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.

Most threads on Mil aviation should actually be on AH&N.

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.

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

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.

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 ....

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

https://cimg5.ibsrv.net/gimg/pprune....78b3719f56.png

NOT REAL HELICOPTER

https://cimg1.ibsrv.net/gimg/pprune....344e9cd7f2.png

https://cimg9.ibsrv.net/gimg/pprune....37f0a65ba8.png

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.

”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?) :rolleyes:

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.

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.

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".

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.

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

SASless

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:

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?

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.

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.’

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.

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.

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


https://cimg6.ibsrv.net/gimg/pprune....b0758aade.jpeg

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.

I think it is an air-to-air shot of a Chinook flying straight and level, in formation with another aircraft. then the film is skewed port by 30(ish) deg and superimposed on the footage of another flypast with the aircraft flying just out of the frame at the top, leaving just enough room to put the Chinook imagery below that of the aircraft, apparently, closer to the ground.

4468, It would appear we are having a Apples and Orange's discussion when we each talk about our recollection of the Chinook.

I qualified my experience ending with the C Model having flown a lot of A models, some B models, and C-, full C models in the standard US Army Configurations.

Chinook 240 kindly provided a copy of the CH-47 Familiarization Manual that addresses the LCT system which I knew as the Speed Trim System.

DASH was not installed on any of the Models I flew.

That was part of the upgrade to the D Model.

The D Model Operators Manual I linked has a description of the AFCS Components in Section 2-5-7 (d) and (f) where the Differential Air Speed System (DASH) is discussed and mentions that it incorporates airspeed hold above 40kts and also provides positive stick gradient through the autopilot.

So....does this clarify things a bit for you.

You are talking D Model or later when you talk DASH and PSG services it provides.

That does not apply to earlier models of the US Standard Configuration A-C Models.

Here is the Army Operators Manual for the D Model.....perhaps not the latest edition but good enough for our purposes.

http://www.chinook-helicopter.com/Pu...520-240-10.pdf

If you move to page 51 of the Fam Manual provided by 240.....you can see a discussion and diagram of the LCT or Speed Trim System as I have described it in an earlier Post.

You can see the DCP Link in the diagram.

That is controlled by the Trim Wheel in the Cockpit which moves the cyclic fore and aft and its position is indicated by a display. marked in "Inches of Displacement".

The use of that Trim Wheel is for comfort purposes primarily and has limitations for on-ground use.....in the A-C Models it was a Two inch Aft of center Cyclic movement with the DCP adjusted to Zero.

The other half of the DCP actuator acts to facilitate a Positive Stick Gradient.

In my Operational Unit anytime that Actuator (either half of it) went U/S it was noted in the Maintenance Record of the Aircraft and labeled "In-Operative"....with no limitations on Flight applying.

In the D Model Operators Manual we see an Altitude Hold Capability which was not present on the A-C models.

Thanks to SAS for a PM- could not recall the name of what was, in the CH-47A I flew from 1963-65 called the Differential Collective Pitch system, whose function was similar to the Pittch Bias Actuator in the S-76. The early UH-60’s had them as well, until the Army decided to take them out because: 1) they were a maintenance headache, and )2 the pilots couldn’t tell whether it was on or off anyway ( something we had told them a long time before that ).
The speed trim really wasn’t put in to provide a level fuselage attitude-that was only partly true-the other part was that it was there to reduce the main rotor shaft bending loads at speed and thereby allow for a higher shaft component replacement time. You might recall some difficulties the USMC had with pilots using the CH-46 speed trim manually to control attitude during approach to an LZ in Vietnam. Think they had at least one shaft failure as a result.
( Background: after flight school I had wangled an assignment to the Test Board at Ft Rucker. The prototype 47A’s they had ( 3 of them ) had been landing at various spots around Ft Rucker, and maybe it was the landing on the golf course* that got me, a 2nd LT into getting a Chinook checkout, and by two of the Boeing Test Pilots to boot, as they were there to assist the program. One could ask them questions and get very straight and detailed answers. It was ironic that after a short tour there and a visit to SE Asia, I wound up at Sikorsky, and while the CH-53A could fly rings around the CH-47A I’d get into some “interesting” conversations over the years with SA folks who failed to pay attention to the gradual and effective improvement programs the Army and Boeing effected into the Chinook product line.
*there were two explosive failures of the nose gearbox, traced to a gear resonance and which caused nose gear box failure and parts etc being ingested into that side’s engine with very noisy further results. Another ship had what was referred to as floating SAS links, resulting in a ship rolling on its side. Lastly, there was an aft shaft bearing failure, in which the crew was able to get it on the ground on the north part of Cairns AAF, but th blades hit the tunnel and the engine controls were cut, so they were shut down via a fire truck directing their turret into the inlets, one at a time. I happened to be landing at the north helipad and folks were running across the path to my tie down in front of me, so I turned the D model around and watched all of this one. No injuries. Anyway, the field grade types started finding other things to do. Except one: we had a civilian Joe Givens who was an O-6 in a USMC fighter sad at NAS New Orleans and he still flew them. Turned out later that this Joe Givens was in the same F4U-4 squadron in the S. Pacific with Byron Graham, my future boss as Ch Exp pilot at Sikorsky. Sorry, had to keep the record straight re field grades.

Anyone care to calculate the aircraft speed, based on known dimensions of objects in the frame?

If you were to plan a ultra low level flyby, what speed would you choose? (clearly the troops were given enough warning to climb the containers)

Can we use the rotor rpm to crosscheck the above (probably not but worth asking)

Numerous cameras in use but only one video published thus far?
The plastic cone positioned camera right is unmoved?
No markings or insignia discernible?

On the other hand...
The reflection of blades in the puddle is convincing.
At the end of the clip, the image slightly brightens when the operator sticks his finger into frame. The aircraft brightens to the same degree. (requiring an exceptional commitment to detail, if it were a fake)



Mjb

The Big Boys tell me they go as fast as the old girl will go....stopping at the first Red Line reached....and not worry about the Dental Work.

Of course I know naught of such childish, dangerous, unprofessional antics as seen in the Video....why that would inject some fun into helicopter flying and that just isn't the done thing anymore.

I might be wrong, but don't the engines sound rather normal when the aircraft passes the camera despite the video being slowed way way down?
I would expect them to have a much lower sound.

SASless; Seems so. As a friend has said, back in the day we were "Professional Hooligans"

I was part of that era too. It claimed lives unnecessarily, not funny and in no way professional.

Maybe a sense of humour failure. The critical word was "professional". Professional enough to keep the right side of dangerous. It was the amateur hooligans who got into trouble.

You suppose some folks got "old" for good reasons besides blind staggering Luck?

When I was instructing I would use the mantra "You've got to be good to be gash". After a theatrical pause I'd continue "But if you were good, you wouldn't be gash!"
There are parallels with the F16 female OC thread. I had been told "Rules are for the guidance of the wise" and, while I can understand why such a comment would be made, these days particularly, rules are for everyone, with no exceptions.

This thread has headed down the proverbial rabbit hole me thinks.

It started off discussing whether a video was real or fake....and never decided that despite lots of effort....some good....some we could have done without.

Now we have seen a shift to bashing unseen.....unrecorded....purely imaginary similar exploits of airmanship.

The ultimate comment could be translated into an old adage that took quite a bit of bashing itself after a bit of unpleasantness......"Befehl Its Befell!".

Time to move on I would suggest.

But isn't that part of the living history that is PPRuNe? Over the years some of the most enjoyable reading has veered significantly from the thread starters words, gone down numerous rabbit holes and sometimes recovered and sometimes not. There's never a dull moment [well, sometimes] and on the whole it's all a good stirring of the pot of memories.

But that is so unfair to those of us with failing memories!:uhoh:

I'm not sure anyone would put the rotor blades this close to the dirt, but it was painted to make the viewer uncomfortable looking at it....

https://cimg9.ibsrv.net/gimg/pprune....5a1d731a3.jpeg