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Chinook low flyby vid doing the rounds on Facebook......

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Chinook low flyby vid doing the rounds on Facebook......

Old 1st May 2022, 18:13
  #41 (permalink)  
 
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Originally Posted by 4468 View Post
That ‘fly past’ was flown at considerably higher speed than the slowed down video suggests. As we all know, rotor downwash trails the aircraft. The camera movement does not allow sufficient dwell time on likely areas to determine whether any downwash was present.
Yeah, except that for a considerable portion of the video the area behind the aircraft is visible. Also, how much does downwash "trail" at 15ft exactly? Also the long video shows the aircraft departing into the middle distance which is clearly long enough.

​​​​​​Not a slam dunk, but suspect.
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Old 1st May 2022, 20:34
  #42 (permalink)  
 
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Originally Posted by Ripton View Post
One for the Which Aerodrome thread in Aviation History and Nostalgia?
Most threads on Mil aviation should actually be on AH&N.
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Old 1st May 2022, 22:05
  #43 (permalink)  
 
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Originally Posted by DaveReidUK View Post
Actually, it's all about frame rate.

Even a minute variation in rotor RPM between the front and back rotors could produce the effect where one or other appeared to be moving in the opposite sense to actual.

Watch any film or video of a helicopter in flight and it's very difficult to discern the direction of rotation from successive frames.
None of the weird backwards blade effects apply to high speed footage with a frame rate far in excess of rotor RPM. The footage looks to be taken at maybe 120-240 fps (default options on most cell phones, gopro etc), the rotor RPM of a Ch47 is 225 or something, or just under 4 per second. In other words, even at the lower end estimate (120fps), the camera records 30 individual frames of the blades rotating, which means that there can absolutely not be any effect caused by the frame rate that would distort the visible rotation or make it seem opposite. A frame is taken every 12 degrees angle of rotation, or every 6 degrees at 240fps, while the blades are obviously spaced roughly 120 degrees from each other.

In fact one could easily determine the frame rate this was shot at based on the expected rotor RPM, by measuring said angle.
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Old 1st May 2022, 22:40
  #44 (permalink)  
 
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The video that we are looking at (from aviationsorcenews) is only 30fps.

If it was originally faster, then 3oo4 or 7oo8 frames have been discarded.

IB

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Old 1st May 2022, 22:53
  #45 (permalink)  
 
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Even a minute variation in rotor RPM between the front and back rotors could produce the effect where one or other appeared to be moving in the opposite sense to actual.
Perhaps the worst nightmare of every Chinook Pilot is for the two Rotor Systems to have "any" variation of RPM between the two Heads.

If that happens....Chinooks become the most efficient sausage making machine known to Mankind.
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Old 2nd May 2022, 02:45
  #46 (permalink)  
 
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I'm not persuaded that the published clip is genuine; more likely a very clever and imaginative bit of editing - and well done at that. The flypast could almost certainly be valid. What's happening on the ground is probably valid. However for it all to be one authentic clip - shot in real time does not pass scrutiny for many of the reasons already stated above. There is also a surreal aura to the entire sequence; the lighting, the speed (obviously slowed down - but why?). It just doesn't ring true.

There is one further question mark. As far as I am aware, this is the only published clip of the event. If so, that seems very strange given that almost everyone on the ground would have been filming, and we should be seeing multiple contributions from other sources at other locations in the occurance. They may yet appear, and if they do - then I'll moderate my reservations, but until that happens, well, I will hold by my stated reaction so far. Sorry - nice bit of work but ....
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Old 2nd May 2022, 07:51
  #47 (permalink)  
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apart from one set of the blades being put on back to front... forget about the visuals of the direction of rotation, the position of the root fitting and the blade gives the direction of rotation.

The CH 47 is a longitudinal intermeshed copter layout with overlapping rotor disks. The rotors synch like two gears over the center of the fuselage as it is cheaper than paying for a new helicopter every time you add a cyclic input. The front disk advances on the RHS of the helo, the rear disk advances on the LH side, so both sets of rotors crosses the fuselage from the left to the right side.

What side of the blades are seen? The Ch47 has the front disc set at a 9-degree forward cant angle, and the rear disc is set at 4 degrees. longitudinal control comes from altering the collectives at each of the rotors, not altering the disk TPP. So the highest point on the rear rotor disk is always at the rear of the helicopter., same for the front disk. variation of the disk from the offset comes from flap back, which is speed dependent. Roll comes from lateral cyclic input to the disks, and yaw comes from mixing the lateral cyclic between the rotors. That all means that the rear disc's highest point is always to the rear of the helo, so in the 2nd and 3rd image below the rotor blade root fitting is showing a rear rotor that is advancing on the right side of the helicopter, and that is wrong. The frame rate does have the direction of rotation reversing but it also changes partway through the video or appears to, which it shouldn't, where the Nr and the frame rate haven't altered. Sticking with it being false.


A REAL HELICOPTER



NOT REAL HELICOPTER







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Old 2nd May 2022, 08:26
  #48 (permalink)  
 
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I agree with FullOppositeRudder, I think it's produced from a flyby of something - much higher - and a flyby of this Chinook, real or not, in other circumstances. Amongst other things the shadows are wrong.

Someone is sat with their legs out of the back, by the way.
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Old 2nd May 2022, 17:39
  #49 (permalink)  
 
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The Ch47 has the front disc set at a 9-degree forward cant angle, and the rear disc is set at 4 degrees. longitudinal control comes from altering the collectives at each of the rotors, not altering the disk TPP. So the highest point on the rear rotor disk is always at the rear of the helicopter., same for the front disk. variation of the disk from the offset comes from flap back, which is speed dependent.
”variation of the disk from the offset comes from flap back, which is speed dependent”?

So why does the chinnie have a level fuselage attitude throughout the speed range then, and how does ‘positive stick gradient’ work? (Both disks will tend to flap back at different rates and create differing translational lifts won’t they?)

I may look once again at the video, however even if it is a ‘deep fake’ I didn’t see anything wrong with either the orientation or rotation of the blades.
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Old 2nd May 2022, 18:07
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From memory (RAF Chinook engineer late 90s) a pair of airspeed-scheduled servos (DASH?) pitch the 2 discs forward to counter flap-back as speed increases, so the fuselage remains horizontal. Pilot has no direct control; its not part of the primary flying controls.

Perhaps someone with more recent experience (and more grey cells) can elaborate.
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Old 2nd May 2022, 18:14
  #51 (permalink)  
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Originally Posted by 4468 View Post
”variation of the disk from the offset comes from flap back, which is speed dependent”?

So why does the chinnie have a level fuselage attitude throughout the speed range then, and how does ‘positive stick gradient’ work? (Both disks will tend to flap back at different rates and create differing translational lifts won’t they?)

I may look once again at the video, however even if it is a ‘deep fake’ I didn’t see anything wrong with either the orientation or rotation of the blades.

Good point!

"Differential Collective Pitch Longitudinal control is achieved by differential collective pitch (DCP); moving the cyclic stick forward decreases the pitch of the forward rotor and increases that of the aft rotor and vice versa. A differential airspeed hold (DASH) system ensures that a positive stick gradient is maintained throughout the speed range. Longitudinal cyclic trim is incorporated to enable the aircraft to be flown throughout the speed range in a substantially level attitude, thereby reducing drag and stress on the rotor shafts".


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Old 2nd May 2022, 18:40
  #52 (permalink)  
 
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I shall date myself by saying my last experience on the Chinook was with the mature C Model having cut my baby teeth on the earliest A Models with the fixed landing gear (one of them I learned to fly Chinooks in is now in the Fort Rucker Museum).

In my time the Speed Trim system (being called DASH) by some here.....consisted of electrical servos that would controlled by air pressure created through the Pitot Static system with the changes in airspeed.

The system was switched ON by Checklist....and once turned own did its own thing....with Cockpit Instrument Panel Indicators (one for each Rotor Head) showing the position of the Servos. In the event of a failure of the automatic system there was a Manual Option that required constant input by the Crew and also provide a means to manually retract the Speed Trim Servos before Landing.

If the Crew put the Trim into Manual and forgot to manually retract the Servos....an extraordinary stress was applied to the Aft Vertical Shaft on the Aft Transmission and also caused an unusual landing attitude.

The purpose of that system was to level the fuselage during cruise flight and eliminate drag.

Lots have been said about the "Positive Stick Gradient" requirement imposed by the Army and FAA (the S-76 was labored with similar problems) but in real life use....most Pilots cannot identify when that system is working or not unless they see a Instrument Indication to that effect.

Monkey Memory accrues from comparing a cyclic stick position to Pitch Attitude/Air Speed and then moving the Cyclic to achieve the result desired....then repeated throughout the flight.

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Old 3rd May 2022, 04:59
  #53 (permalink)  
 
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Originally Posted by Ivor_Bigunn View Post
The video that we are looking at (from aviationsorcenews) is only 30fps.

If it was originally faster, then 3oo4 or 7oo8 frames have been discarded.

IB
It's recorded at a high frame rate and then played back at 30fps to create the slow motion. For example, playing a 120fps recording at 30fps creates 0.25x playback speed which is in the ballpark of what we are looking at. If it was recorded at only 30fps and then played back at reduced speed for slow motion, it would look like a slide show at 8fps
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Old 3rd May 2022, 17:06
  #54 (permalink)  
 
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SASless

Lots have been said about the "Positive Stick Gradient" requirement imposed by the Army and FAA (the S-76 was labored with similar problems) but in real life use....most Pilots cannot identify when that system is working or not unless they see a Instrument Indication to that effect.
I’m afraid I don’t understand this?

Nobody can be in any doubt whatsoever whether they have a positive (or negative) stick gradient. Absolutely no “Instrument Indication” required. (Or available?)

Unless we are talking about two very different things?

Here:

In my time the Speed Trim system (being called DASH) by some here.....consisted of electrical servos that would controlled by air pressure created through the Pitot Static system with the changes in airspeed.

The system was switched ON by Checklist....and once turned own did its own thing....with Cockpit Instrument Panel Indicators (one for each Rotor Head) showing the position of the Servos. In the event of a failure of the automatic system there was a Manual Option that required constant input by the Crew and also provide a means to manually retract the Speed Trim Servos before Landing.
You are clearly conflating DASH with the LCTs. Two very different systems.

You also spoke about “A Models with the fixed landing gear”. I never flew the A model, but I was unaware there was any significant difference in the gear between the various models?

Last edited by 4468; 3rd May 2022 at 17:21.
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Old 3rd May 2022, 17:32
  #55 (permalink)  
 
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https://en.wikipedia.org/wiki/Boeing..._evaluated.jpg

Note the double wheeled gear on the aft end.

Later versions of the A Model and subesqeunt went to a single wheeled version with each aft gear being able to castor independently with one strut having a power steering actuator controlled by the Pilot.

What experience do you have on the Chinook?

The difference between the various Models, Marks, Operators and in between would create a number of differences in our frames of reference.
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Old 3rd May 2022, 18:19
  #56 (permalink)  
 
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What experience do you have on the Chinook?
A fair amount actually. Certainly enough years of operations to understand the difference between DASH, (which gives utterly unmistakeable “positive stick gradient” with no cockpit indication - when I flew them) and LCTs which give a level fuselage attitude and present indications in the cockpit. “One for each rotor head.” These are what had a manual mode. Not DASH.

Maybe I misunderstood your post?

As I explained, I never flew the A model, so was unaware of the ‘fixed gear.’

Last edited by 4468; 3rd May 2022 at 18:55.
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Old 3rd May 2022, 18:57
  #57 (permalink)  
 
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Loving the thread drift into how Boeing’s finest product flies!

I’ve also seen similar vids of a Dutch CH47 conducting equally low fly pasts. Can’t say whether this one is fake or not but I could believe it’s real.
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Old 3rd May 2022, 19:02
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Am I right to think DASH works with the autopilot system.....something the Aircraft I flew did not have.

The A's and B's had SAS only, and the C's had SAS and PSAS.....and DASH was unheard of at that time.

Are you calling LCT the Speed Trim system I described.?

Was your experience gained in the RAF version of the aircraft and if so....how did they differ from US Army versions?

I seem to recall the RAF had quite some problems wtth MOD Air Worthiness Approvals that centered around the Avionics.




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Old 3rd May 2022, 19:27
  #59 (permalink)  
 
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Looking at this old B model tech manual diagram, I would say the speed trim is what we would now call the DASH, which is fully automatic, has no manual override or position indication in the analogue cockpit display we currently have. LCTs are still LCTs. http://www.243rdfreighttrain.org/Chi...ion_Manual.pdf


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Old 3rd May 2022, 21:08
  #60 (permalink)  
 
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1st thought was fake, but looking at the reaction of the entire audience there is definitely something flying low over head, Chinook or not.

Karup Air Base has a T-33 on a short stick somewhere, NATO Hardened shelters on open ground & the truck at bottom right, towards the end of the video looks like it's a 4 axle in camo, possibly a Dutch military Scania R124 with a trailer.

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