how?

optical illusion based on the frame capture rate of the camera equaling the rotor rotational speed. Much like how a strobe light makes moving object appear still.
 
Rotational speed of the blades matches the film image speed, making it appear the blades are not moving.
 
Thats what I thought too but... those blades are just TOO still, the whole time, and when they do move, there's NO appearance of any optical illusion at all...

Thats freaky - if that really is film speed matching up, thats just amazing...
 
I thought the same thing, but the little rotor on the back moves. I know nothing about helicopters. Don't the two rotors turn at the same RPM?
 
I'v got a video of me taxing my plane and the prop is spinning at 1rpm but I think that video was doctored. It prob started with a slow moving rotor and somebody thought "wouldn't it be cool if the rotor was still". The manuevers being done should have varied the rotor rpm at least to a small degree to make it move some.
 
deafsound said:
I thought the same thing, but the little rotor on the back moves. I know nothing about helicopters. Don't the two rotors turn at the same RPM?
Click to expand...
No they don't. They do, however, have a fixed RPM ratio between them. I don't remember the exact number, but my recollection is that in our R44 the tail rotor RPM is around 4 times the main rotor RPM. This gives them roughly the same rotor tip speeds (at 102% RPM, 705 FPS for the MR, 614 FPS for the TR -- you don't wanna stick your fingers in that!).
 
RotaryWingBob said:
No they don't. They do, however, have a fixed RPM ratio between them. I don't remember the exact number, but my recollection is that in our R44 the tail rotor RPM is around 4 times the main rotor RPM. This gives them roughly the same rotor tip speeds (at 102% RPM, 705 FPS for the MR, 614 FPS for the TR -- you don't wanna stick your fingers in that!).
Click to expand...

Bob or shall I call you iHoover ;)

Looking at your avatar it would seem that pretty much main rotor speed does not change very much at all during flight. So we should see some tilting of the main disc in that video as forward speed is affect should we not?
 
smigaldi said:
Bob or shall I call you iHoover ;)

Looking at your avatar it would seem that pretty much main rotor speed does not change very much at all during flight. So we should see some tilting of the main disc in that video as forward speed is affect should we not?
Click to expand...

iHooooover... ;)

Rotor RPM is kept in a very tight range. The avatar is from our R22 -- notice that the green arc only extends from 101% to 104% -- it's even tighter in our R44. The job of the governor (or the pilot's wrist if there isn't one) is to keep RPM essentially constant. The governor, BTW, is kind of the direct opposite of what you have in a constant speed prop, where the pilot controls RPM and the governor controls pitch. In a helicopter, the pilot controls pitch with the collective and the governor manipulates the throttle to maintain RPM at whatever the governor is set for (not under pilot control in flight, but turbine helicopters generally have a flight RPM and a ground idle RPM setting). High RPM will possibly damage the rotor system, low RPM (below say 80% depending on DA) will kill you :hairraise:

As to tilt, I agree that if the ship is in forward flight you would see the rotor disc tilted forward wrt the horizon -- this is necessary to provide forward thrust (in a hover, the ship goes in whatever direction the disc is tilted).
 
So with rpm in such a tight range it would be possible if the frame rate of the video camera and the rpm of the rotor were just right that it would barely if ever show movement? In other words we should not expect to see the main rotor rpm increasing and decreasing as the helo maneuvers.

As for the tail rotor while it at an rpm proportional to the main rotor form the video it appears to have its rpm vary at a much great extent than the main rotors. Is that possible? Does the tail rotor change rpm in a greater range?
 
smigaldi said:
So with rpm in such a tight range it would be possible if the frame rate of the video camera and the rpm of the rotor were just right that it would barely if ever show movement? In other words we should not expect to see the main rotor rpm increasing and decreasing as the helo maneuvers.

As for the tail rotor while it at an rpm proportional to the main rotor form the video it appears to have its rpm vary at a much great extent than the main rotors. Is that possible? Does the tail rotor change rpm in a greater range?
Click to expand...

Scott, in general the governor will hold the RPM so constant that in practice you don't see any deviation of the tachs throughout any sane maneuver. In autorotations however, because the governor has no role to play, maneuvers will affect rotor RPM and the pilot has to anticipate that. Hauling back on the cyclic, lowering the collective, and turns will all increase RPM.

Because the TR spins at 3-4 times the MR RPM, a small fluctuation in MR RPM will be 3-4 times greater in TR RPM.
 
I was going to post that video but I couldn't find I way search for it on YouTube

LISTEN to the sound. That "non moving" rotor still breaks the air like a moving one.

What gets me is through all of the maneuvering the rotor RPM apparently never changes.
 
RotaryWingBob said:
Scott, in general the governor will hold the RPM so constant that in practice you don't see any deviation of the tachs throughout any sane maneuver. In autorotations however, because the governor has no role to play, maneuvers will affect rotor RPM and the pilot has to anticipate that. Hauling back on the cyclic, lowering the collective, and turns will all increase RPM.

Because the TR spins at 3-4 times the MR RPM, a small fluctuation in MR RPM will be 3-4 times greater in TR RPM.
Click to expand...

Very educational. Thanks


Then one last question.

Based on this information and your experience would you say that it is resonable to infer that in the video all rotors are turning at an appropriate rotational speed and because the of the frame rate capture or film shutter speed of the camera what we are seeing is an optical illusion?
 
I was initially going to go with the optical illusion thing..... I think it is because the Hind is so ugly it is simply being repelled by the ground :-)

Pete
 
smigaldi said:
Very educational. Thanks


Then one last question.

Based on this information and your experience would you say that it is resonable to infer that in the video all rotors are turning at an appropriate rotational speed and because the of the frame rate capture or film shutter speed of the camera what we are seeing is an optical illusion?
Click to expand...

You're welcome, Scott.

I have to assume that everything was spooled up to normal flight RPM, and that the frame rate/shutter speed is causing the illusion.

I don't know of any ship which is capable of recovering from a low RPM condition. Basically, at low RPM the MR blades stall, and once that happens you can't even initiate an autorotation. At the Robinson pilot safety school I saw a home video of an R22 Mariner (R22 with floats) that got into a low RPM main rotor blade stall situation. That ship went down like the proverbial rock, and the occupants became the aviation equivalent of road kill. So I don't see how this can be anything other than illusion.

One other point. Turbine helicopters couple directly from the shaft to the drive system. The only way the rotors can stop is either because the turbine has stopped, or in the case of an autorotation (where the rotor system freewheels), if the pilot botches it by failing to lower collective. In either case he's dead...
 
smigaldi said:
Very educational. Thanks


Then one last question.

Based on this information and your experience would you say that it is resonable to infer that in the video all rotors are turning at an appropriate rotational speed and because the of the frame rate capture or film shutter speed of the camera what we are seeing is an optical illusion?
Click to expand...

Either that or it's a very complicated photoshop effort. I did wonder if the video was captured using something to sync the frame rate with the rotor. I could see how that might be done by monitoring the variation in light coming through the main rotor. One other option would be to capture at a high rate and then resample at rate that matched the rotor RPM.
 
lancefisher said:
Either that or it's a very complicated photoshop effort. I did wonder if the video was captured using something to sync the frame rate with the rotor. I could see how that might be done by monitoring the variation in light coming through the main rotor. One other option would be to capture at a high rate and then resample at rate that matched the rotor RPM.
Click to expand...

That would be my guess. I'm willing to bet this is pretty trivial with some knowledge and Adobe Premier.
 
How can it fly? The military now has an anti-gravity device. They put it in a helicopter body with fake rotors to fool the public.
 
Wow did you read some of the responses to that video on that site?
 
Further evidence it's an effect of the video: How many helicopters do you know that have 5 blades on the main rotor? The most a quick Google Image Search comes up with is 4.
 
bluesky74656 said:
Further evidence it's an effect of the video: How many helicopters do you know that have 5 blades on the main rotor? The most a quick Google Image Search comes up with is 4.
Click to expand...
Well, the Hind helicopter is one that comes right to mind! :rolleyes:

Hind's (of which this is one) actually DO have 5 main rotor blades. :)
 
You must log in or register to reply here.
Back
Top