Sorry - forgot to comment on the 'picking up to the hover' question.
There is no need for the pilot to move the pedals when lifting to the hover, for two reasons -
a) the AFCS keeps the heading constant
b) the two rotors have no overall torque effect to be cancelled.
Having said that, the two rotors do not carry the load equally, even with a mid-CG. I've been told the aft rotor carries about 55% of the load, and the forward one, 45% (obviously). The rotor configuration means that you can lift the nose of the machine into the air quite precisely, and then control pitch attitude with power, not cyclic (within a fairly small range).
In fact, two wheel taxiing a tandem (either CH-46 or CH-47) is quite an art and a bag of fun. Three wheel taxiing the CH-46 is very challenging and needs a whole new set of skills.

45%-55%
 

The CH-47 spends a great deal of time in a hover for cargo hookup and drop off and much of its’ time below 60-knots at which point forward cyclic would be added by the black boxes. That being said when the rotorheads are returned to the factory many if not all life cycle components are scrapped. They are excessively worn and many of the parts have stress cracks beyond the point of repair.

On a Sikorsky rotorhead most of the parts can be refurbished or overhauled when the heads are returned to the factory or an overhaul facility.

Although the question was not asked I provided an answer.


:E :E

Shawn, I have to admit that I'm a little confused with your responses. The question was about tandem rotor configuration, and you exclusively talked about augmented modes of flight. The right AFCS in any machine can reduce the workload down to what you're mentioning.

In the flight test community, is it the norm to ignore the limitations of a configuration if a computer can compensate?

I hear you on the two vs three wheel taxiing of the H46. Last trip down memory lane this summer in Trenton when we say good bye to the last of the labs.

Matthew.

Thanx for the relpy guys and sorry for my delay in posting, work keeps getting in the way of my leisure.
I did a search and found some good stuff which cleared up a couple of other questions I had, but one more if you would be so kind.
Assuming the all the electronic gismos are turned off to give a true representation of the flight characteristics;
Say you pick up to the hover in nil wind, I understand that the torque from the collective pull is blalnced, but say you add fore or aft cyclic, does this then mean you would have to add pedal to correct for the difference in torque? And with the gismos turned on is this effect masked completely or partially?

Both the CH-47 and CH-46 have duplex AFCS which means that it is an emergency situation to have both fail. Hence, you probably wouldn't spend much time taking off with them both off.
The AFCSs are sufficiently complex and layered so that parts of it, like the LCTs, are working even if you shut off the attitude hold part.
In fact, lifting off to the hover can be accomplished in both machines by merely lifting up the collective - no need to come to the rear wheel position (except perhaps caution and good airmanship). I've seen an awesome demonstration of a 'jump' takeoff in a Chinook that literally didn't blow any snow around - a quick and large pull on the collective and we were rocketing up.

Spaced

The RAF demonstrate the phenomenon of the rear rotor experiencing ETL when turining about the nose, to students on the Chinook conversion course. It requires quite a high turn rate, well beyond what you would normally input for positioning, and is not difficult to control. It happens with the AFCS in too.

Current models of the H46 and H47 do have AFCS.

All the other tandem rotor helicopters also demonstrate tandem rotor flight characteristics. Some without any significant SAS/AFCS.

I've flown tandem configuration without augmentation many times in a non-emergency situation. Once in the hover, a change in one axis requires retrimming all other axes. There is no anticipation of a change in pedal due to incresed torque or due to pitch changes. However, the inherent instability in the configuration requires so much control that it may mask any anticipated control inputs.

Lifting to the hover requires significant longitudinal cyclic changes, even more so if you're doing a no hover or jump takeoff. This is to avoid putting excess pressure on oleos and because droop stops can be contacted with low power settings and undue cyclic movement.

The H46 sits level while taxiing, nose up in the hover, and level to nose down in forward flight. To take off without hover requires you increase collective while moving cyclic aft, to avoid pressure on the nose wheel oleo, then continue to increase collective while adding forward cyclic to set up for forward flight. The point that you change cyclic movement is approximately when it feels like the nose wheel is unloaded. During this evolution, you initially set your pedals as power is increased to eliminate yaw, then add a small amount of right pedal as you get into forward flight. I think this is more due to aerodynamic characteristics rather than any difference in torque between the heads.

http://www.lafhelicopters.com/pictur...storique32.jpg


Hi Spaced,

You are probably aware of the Leflamme, so the link below is for others who might not be.
Laflamme Tandem Helicopter

Are you wondering how this tandem might perform after they add the rear rotor and remove the test stand?

Thanx for all the info guys, cleared alot up.

Dave, yeh Ive seen that one, seems as though the Canuks like 2 rotors, eh?
That was what actually got me curious as to the performance of the tandem layout, as far as I know its the only small one thats been tried since about '52. I thought it was interesting to seen how they were trying to overcome the yaw instability by sweeping back the Hstab, putting the Vstab further aft. I would guess that they have moved the CG forward as well to try and keep the back behind the front, it will be interesting to see how well it flys. I didnt realise how dependant the tandem layout was on artificial stabilisation, sounds like the first few flights will be interesting.:ok:

I would predict that M. Laflamme is going to have severe directional control problems with his machine.
Way too much side area head of the front rotor, not much behind the rear one.
There is a very good reason why the Boeing machines have such high rear rotors, with such large fins. In fact, both of them have a number of very subtle but powerful aerodynamic fixes, such as strakes on the front pylon to break up airflow and stop the front pylon acting as a wing with any sideslip, blunt back fin on the CH-47 and cambered fin on the H-46, and so on.
And that's without the AFCS and sideslip ports sensing sideslip and yaw rate gyros to sense yaw rate and keep the whole thing pointing frontwards.
Good luck to him, but this is a pretty complex business.

Wow, when I saw this, I though somebody wanted to get to know ME better!!

The AFCS in the CH47 (the only tandem I have flown) is, for a helicopter, excellent. It enables any pilot to pole a Chinook much the same as any other helicopter. Nothing is really any different (with the exception of ETL on the aft rotor during rapid spot turns)

HOWEVER, with the AFCS out they are PIGS to fly. Especially on instruments!

With the aft rotor producing the majority of the power, and very little inherent stability, the aft rotor constantly tries to overtake the forward rotor ANY WAY IT CAN!

I haven't got time to talk about 'Positive Stick Gradient', but any orthodox helicopter pilot would find the idea of bringing the cyclic back into their stomach, as the speed increases, not to mention, pushing it fully forward as the speed decreases, absolutely bizarre. (This is only AFCS out!)

All in all though, when it's working, it's the dog's bo**ocks.

As a footnote, anybody interested in Chinooks, may care to look at the thread in the 'Military Pilots' section, regarding a Royal Air Force accident on the Mull of Kintyre, Scotland. The 2 (tragically deceased) pilots need all the support they can get.

Actually, when taxiing the "46", its easier to beep the NR down to about 90% and steer. Why, I have no idea.

I was also told that tandem rotors are less efficient than single rotors due to the weight of the two transmissions as well as disturbed airflow over the fuselage and the overlapping rotors - I would like to see a comparison from someone in the know.

As for blade stall, our NATOPS says that you don't know which way the helicopter will roll because you don't know which rotor system will stall first - ahhhh, the joys of the "46" (and probably the "47" although I have never flown a Chinook.

46driver, you're right about the efficiency concerns. While there isn't a tail rotor robbing power, the final word should be performance. The Canadian H46's and Sea Kings use identical engines, have similiar AUW, yet the Sea Kings have far superior single engine performance. The useful loads are comparable, but I think it's slightly better with the Sea King (hard to tell because of the amount of mission kit in each).

I don't have the details but I'm guessing that the H46 outperforms the Seaking in the hover only at extreme weights where the tail rotor losses are highest and when the wind is not favourable for the tail rotor. Also, the H46 cruises faster except at highest weights.

In the end both configurations are practical, the mission will dictate which is best.

Chinook - Ground resonance (video)
 

Click here

It's quite a big file. Best to save first if you're on dial-up.

It was worth waiting on:uhoh:

Any Chinook driver want to make a comment?

I cannot get it to open...but I ask the question...was it for sure ground resonance or was it a blade phasing problem? Having several thousand hours in the Chinook...I would love to see the video...although viewing it might add to my bar bill!

Sasless . . . .

Click HERE, and download the Quicktime player required to view this movie, its the best and most common player around and its worth having on your computer.

I saw something just like that in person a few years back as some soldiers were playing soccer and they kicked the ball vertically and it landed on the turning rotor of an idling allouette, and after that pretty much the same happened, the XMSN was ripped off the fuselage.

That was clearly a test operation, looks unmanned and without full landing gear. I think it is controlled from a remote site I think the aircraft is heavily loaded (shortened oleos) and tied down to simulate shipboard operations. The crew usually starts the cycle with cyclic stick circular motions at the ground resonance frequency (a beep tone in the headset helps the test pilots get the right frequency), I can't see them in this clip, but they could be small, or it could be that they are not needed, as the resonance was self exciting.

Note the engines cut back about 1/3 of the way in, this is the testers probably trying to get control of things, the second big white puffs are the engines actually being destroyed by the excessive vibrations, possibly as the shafts were cut due to excessive motions. It is the cut shafts that mistime the rotors and cause the blade meshing.

Looks like things got out of hand. Anyone recognize the footage/location/warstory? Probably a Boeing Philadelphia film, as they would do this kind of work for the Army (paint job), could be quite old. My guess - a ship tie-down program for the Special Ops folks.

If you have trouble downloading, try right-clicking on the link given, and pull down the "Save Target as" selection, then put the file onto a known file , amybe the desktop. It can be played from there easily. It is 12 Megs, though, so dial-up is daunting.

(edited after seeing the side view film)

There is also a video of this shot from the side.

Because the video part of the site is currently unavailable, in part perhaps due to Draper posting a link in his JB thread, it's exceeded its bandwidth limit for the month !

I seem to recall reading that the airframe involved was one which had barrel rolled in the US somewhere after a hydraulics contamination, and was being used as a test airframe at Aberdeen Privong Grounds, Maryland.

Side View
 

I've uploaded side view here if anyone's interested.