Which helicopter has the most peculiar flight characteristics?

[Graviman]

I can only speak from the perspective of piloting an R22, but i like to discuss and understand the handling quirks of more expensive to operate machinery. I'd like this thread to try to capture as many quirks of various machines as possible.

To start things off: for me the biggest quirk of the R22 is the need for the lateral cyclic bungee. This is required to trim the cyclic for inflow roll, but is often not taken back off when you return to the hover. This quirk means that hovering can require more intense concentration on ref points, as the helicopter is continuously suggesting you put in a port roll to cancel the force. R44 pilots, with their exotic hydraulics don't have this.

Any other quirks y'awl would like to share? :)

[tegwin]

Have you considered lateral drift caused by the tail rotor thrust?

On an R-22 /R-44 the rotor mast is fitted at an angle to compensate for the lateral drift...(cheap solution but requires more power)

In more expensive machines I believe the actual swashplate linkages and rotor system are designed to compensate for the drift...(Expensive solution but requires less power)

[brett s]

The Hiller control system - a lot more control lag than anything else I've flown.

Felt pretty strange for the first few minutes until my brain got in sync with it on that first flight, then a non-issue - used to swap between a Hiller 12 & Bell 47 pretty often.

[Graviman]

This is spot on what i am after - thanks guys!

I'm trying to build a picture of the sorts of quirks pilots have to compensate for. This gives me an idea of what control systems need.

Another good one is Sasless comment's about operating the Chinook at Maximum Possible Weight, and having to operate rotor thrust close to where stall would pull down Nr. :uhoh:

The other one i have read about is Shawn Coyle commenting on torque limiters forcing rotors into a less efficient Nr, just when you need them to be efficient.

Brett, how did you compensate for the Hiller system? Did you just lower your input frequency, or were you actually trying to anticipate the machine (like a fast roll).

[brett s]

Nothing conscious - at first I'd make a "normal" cyclic correction, then when nothing happened right away I'd add a bit more & then end up overcontrolling. Typical PIO...and to add to the mix, the Hiller 12 had about half the throttle travel of a Bell 47.

The first few minutes were a lot like learning to hover again - I had about 150 hours TT, mostly in Bell 47's & R22's. But once it clicked, it was just like anything else.

[manfromuncle]

The R22 wants to roll to the right when in forward flight. Pain in the a*rs on trial lessons.

[jeepys]

The R22 eggbeater wants to roll left right back front all the time.

[Matthew Parsons]

Bell 412 with engine governors that are not intended for aggressive flight. When you put in a pitch or roll input, you have to anticipate collective changes to prevent an overtorque.

Any tandem rotor helicopter, the lateral-directional stability (without augmentation) is just not intuitive. Multiple pedal inputs to maintain coordinated flight on roll entry, roll capture, and then maintaining in a turn. Repeat when exiting. H46 with an early SAS still required a lot of pilot input to help with this, but I didn't notice this too much on the British Chinooks.

Mi-2 with extremely high control forces required leading cyclic with trim, unless you are transformer-strong.

OH-6 in an entry to an autorotation required moving my elbows in unusual manners due to the large control inputs and restrictive cockpit space.

Those are some of the highlights I can think of. Remember that what is a peculiarity in one role, may be completely unnoticed in another, or even completely unacceptable.

Matthew.

[EN48]

Some folks have all the fun! With only about 100TT in helos, mostly in an Enstrom 480B (and some in R22/R44/206), all I can say is that the E480 seems entirely devoid of unusual flight characteristics. Probably way too tame for most here, but I am impressed with how well the engineers did in designing out or toning down many of the "gotchas" forund in some other makes/models.

RB

[manfromuncle]

Oh, and another one. Not strictly a 'flight' characteristic, but can catch you out.

In a Schweizer 300, engine started, but clutch not engaged, take your hand off the lever (to put headset on for instance)...and the lever friction is mistakenly off.... the lever shoots up to full pitch and overspeeds the engine.

[rjtjrt]

R22 at 80kt cruise/20inch MP wants to climb and must hold forward cyclic.A mild pain to constantly hold forward cyclic against this tendency and no way to trim it out.
JR

[charliegolf]

Shy,

Puma HC1's roll/yaw divergence is always good for a laugh.


I always thought that was a fault with you, not the Puma :ok:

CG

Did you go to the reunion? I couldn't:(

[A.Agincourt]

'Gazelle' has the most marked 'secondary effect of control' than any other helicopter I know. Which is the secret of the beast.

[Hughesy]

R22's seem to like to try to fly with no or one blade. :O

[Bertie Thruster]

Seaking;single engine failure,max power,max droop,about 40kt ias flat out, just above the water.

Lowering the collective to climb away!

[Semi Rigid]

manfromuncle- that's called a working correlator!

[ShotOver]

I just remember a strange flight characteristic when doing a return to target, and if you didn't reduce power in the verticle climb the helo would actually climb vertical while starting to fly backwards. But that was in a different platform.......alot of power. Then you get used to it. -64

[Graviman]

Wow this is great, guys! Too many to respond to all, but all read and appreciated.

The more i understand about the constraints of helicopter design, the more i am beginning to realise the benefit of various electronic augmentation systems. These vary from sorting out the engine limits when a Cat A machine goes OEI (including 30 sec power), to overcoming stability problems which aerodynamics alone can't solve (like Chinook yaw). In future this will also likely extend to modifying all control inputs to make machine do what is intended rather than demanded (like optimise collective for engine power available and rotor Nr, or feedforwards of cyclic to anticipate manouvre rate).

Howabout for the various roles that a helicopter is asked to do? Operationg below HV curve or long lining? What aspects of flying the machine are hardest to train for?

[Shawn Coyle]

This is really an interesting thread.
Some philosophical points - there is no perfect design, nor a perfect operator.

Pilots end up compensating for the shortcomings of a machine in order to carry out the necessary task. How much you have to compensate depends on the machine.

A great example is trying to hover very close to an object - say trying to put workers onto the top of a power pylon. It is very easy to do in some machines, as the controls and rotor head together make the machine very responsive immediately. Same task in a different helicopter - say with an underslung rotor head - is nearly impossible to do. (Some pilots can do it, but I certainly can't).
Those who can do it are compensating very well. But many can't compensate that well.

Perhaps another thing to look at are - what are the strangest types of flying that you've heard of being done in a helicopter? For me, the two most interesting from a flying qualities point of view are:
a) trying to hold a mirrored box on the front of the the helicopter directly over a vertically pointed laser beam while climbing to a thousand feet AGL so that surveyors miles away can take a shot at said box on the front to extend their survey points. Legend has it that only one in a hundred pilots could it.
b) towing a mine-sweeping sled at a constant groundspeed and over desired track. Turns were led with opposite pedal, if memory serves me right.

In both cases, pilots were working like mad to complete the task.

But, please more things we find unusual about these machines. I'll see if I can explain some of them.

[organ donor]

Collective bounce on a 205 I think is unique to that particular machine.

[Fareastdriver]

It’s got to be the Sycamore, especially the HAR14 version. This was the basic training helicopter for the RAF until 1965 when they changed to the Sioux because they were crashing so many. It didn’t have a squash plate. The rotor was controlled (????) by a rod that went stuck out of the top of the rotor shaft and bent metal guided the blades. Skeeter and Scout similar, I believe.
It had wooden blades, not interchangeable, they came as a set. A replica of a 20mm cannon shell was the mass balance which stuck out of the tip. You couldn’t hover for very long. 23 inches MAP was the 30 min rating for the Leonides engine, 30 inches for 5 and you were at 27-28 all the time.
Control authority was chronic, No hydraulics, dual controls but only one collective so if you were in the left hand seat it was left hand for cyclic and right hand for collective. You had aeroplane type trim wheels to put Q force onto the stick and you had to anticipate it before a maneuver so that you did not have to re-trim it with you hands full. The collective/throttle cam for the engine was way out of sequence so when you lifted of you had to open the throttle and as you got airborne you had to roll it off. The hand throttle was horizontal like a motorbike but it went the wrong way. You rolled the top forward to increase and vice versa. Right hand seat you had you hand on top if the throttle, left hand seat you had your hand underneath it.
There was 62 pints of water meth in two tanks. One underneath the cabin and a smaller one in the pylon. When you started running out of fore or aft cyclic you pumped water from one end of the aircraft to the other.
Despite this it would cruise at 110 knots, trimmed out, hands off, in perfect comfort.
Practice engine offs were something else. You rolled back the engine, dumped the lever, pulled it up 2 notches, (a collective pitch indicator) set the engine to 1500rpm and went for you landing site, always an airfield. When you were certain you were going to reach it you pulled the throttle back and pulled the slow running cut out. (The 1500 rpm was to stabilize the cylinder head temps before shutdown) At about 100 ft you would fully flare and when it stopped you added another 2 notches and descended vertically. When the ground was just about to swallow you you wacked in all your collective and a bit of forward cyclic to get a couple of knots of groundspeed. You had to do this because at zero speed the undercarriage would splay and roll the tyres off the rims.
You then had to restart the engine. Droop stops on the Sycamore were horrendously unreliable and the blades would punch holes in the boom so you had to get the engine cranked up before the rotor RPM went critical. I never did an engine off during my training, the instructor had his hands on the controls all the time leading me.
Shutting down after a flight you had a mirror so you could check the soldiers (droop stops) on shutdown. Should one not go in than you rev the rotor up and down to persuade it. If that didn’t work you called out the fire crew.
One of them would kneel down with a big hose aiming it just over the top of the boom near the pylon. As you shut the engine down the blades would bounce off this jet of water and wouldn’t hit the boom. The blades had then had it. Being wooden and full of water they would warp and have to be replaced.
It was probably the best helicopter trainer ever used. If you could fly it you could fly anything.


Before any Knowall jumps on me saying they have seen a Sycamore with twin collectives etc there were versions produced with this but the HAR 14 was the standard RAF Sycamore.

[Cim Jartner]

The Puma HC1 has a nasty habit of rolling over to the left when you land with the throttles back at ground idle!!! :}:ugh::{:O:eek::E

[Shawn Coyle]

Collective bounce?
The S-61 had and the UH-60 has collectives very low in friction. Whole body resonance mode if you do the wrong thing.

[Sailor Vee]

Collective bounce?
The S-61 had The 61 collective bias used to be easy to correct, BUT, someone filled up the nose bay with avionics and now it's one helluva job getting to the area to do so, if the bias is set correctly, the collective is a dream, you just have to get used to the 'Sikorsky Shuffle' if you're slow in getting the SSL(s) forward on engagement!!

[JimEli]

Ever land an Astar without the spring steel extensions touching?

>
Perhaps another thing to look at are - what are the strangest types of flying that you've heard of being done in a helicopter? For me, the two most interesting from a flying qualities point of view are:
<

Flying an ADS-33 course in an EC-120.

[Fareastdriver]

Pumas rolling over after an EOL was due to the mechanical collective/yaw interconnect especially when close to zero speed. After the flare and whilst cushioning the collective would inpart a strong anti torque (Right pedal for Americans benefit) to the tail rotor that would be counterbalanced by the autopilot heading hold. When the alternators dropped off so did the autopilot so it yawed to the right.

The only way to counteract this was to banging on full left pedal this instant the alternators tripped. If you thought about it you were too late, if it had started yawing to the right you would never get it back but if you put too much on that was correctable.

I was shown how to and would demonstrate this in the mid 70s with two tanks in the back to ensure a proper needle split equivalent. Was not allowed to land it but without anticipators it would do the whole thing to a 3 foot sloppy hover with about 200 Rrpm.


THEN THE ENGINES WOULD WAKE UP.


On the yaw/roll divergence it was never noticed on pre polivalent aircraft. When we collected XW 209, the first one, we received concerned comments from Boscombe who were worried about it's aerodynamic stability with the increased area above the cockpit. We had other problems at the time so we didn't investigate it. The 332L has a forward plug in the fuselage and compared with an early 330 is is a pig to fly. The 332L has an extra fin forward of the skid to correct for the plug, maybe a Puma could do with the same.

[Shawn Coyle]

Although I only got 1.5 hrs on the Puma, and never did experience the roll/yaw divergence, one of my mates did on his ETPS preview, with an experienced Puma QHI. It happened when they rolled into a turn and added some pedal to keep the ball in the middle - surprised them all.
Evidently it only happens with a rear CG- lots of surface area ahead of / above the CG, and the pedal input added the necessary sideslip to cause the machine to roll pretty smartly.
That's why the later versions have the added area at the fin at the back.

[PlasticCabDriver]

The result was usually: pause two three WOO!! - the old girl would rapidly roll on a whole $#1t load more bank, all by herself. The student seldom asked for the demo to be repeated

She still does! And the students still don't!:ooh:

[busdriver02]

The more I read this forum, the more I think I'm incredibly spoiled flying the Blackhawk.

[ShyTorque]

You are! :ok:

[busdriver02]

Of all the things you guys talk about that can bite you in the ass with your various helos, I can think of more than one occasion in my limited 700 hours in the hawk that I've done the same thing or likewise ham-fisted and stupid, and she takes it in stride. Even flying around at max gross almost all the time, the 60 seems to have no bad habits at all. I'm inclined to think NickLappos really does know what he's talking about.....

[Shawn Coyle]

The Blackhawk is proof positive that pilots care more about performance and power than handling qualities.
I don't know what's been changed in the newer versions, but the A model definitely had some unusual features -
Like a heading hold that required you to take your feet off the pedals.
A beeper trim system for the stick that was of no use (and no-one ever used it to change anything when the trim release worked much better).
Running out of collective in forward flight
Droop stops that couldn't stand a tailwind start up.
A tailwheel lock that could be nearly impossible to get in.

But performance wise - very nice. And having said some bad things about it, if I had to go to war in a utility helicopter, I'd want it to be a Blackhawk.

[JimEli]

Why stop there?

1. Uncomfortable nose high hover attitude.
2. Limited forward/down cockpit visibility (which makes most new pilots fly approaches out of trim).
3. A stabilator that for years, scared the bejesus out of many pilots.
4. A rotor system that refuses to smooth (I swear somebody walked out of the plant with the master blade).
5. Transient rotor droop like a mother (A model).
6. It leaks like a sieve and the heater works intermittently at best.

But you can’t have mine.

[TwinHueyMan]

100% agree on all the Blackhawk topics, except:

"A beeper trim system for the stick that was of no use (and no-one ever used it to change anything when the trim release worked much better)."

Our guys use that to good effect on instrument holding! Works far better than fighting the trim or releasing it alltogether. The new guys don't use the bleeper, but the old guys do, so it just adds more distance between the good old guys and the youngin's!

Mike

[Graviman]

I'm learning a lot from all these posts! :ok:

The Sycamore makes me appreciate how much development has gone into the R22, which seems to have no suprises (outside of what you would expect for such a nice simple machine). Has anyone ever considered the R22 as a trainer? :uhoh:

What causes the Puma to have it's WOO moment? Is this the beginnings of dutch roll, or is it just the amount of cyclic needed to hold it in the heading hold side slip?

I'm actually amazed the S-70 is not as vice proof as i thought. I'd like to hear more about how that stabilator, and the beeper trim, catches youngin's (and us wannabe's) out.

I'm sure i'll have many more questions - i'm just enjoying all the input..

[Boslandew]

I flew about four hours on the Westland 30 back in the 80's- BAH had two or three of them. I was told, although I never had it confirmed, that as a throwback to the naval Lynx, the W30 had a collective input on cyclic so that when you pulled full power for a takeoff from the back of the frigate it made a rear cyclic input to ensure the aircraft didn't hit the frigate hangar. This input was evident throughout the speed range so that trying to get it to fly at the cruise speed for the day, typically 115 knots, meant endless trimming.
The engines had individual speed levers in the ceiling and, instead of a collective mounted beeper trim, a torque balancing control between them. Pulling to the hover almost always produced a large torque split which required taking your left hand from the collective, twisting the balance lever toward, as I remember it, the high engine, before grabbing the collective again. The SAS was a bag of nails involving 'half-lane switches' for each control axis which had to be activated in the event of a control malfunction. The height hold could not be used below 400 ft. Range with full pax was about 32nm.
On the plus side it had an enormous power reserve, an excellent gear box and a cabin section second, in those days, to none. The half dozen or so regular pilots swore by it and it performed very well operating the Penzance- Scillies service for a while.
Finally, I can remember a colleague converting unwillingly from the S61 to the first Puma's the company bought. "B-----y" thing, he said. "One day you pull collective and the nose goes up, the next day, you pull and it goes down!!"

[Boslandew]

The commercial Chinook Flight Manual contained some very ambitious claims for operating speeds. At MAUW, rarely WAT limited, the book might say 130-135 knots. The aircraft vibrated a lot and each pilot seat and the entire 44 pax cabin were mounted on vibration absorbers, the left-hand seat being the worst for vibration.
With each flight from Aberdeen scheduled for 5-6 hours, on occasion 8.50hrs, one didn't wish to tarry so sought the maximum practical cruise. This meant accelerating once level using the 'cocked hat' until the co-pilot began to look blurred then reducing speed by a few knots until he came back into focus and stopped complaining produced the cruise speed, typically 115 knots.

[Redplexus]

Laughed out loud at the Chinny cruise speed setting method. Of course the Hughes 300 is vastly superior to any of this mil tin-pannery - it cruises at a speed only fractionally lower than that at which it falls gracelessly out of the sky. Also is less accepting of rear-quartering breezes in the hover than Heather Mills of a divorce settlement. The thing will routinely throw the unwary into a right pirhouette with only a 3-5kt gust on take-off.

[brett s]

While Chinooks definitely vibrate, I find it odd that you were having as much as you decribed at those rather low cruise speeds - we used to cruise at 120-130 kts regularly in CH-47C & D models, and faster if in a hurry & light.

Didn't start getting annoying until 140+ kts in most with the current composite blades if a decent track & balance was done, the C models when they still had metal blades were even smoother.

Wonder what the differences were between the military & civilian models were that could account for that, or was it just poor track & balance?

[DonMateo]

The Hughes 500 and the r22 also have the collective assist spring, because of the lack of hydraulics. Not a good idea to get out with the aircraft running in any of these aircraft.

[DonMateo]

The BO105 is a rather unique aircraft design. It doesn't like to fly in trim, much more stable 1/4 to 1/2 ball out. Lots of vibrations coming down through translational. You need to always keep in the back of your mind that if it becomes necessary to enter autorotation, from cruise flight, you MUST first pull aft cyclic before you lower collective. If collective is lowered first, the aircraft will point its nose straight down at the ground and it will be impossible to pull up the nose without pulling collective. One other thing, you must be very careful of a descending right turn, the right wing can drop and you quickly run out of left cyclic. Full left pedal is required to get out of the spiraling dive.

[ShyTorque]

Yes, I've seen that Red Bull Bo-105 pilot on Youtube trying to fly straight and level, he just can't manage it.. ;)

[oldbeefer]

ShyTorque said:
'That was always a good demo to give to a nervy student pilot, but still not as good as a well-flown and sporty Puma yaw/roll divergence demo'.

Yes, Shy, but if the CFS QHI demonstrating Jackstall in a 60 degree descending turn chooses to do it to the RIGHT, the result is interesting to say the least - I didn't realise a Gazale could go through the inverted position, but it did! Needless to say,I demanded an instructor change.

[ShyTorque]

And a change of underpants no doubt.... :p

I got the ex Puma QHI Whirlwind retreating blade stall demo at 100 ft agl running in to Chetwynd, thankfully the aircraft rolled upright or I wouldn't be here to recall it.

The climb to 10,000 ft wearing the parachute, to see the official version just didn't seem as exciting somehow. :bored: :p

[Boslandew]

Brett S
My memory may be playing me false after 20 plus years but only by 5 knots or so. Every takeoff from Aberdeen was within a whisker of 48,500 lbs MAUW and it was only after 4-5 hrs flight with 10-12,000 lbs fuel burnt that 135-140 knots was comfortable. 140 Knots I think was VNO. Bear in mind that the pax were civvies who weren't wild about the Chinook in the first place (because of vibration mainly) or about 2-3 hr/275 nm legs. Also, each aircraft usually flew 10-11 hrs a day weekdays, two round trips to the Brent field, with additional Saturday flights, so track and balance might not have been optimum.
I don't know if the military versions had the vibration absorbers fitted - I know that BV made quite a few mods to the 234 to get the FAA/CAA certification. I'm not sure that they were entirely successful - I can remember that flight in the co-pilots seat could be uncomfortable for the first hour or more. BV produced a vibration graph which rose smoothly from 30,000-40,000 lbs, increased more steeply to 45,000lbs and became very steep indeed above that. I can also remember that the vibration induced when NR were reduced from T/O, 102% to cruise, 100% was very noticeable and persisted for up to 2 minutes.
Incidentally a friend of mine ditched a Chinook with major hydraulic contamination causing Nr variation between 96% - 104%. He said the vibration caused during the descent was so extreme that he was convinced that the instrument panel was about to tear loose.

[brett s]

The US military Chinooks used the cockpit vibration dampers as well - hated changing them, rather heavy & in awkward positions to install or remove - under each pilot seat & one in the nose.

It's probably just the weight you were operating at - C models were limited to 46,000 lbs while D's were 50,000 at the time I was involved (in the 1980's, it's been a while for me too!), but most of the time when really heavy it was with external loads that limited our cruise speeds.

[busdriver02]

I guess I just got used to the 60's quirks since the only other helo I've flown was a brief 100 hours in the Huey. I also use the trim beeper for instruments, but almost hold down the trim release when flying low level. The heading hold seems to wander a bit in a hover, but I don't know if that's due to flying at 20-22k all the time or the Atari brain. Transient droop is no problem in the Lima/Golf though.

[Boslandew]

Brett S
Just out of interest can you remember what the VNE/VNO was on the military Chinooks? I seem to remember that they were both the same for the 234 - 140 knots. I certainly never flew one above that.
Regards
Boslandew

[brett s]

170 kts Vne for the CH-47C & D, and I've seen 160+ :)

[Boslandew]

Sapristi!! Had no idea they'd go that fast. Would have knocked a bit off of a Brent trip.

[Graviman]

Just a silly question: What sets the limit for Vne in a CH-47 C & D? I assume it is not fatigue life in the airframe. Does the machine eventually enter an uncommanded yaw, as both rotors reach retreating blade stall? I can't imagine longitudinal authority would be that affected.

Another question: Why is rear rotor more heavilly loaded than front? Is this to limit the possibility of VRS in the rear rotor, so there is always authority to lower the nose?

Thanks in advance, guys - i'm enjoying reading these...

[500e]

My understanding of the vibes was all left seat crew had special fillings in teeth to prevent them falling out @ 140.
Strange they went blurred would have thought you vibed in unison

[Matthew Parsons]

Graviman, the aft head is attached to a very long mast, and at some point you get concerned with how much of a bending moment you generate on that mast. That was normally the speed restriction on the H46.

As far as loading the aft head, as the fuselage pitches further down, you get fuselage drag due to forward flight that creates a pitch up moment, so you need to either tilt the heads to compensate or increase the lift on the aft head (or a bit of both). Because the aft blades fly over the cabin, there is a limit to how far forward the aft head can tilt.

[before landing check list]

On the MI24 in a steep turn the inside wing had a habit of stalling thus inducing a roll toward it. It was correctable with pedal only at that point.

[NickLappos]

THE most peculiar helo I ever flew was the CH-54 Skycrane, which was optimized for a perfect hover, so the main rotor shaft was tilted to the left by 3 degrees, and the shaft was perfectly vertical longitudinally. This meant that it hovered perfectly level (no left wheel down) and nose level. This also ment that when it was in "high speed cruise" at 125 knots, the nose was 15 degrees nose down, and the roll was about 3 to 5 degrees to the right. Without a shoulder harness lock, the pilot was in danger of falling in the cockpit and pressing his face firmly into the windshield!

An hour of cruise in the Crane was worth two hours leaning back and walking in circles afterward while rubbing one's sore shoulders! "Pack Marks" on the skin were the signs of a dedicated Crane Driver!

[denlopviper]

how come i havnt seen anything on the CH-53 or the B-222?:}

[ProfessionalStudent]

The CH47 really doesn't like being at 50000lbs (22.7T), but burning off just 7-800lbs makes a massive difference to handling. It'll still bite you in the arse if you're a bit club-fisted, but it doesn't feel like flying a mattress any more...

[Graviman]

There has been some great feedback on this thread! :ok:

Matthew CH-47 C&D being Vne limited by rotor shaft bending moment is interesting. To my mind this layout has a great deal of development potential, by designing stiffer/stronger rotor systems. If some payload is sacrificed Vne could be pushed up as an interesting challenge to V-22.

Nick, CH-54 really does highlight the compromise made between cruise and hover. I imagine that designing in flexibility in the main shaft would have increased mass, and reduced control effectiveness. Judging from success of intermeshing Kmax, in a simailar role, i will be interested to see whether X2 derived designs impact on this role. It is interesting to note that the latest MI-26 is FBW.

Later thought: I'm actually puzzled why CH-54 did not use an inverted aerofoil horizontal stabiliser to improve cruise trim, unless that would have overstressed main rotor hub. This would have also helped the lateral roll to starboard in cruise by keeping TR at similar height to MR.

[Matthew Parsons]

CH-47 C&D being Vne limited by rotor shaft bending moment is interesting. To my mind this layout has a great deal of development potential, by designing stiffer/stronger rotor systems.

I agree completely. Check out the Boeing 360.

Matthew.

[Graviman]

http://www.aviastar.org/helicopters_eng/boeing-360.php

http://www.aviastar.org/foto/boeing-360.jpg
First flown in 1987 the Model 360 is a privately developed advanced technology rotorcraft, designed to research the company's other rotorcraft programs. The helicopter features advanced aerodynamics and extensive use of composite materials including the fuselage, rotor shafts, blades and hubs. Powered by twin Avco-Lycoming AL5512 engines (4200shp) the Model 360 has a 370km/h cruise speed. The aircraft's advanced cockpit features cathode ray tube displays, multi-function callouts, digital automatic flight control system and other improvements to reduce pilot workload.

[Shawn Coyle]

One of the interesting things about the CH-46/47 is the Longitudinal Cyclic Trim (LCT) system. This tilts the rotor heads as a function of airspeed and density altitude, and keeps the fuselage more or less level. While it's included in the basic AFCS box, it's considered as a separate function.
There are indicators in the cockpit to show the position of the actuators, and one of the post-take off checks is that passing 60 KIAS, you have to see that the LCT's are programming. If they're not, better slow down before you reach the limit of 100 KIAS with the aft head 'retracted'.
The reason for this is the flapback of the rotor blades on the aft head induce quite a bit of stress on the upper bearing for the rear rotor mast (which is quite long from the transmission to the rotor head).
The standby check for the aft LCT not extending is that at 100 KIAS, the vibration gets markedly reduced as the rear rotor disk is not above the front one and not getting all the nasty vortices it would normally get.
When British Airways got their CH47s in the early 1980's and saw the change in vibration levels with the aft LCT retracted, they asked for this to be a normal feature as it reduced the vibration level and their customers would like it. Not sure if that ever happened, but it shows the complexity of the problem.
If the upper bearing were made larger / stronger, more capable of taking the loads, it may help with the vibration, but probably would have put the problem somewhere else....

[Fareastdriver]

Anybody remember the Belvedere. I didn't fly it but used to operate from the same bases as I did. One engine at each end with a synco-shaft to keep the blades apart and allow it to fly on one engine.

It had Avpin starting so they kept the ladders at the cockpit doors whilst starting the engines so that the crew could leap out in a hurry.

Should the synco-shaft go apparantly it could continue flying because the rear rotor was always above the front. It was bit noisy on shutdown, though.

They were eventually scrapped at Seletar about 1970 when 66 Sqn folded. When they pulled out the war reserve from the MU to join them they discovered it had the same serial number as one of the squadron's aircraft.

One had an engine fire at the front and another at the back. They joined the good halves together and it flew again.

The gearboxes were made by the Swindon railway works.

Maybe they were all mythes.

[before landing check list]

http://en.wikipedia.org/wiki/Bristol_Belvedere

No myth.

[Graviman]

Interesting to understand why CH-47 produces so much vibration, Shawn. I guess future developments of the tandem layout will try to seperate the rotors more, vertically if not horizontally. Have there been any incidents involving the synch shaft failing? I imagine this would be the achilles heel, compared to Fareastdriver's Belvedere...

Did the Belvedere offer any advantages in longitudinal and yaw stability? It looks longer, and seems to have more rear empenage.

[thekite]

I had the experience of the heli snapping nose down into a vertical dive when I was at no great altitude to start with.

Flying with geological teams in the South Austalian deserts, I will never forget the geopick (geologist's rock pick) rising off the lap of the Geo's assistant, turning over and over and heading towards me - in those days i dared not wear a helmet - before changing direction and slashing into the windscreen....

Instant engine and rotor overspeed. Being both LAME and pilot I had sufficient kit with me to carry out an ad hoc overspeed on it all after the immediate and sweaty landing, while unseen Toyotas grumbled about trying to find us.

Pitts Special piliots, eat your hearts out! In more recent years I owned a Yak 52, but I could never have emulated that horror bunt, nor wanted to.

Ericferret, of this network gave me the answer, thirty years on!

Turned out that brinelled pitch bearing housings can catch a slight collective decrease until a more definite movement causes a huge pitch dump. And there is your vertical dive.

thekite

[Matthew Parsons]

Graviman, sync shafts have failed. There is little the pilot can do at that point as the aft head is still powered and the forward head is not. A pitch down will occur likely before the pilot realizes the speed difference. The pilot would move the cyclic aft to correct which increases collective pitch on the front head, leading to a greater rate of decay.

Collective down, level fuselage, and engines off gives you the best chance of a survivable landing.

Achilles heel? I wouldn't jump that far. Hard to say without the stats, but I'd guess there are other dangers that would catch you out first.

[Fareastdriver]

The front gearbox failure on the British airways BV 234 some twenty years ago was effectively the same as a transfer shaft failure. Only by some miracle did two out of forty six survive. The captain, one of the survivors, had no idea of what went wrong, it just fell apart around him.

[brett s]

Rotors losing sync on a CH-47 leads to near instantaneous disintegration as the rotor systems collide - there have been a few over the years, mostly on the ground luckily. Below is a link to one such accident in 1982 with 46 fatalities:

http://www.chinook-helicopter.com/history/aircraft/C_Models/74-22292/74-22292.html

[Graviman]

Just going through this thread again, to extract any more info:


Mi-2 with extremely high control forces required leading cyclic with trim, unless you are transformer-strong.


Was this to do with the centreing springs of the SCAS or just poor hydraulics?


Puma HC1's roll/yaw divergence is always good for a laugh.


Reread Shawn's later post about rear CG and insufficient tail fin area. So what caused the roll divergence?


R22 at 80kt cruise/20inch MP wants to climb and must hold forward cyclic.A mild pain to constantly hold forward cyclic against this tendency and no way to trim it out.


Flying R22 certainly teaches you the basics of what SCAS should do!


'Gazelle' has the most marked 'secondary effect of control' than any other helicopter I know. Which is the secret of the beast.


I'd like to understand more about these.


Seaking;single engine failure,max power,max droop,about 40kt ias flat out, just above the water. Lowering the collective to climb away!


Is this easilly learned, or would it catch the unwary?
Should control systems handle this to reduce pilot workload?


I just remember a strange flight characteristic when doing a return to target, and if you didn't reduce power in the verticle climb the helo would actually climb vertical while starting to fly backwards.


This is similar to a fixed wing chandelle? A quickstop with a coordinated turn at the top?
Is there any mod to the control system would would help with this manouvre?


But, please more things we find unusual about these machines. I'll see if I can explain some of them.


I finally understood SCAS from your book. I'd be interested in any other light you can shed here.


Collective bounce on a 205 I think is unique to that particular machine.


This implies this machine needs some damping in the collective control?
Ah, Shawn Coyle, Sailor Vee and ShyTorque clear this up later...


...Sycamore...HAR14 version...
...You couldn’t hover for very long...
...No hydraulics...
...aeroplane type trim wheels to put Q force onto the stick and you had to anticipate it before a maneuver so that you did not have to re-trim it with you hands full....


I enjoyed this post! Why was hover so difficult? How did the trim wheels work (drove the water tanks)?


Pumas rolling over after an EOL was due to the mechanical collective/yaw interconnect especially when close to zero speed. After the flare and whilst cushioning the collective would inpart a strong anti torque (Right pedal for Americans benefit) to the tail rotor that would be counterbalanced by the autopilot heading hold. When the alternators dropped off so did the autopilot so it yawed to the right.


Hmmm, now i realise why pedal collective co-relation is not such a good idea.


The Hughes 500 and the r22 also have the collective assist spring, because of the lack of hydraulics.


Why is this there? R22 seems to need a gentle up collective readjust every 10 seconds - friction not effective?


On the MI24 in a steep turn the inside wing had a habit of stalling thus inducing a roll toward it. It was correctable with pedal only at that point.


I'm suprised it topped out at 180KIAS, if it is a compound heli. Or are these blunt wings, like that "winged" version of the Black Hawk?
http://en.wikipedia.org/wiki/Mil_Mi-24


Flying with geological teams in the South Austalian deserts, I will never forget the geopick (geologist's rock pick) rising off the lap of the Geo's assistant, turning over and over and heading towards me - in those days i dared not wear a helmet - before changing direction and slashing into the windscreen....


:uhoh:


(Chinook)...sync shafts have failed. There is little the pilot can do at that point as the aft head is still powered and the forward head is not. A pitch down will occur likely before the pilot realizes the speed difference. The pilot would move the cyclic aft to correct which increases collective pitch on the front head, leading to a greater rate of decay.


Should an automatic control system be designed to provide automatic responses for failures of this type? I'm thinking tail rotors, engines, transmissions here too. There were some interesting thoughts along these lines for the FBW Comanche.


I forgot what a goldmine this thread turned out to be. Thanks to all who contributed! :ok:

[Evalu8ter]

I used to use the Mi24/35s "interesting" flight characteristics as part of the tactics course when discussing HvH. I believe the US army found out about this in the Cold War when AH-1 drivers would meet their Mi24 opponents at the IGB. Pilots being pilots, these "meetings" often ended up in showing off. I believe that a "wing-over" competition ended up with at least one Hind crashing trying to out-do the Cobra.

Chinook stab-out is always fun, with the cyclic seemingly operating in reverse in pitch and the back end always trying to overtake the front. Oh, and the excessive rearward speed tuck-under was always a charmer!

[Graviman]

Thanks Evalu8ter. HvH must be something else...

By "Chinook stab-out...cyclic seemingly operating in reverse in pitch", you mean negative static stability? IE faster stable flight requires final cyclic position to be in a more rearward position, but initial pitch input is still in the correct sense. I can understand how that would feel strange/marginal to fly.

This is to do with the fuselage drag Matthew Parsons mentions? I'm confused since i though this was pitch-up drag.

[Shawn Coyle]

Stab out in the Chinook did in fact have negative static stability. The fix was a thing called the DASH (Differential AirSpeed Hold). This put a large actuator betwixt pilot and rotors which extended with airspeed (not the same as the LCT - Longitudinal Cyclic Trim).
There was one Canadian crew that discovered the reason for the warning in the flight manual about AFCS off flight 'Turn it back on at the same airspeed it was turned off at, or wait till you're on the ground with the rotors stopped to re-engage.'
What happened was that they turned off the AFCS at relatively high speed, with the DASH extended. They then returned to the hover, and switched the AFCS back on. The DASH then went to the retracted position as it tries to match the airspeed then existing, which of course means the nose is going to drop pretty dramatically without the crew moving any flight controls....
Read and respect what's in the manuals!
I wonder how many other CH-47 crews this has happened to???

[Graviman]

I think Chinook is definately winning the "Most peculiar flight characteristics" award! :cool: