Which helicopter has the most peculiar flight characteristics?
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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....
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....
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.
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.
Last edited by Fareastdriver; 10th Dec 2007 at 01:46.
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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.
Did the Belvedere offer any advantages in longitudinal and yaw stability? It looks longer, and seems to have more rear empenage.
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Hillers: getting back to wierd little helicopters:
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
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
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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.
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.
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.
Last edited by Fareastdriver; 11th Dec 2007 at 09:02.
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/hi.../74-22292.html
http://www.chinook-helicopter.com/hi.../74-22292.html
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Reread of thread..
Just going through this thread again, to extract any more info:
Was this to do with the centreing springs of the SCAS or just poor hydraulics?
Reread Shawn's later post about rear CG and insufficient tail fin area. So what caused the roll divergence?
Flying R22 certainly teaches you the basics of what SCAS should do!
I'd like to understand more about these.
Is this easilly learned, or would it catch the unwary?
Should control systems handle this to reduce pilot workload?
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?
I finally understood SCAS from your book. I'd be interested in any other light you can shed here.
This implies this machine needs some damping in the collective control?
Ah, Shawn Coyle, Sailor Vee and ShyTorque clear this up later...
I enjoyed this post! Why was hover so difficult? How did the trim wheels work (drove the water tanks)?
Hmmm, now i realise why pedal collective co-relation is not such a good idea.
Why is this there? R22 seems to need a gentle up collective readjust every 10 seconds - friction not effective?
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
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!
Originally Posted by Matthew Parsons
Mi-2 with extremely high control forces required leading cyclic with trim, unless you are transformer-strong.
Originally Posted by ShyTorque
Puma HC1's roll/yaw divergence is always good for a laugh.
Originally Posted by 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.
Originally Posted by A.Agincourt
'Gazelle' has the most marked 'secondary effect of control' than any other helicopter I know. Which is the secret of the beast.
Originally Posted by Bertie Thuster
Seaking;single engine failure,max power,max droop,about 40kt ias flat out, just above the water. Lowering the collective to climb away!
Should control systems handle this to reduce pilot workload?
Originally Posted by 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.
Is there any mod to the control system would would help with this manouvre?
Originally Posted by Shawn Coyle
But, please more things we find unusual about these machines. I'll see if I can explain some of them.
Originally Posted by organ donor
Collective bounce on a 205 I think is unique to that particular machine.
Ah, Shawn Coyle, Sailor Vee and ShyTorque clear this up later...
Originally Posted by Fareastdriver
...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....
...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....
Originally Posted by 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.
Originally Posted by DonMateo
The Hughes 500 and the r22 also have the collective assist spring, because of the lack of hydraulics.
Originally Posted by 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.
http://en.wikipedia.org/wiki/Mil_Mi-24
Originally Posted by thekite
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....
Originally Posted by Matthew Parsons
(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.
I forgot what a goldmine this thread turned out to be. Thanks to all who contributed!
Last edited by Graviman; 16th Dec 2007 at 09:45.
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!
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!
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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.
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.
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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???
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???
