a Bell due to respective tail rotor control power provided). Of course, a tandem is equally inclined to be indifferent to heading the same way it's going in forward flight, to the point of preferring to be sideways. A tandem design also requires some thoughtfulness and caution with respect to the interaction of the cyclic and collective systems for pitch and height control. For example, a tandem lesson learned early on is that showing off with an aggressive pedal turn in a low hover might result in the rear rotor going into translation lift while the forward rotor has not, introducing a sudden nose-down pitch that the cyclic may not be able to counter fast enough with differential collective.

[JohnDixson]

TT: nothing to do with the canted tail. If you check the 53D for instance, you get similar results.

Re your post 56, the point is quite true, but somewhat mitigated by the fact that the two tandems in the US military inventory history, the CH-21 and CH-47, were/are both cargo helicopters, thus not expected to have a mission agility capability such as the OH-6 or ( wistfully ) the RAH-66 Comanche.Flew both the 21 and 47 at the test board at FT Rucker 1963-65, and both had more than adequate directional control. Before someone jumps me, I was addressing flight, and leaving the taxiing techniques discussion to another day, but even there a bit of finesse went a long way.

[Myra Leese]

Bob,

If you look out of your crew room window you will see a big new building on the other side of the airfield blocking your view of the beach. Within you will find any number of rotary pilots who would gladly offer you a coffee and then bore you rigid with helicopter principles of flight and tech. However, having re read your posts, best bring a tin hat and a humble pie to go with the coffee!!

I think next week is quiet all round for the station so PM me if you want to come and have a look round.

[Tailspin Turtle]

TT: nothing to do with the canted tail. If you check the 53D for instance, you get similar results.

Re your post 56, the point is quite true, but somewhat mitigated by the fact that the two tandems in the US military inventory history, the CH-21 and CH-47, were/are both cargo helicopters, thus not expected to have a mission agility capability such as the OH-58 or ( wistfully ) the RAH-66 Comanche.Flew both the 21 and 47 at the test board at FT Rucker 1963-65, and both had more than adequate directional control. Before someone jumps me, I was addressing flight, and leaving the taxiing techniques discussion to another day, but even there a bit of finesse went a long way.
John - thanks for the referral to 53D cig range. I’m beginning to suspect a tandem pitch-control limitation on cg. Note that I wasn’t dinging tandem directional control but its directional stability. Another interesting facet of tandem performance was that confined areas were best departed sideways to get the most out of both rotors in the process.

[Fareastdriver]

There cannot be much wrong with twin rotor helicopters. Bristow is buying a load.

https://www.pprune.org/rotorheads/615247-bristow-buying-columbia.html

[JohnDixson]

TT, you’ve been talking to some knowledgeable old heads, I think. The guy who finished checking me out in our 21 had recently returned from VN and taught that technique of gaining additional climb performance by swinging the rear rotor out to the side and away from the forward rotor’s downward flow field when taking off toward a tree line. One time and you get the idea.

You’re also on point re the SAS-off directional stability. We would practice that quite a bit, and it paid off one day when, returning from a trip out to Ft Bliss, the upper seal in the aft transmission failed in a quick, major league way, and the transmission lubricant came pouring down over the both AC generators in the back of that box, shorting both out, and along with it both SAS. The ship started out sideways very quickly indeed: blew in one window on the side that yawed into the wind.

[Tailspin Turtle]

TT, you’ve been talking to some knowledgeable old heads,
Once upon a time, I talked to some of the Bell test pilots and engineers who worked on the HSL program, an early 1950s tandem powered by a P&W R-2800 engine for ASW. When I said HSL, their next sentence included the words “noise” and/or “vibration”. It was quite a learning curve for them, but the joint experience that resulted from solving all its problems forged them into the team that created the HU-1 (not a typo, nephews; that’s why its nickname is Huey).

[SASless]

the two tandems in the US military inventory history, the CH-21 and CH-47

Errrrr......ahhhhh......Three Army Tandem Rotor Helicopters....remember the forgettable H-25 Mule?

[JohnDixson]

Oops-you are correct SAS. Actually the guy I sat next to immediately upon joining SA, Rip Green, had flown the 25, and did not have much good to say about it, but I do not remember what his complaints were. Unfortunately, Rip left not too long after I signed in. But the 25 didn’t last very long at all in Army livery now that I recall. So, while it didn’t impress, the 21 did, and there were still healthy casual bar discussions involving whether it, or the H-34 was a better machine in the mid 60’s.

[John Eacott]

Errrrr......ahhhhh......Three Army Tandem Rotor Helicopters....remember the forgettable H-25 Mule?


Well, then there was the predecessor to the Mule, the HRP-1. 20 delivered between late 1947 and 1949, initially used by the Marine Corps (pay attention SASless) for developing vertical assault tactics, then passed on to the Coast Guard for SAR development. The remaining (attrition rate?) then went to the USN for dipping sonar and minesweeping development: talk about cutting edge!


OK, not US Army but still the lead in service machine to the H-25 ;)



https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/500x288/80-piasecki_hrp_1_51147bbf8cf3addb36294be349a6b5f4518e16db.jpgh ttps://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/483x500/80-piasecki_hrp_3_800d6c7a00e1062c2d415456dc81843483676328.jpgh ttps://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/500x274/80-piasecki_pv_3_1_9ac90d4be9988d4a9b3278e8f91204470d4893dc.jpg https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/827x417/80-piasecki_hrp_towing_tests_jun1953_32342c7c688a6d9cdd82021599 5d62a9e6ce462d.jpg


https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/510x367/80-hrp_1_with_floats_nan3_50_36cb73cfea49cced02b1e480fb547b4b75 48c3d9.jpg

[Ascend Charlie]

And it was his great-uncle Gustav Piasecki Eiffel who also played with metal frameworks, and his nephew John Piasecki Bradfield did the Sydney Harbour Bridge. Another descendant created the Big Banana at Coffs Harbour, but it never took off...

[ShyTorque]

What that doesn't mention is the occasions when they saw an threatening Allied vessel or ship and the submarine cut off the 330 and dived without him.

A quote I saw was: "Whereupon the pilot will drown in the normal way"

[Bob Viking]

Thankyou ladies and gentlemen for your insights. I guess I hadn’t really thought about it too much but I understand the role of the gear box on helicopters now.

So a tail rotor should keep spinning with the main rotor even in the result of an engine(s) failure allowing auto rotation. I guess both sets of blades will keep spinning on the Chinook allowing for the same.

A TR failure or blade collision on a Chinook are both clearly dramatic emergencies. One more so than the other depending on circumstances (apart from being on the ground I can’t really imagine a blade collision ever being surviveable).

So really cost is the main reason we don’t see more twin blade (I apologise if this is not the correct term) designs.

Again, thanks for your inputs. I think I’ll still avoid helicopters whenever possible though. They’re just not right.

BV😁

[Fareastdriver]

A quote I saw was: "Whereupon the pilot will drown in the normal way"

Better than listening to your boat crushing itself to death on the way to the bottom.

[SASless]

Again, thanks for your inputs. I think I’ll still avoid helicopters whenever possible though. They’re just not right.

Actually the helicopter is okayt....it is the Helicopter Pilots that are not quite right!:uhoh:

[Bob Viking]

I would like to state, for the record, that is was not me who said that.

At least, not out loud.

BV

[sycamore]

Bob, I really think you are a secret admirer of the helicopter fraternity,and should really get yourself on a cross-dresser...sorry,cross-over course at Valley...Enough of the mundane,flashing around you`ve done with your arse on fire,come and join the gang that really get down`n dirty...With your `ex-steely `good looks,golden locks and blue eyes ,you`d be an immediate `hit` with the `blue-rinse brigade`,stepping out of a camo or yellow helo at an airshow..Come on,you know it makes sense at your age...get on that skateboard,and seek new challenges.....!!

[Dave B]

Bob Viking
If your fixed wing ever drops you in the sea, you may be glad of a Helicopter being around.
I have to correct thread 23, the Belvedere was made by Bristol, not Westland. It has a bad reputation, not always justified. If you consider it was the late fifties, and you had a machine that had an AUW of 22,000 Lbs, could maintain altitude on one engine, had a cruise speed of 120 Knots, and a VNE of 140 Knots, (from Memory).
The starter system was high maintenance, but did work ok most of the time. Its an urban myth that some pilots tried to start the aircraft from outside of the cockpit as they were frightened of the starter, that would not be possible, as the rotor brake had to be off to start the aircraft. I flew several hundred hours as a crewman, and I never saw a pilot do this.
I only had one starter explosion on my time, and that was a low key event, I was in the left hand seat when the pilot pressed the button, there was a muted thump from the rear engine, and then the fire warning came on, we exited in an orderly fashion.
The main rotor heads were similar to the Sycamore, and were light weight, and low maintenance.
If you are interested, seek out a Belvedere in a museum, note the flush skin panels, the Shobert riveting, and the doors and window hatches that worked correctly, the build standard was better than either Westland, or Sikorsky could attain.
In my time as a Helicopter Engineer, I flew in about Nine Helicopter types, as Crewman, Flight Test Engineer, or passenger, and I can say that the Belvedere was the smoothest.

[Bob Viking]

That is a very kind offer. I shall give it some thought.

For now though I have a joke for you. It’s not exactly politically correct but it’ll serve a purpose.

Whats the worst thing about being a rollerblader?

Having to tell your parents you’re gay.

Now of course, you could substitute rollerblader for all manner of things.

BV

[chopjock]

B V

So a tail rotor should keep spinning with the main rotor even in the result of an engine(s) failure allowing auto rotation.

Remember though autorotation can still be achieved with tail rotor not spinning!

[SASless]

Bob....ease up!

My whole Helicopter career my dear Mum always thought I played Piano in a Brothel......I never told her otherwise for fear the truth would kill the old girl!

[Georg1na]

Dear Bob – you are very clearly of the fixed plank mindset – and that is fine – but I think we would all agree that birds are the finest flyers on the planet and most of them do zero speed landings from Eagles to Tits. (Calm down). You may also have noticed that the wings of birds are made up of hundreds and hundreds of moving parts that all seem to hinge in harmony to achieve both forward flight and all the uppy downy bits in between. I seem to recall that plank wings that you favour have all the sophistication of a slab of balsa with traily bits stuck on the rear.(Calm down sailor).

I will agree that the hundreds and hundreds of bits in a helicopter hardly seem to work in harmony but most of the time whatever they do is fine by me if it keeps me airborne. The less you know about how it works is a huge stress reliever!

[Bob Viking]

I think your criteria for lumping birds in with helicopters is a little tenuous to say the least.

I have yet to see a bird with a giant spinning feather arrangement on his bonce. They have wings like an aeroplane. Admittedly they flap them which, thankfully, does not occur (often) on fixed wing aircraft.

The choice of birds to stop before landing is surely related to their lack of wheels and has nothing to do with a desire to look like a helicopter.

Maybe evolution could solve the wheels problem given another few million years.

BV

[Fareastdriver]

Helicopter follow evolution, not the other way around.

https://www.youtube.com/watch?v=YKHGEo-R_rE

[Ascend Charlie]

Come on, Bob, we all know that an aeroplane is designed to fly, and it is only the drongo pilot who interferes with the controls that makes it crash.

A helicopter is designed to crash, and it is only the brilliant, good-looking, articulate pilot who stops it from doing so by the delicate and deliberate manipulation of the controls, to make it look totally effortless.

And rescue the drongo plank pilot who crashed.

[Bob Viking]

Thats more like it. Your post made me smile.

As an aside, I have had one hands on experience with flying a helicopter (Jetranger). It was from Old Sarum to Sywell and back via the heli lanes in London. Flying forwards was easy. Slowing down and even landing was fine. Bringing it to the hover was easy. Keeping it in the hover did not compute with my fixed wing brain.

For the 30 seconds that it wobbled around uncontrollably I couldn’t get my head around being stationary. The rudder pedals just added to the confusion.

I’m sure I would get the hang of it given practice but, for now, you have my utmost respect. And, yes, you guys do make it look easy. Or is it the autopilot that does that?!

BV🤭

[ShyTorque]

For the 30 seconds that it wobbled around uncontrollably I couldn’t get my head around being stationary. The rudder pedals just added to the confusion.

That's the problem. Helicopters don't have a rudder, they have a pilot with feet that move. Jets just have pilot's footrests.
It's a bit like guiding a wayward horse. Small inputs to keep it pointing the way you want. Once you let it have it's way, it can be a devil of a job to get it back.

One day I hope to get it right... :p

[Georg1na]

"The choice of birds to stop before landing is surely related to their lack of wheels and has nothing to do with a desire to look like a helicopter"

Ah there you are wrong. If they had wheels they would fall off the twig, especially as they would only have two. That would keep them up all night trying to balance!. Perhaps evolution will give them a tail rotor? and brakes? (Oh I forgot - they could have chocks)

Having sat for interminable hours up front in big planes I can never fathom out how you lot cope with the boredom. At least we wizzywinged lot know that something is going to go wrong, and often quite soon !

Did you know that butterflies oops Draonflies - were invented by Igor S but never took off....................no wheels brakes or tail rotor.................or passenger seats................

[Fareastdriver]

With helicopters you start it up and go were you want to go. You don't have to do le Grande Tour of the airfield to find a strip of concrete pointing into wind.

[Uplinker]

I think what the OP was getting at was the criticality and reliability of anti torque tail rotors, as opposed to twin lifting rotors, and is similar to a question I posed on another thread in the light of the Leicester crash.

As a fixed wing pilot, can I ask; how do helicopter pilots inspect the tail rotor and its drive mechanism on the walk around? Are there inspection doors along the tail boom to enable inspection of every shaft joint? I don’t recall seeing any in the helis we used to use for TV work. (Bolkow 105, Augusta 109, Twin Squirrel).

Given that the tail rotor seems to be so critical, why is there only one?. Would it not be safer if there were two separately driven tail rotors, or would that be overkill?

(By ‘tail rotor’, I am referring to the small anti torque yaw control unit fitted to a tail boom on conventional helicopters which have a single lifting rotor.)

[Bravo73]

I think what the OP was getting at was the criticality and reliability of anti torque tail rotors, as opposed to twin lifting rotors

No, he wasn’t. He just came here to troll helicopter pilots.

[Bob Viking]

No I didn’t. That came later.

As as I stated at the beginning, it was an honest question from a FW pilot to increase my knowledge about whirly death machines.

You lot are just too easy to wind up sometimes.

BV

[Myra Leese]

Would that be clockwise or anti clockwise winding?

[Dave B]

Uplinker
To answer your question, tail shaft, and tail gearbox covers are normally screwed in place, and it would not be practical for a pilot to remove these on a walk round. My thoughts on a pilots walk round, are just to see if someone has left a tool on the aircraft, or a panel fastener not done up, otherwise where would you stop. There are many controls etc covered up that a pilot cannot see.
A double tail rotor, that sounds like a complicated nightmare.

[ShyTorque]

Uplinker,

It is type dependent.

The later A109s, at least from the "Power" version onwards (I don't know anything about the very early versions, they are now actually classified as a different type) have a long piano hinged cowling (and a short one) along the top of the tailboom. These are held in place by a series of Dzus "wing nut" headed fasteners so the pilot can swing the cover up and over the shaft to inspect it and it's support bearings. The bearings have tell tale "goo" lines applied across the inner and outer races to check that only the intended parts are rotating and there is no slippage of the races. Some have temperature strips applied, too. The shaft inspection is part of the Check A (Daily check). The "back end" of the gearbox is cowled but limited inspection can be carried out; the linkages are mostly external anyway, on the left side where the rotor itself sits. The tail rotor control rod goes from a hydraulic servo inside the baggage bay (again should be checked as part of the Check A), through the inside of the boom and the rear rose joint attachment point can be inspected through a small hatch adjacent to the tail cone. The underside of the gearbox can be inspected for leakage and general condition at the same place. There is a perspex window on the left side of the tail cone to enable the gearbox oil level to be seen in a round sight glass.

Other, larger helicopters are not so easily inspected.