I’m watching this video of big planes landing on small runways and I’m wondering about the piston plane (the second one) that appears to be landing without power. Are the props barely turning, or is it a strobe effect? It sure looks like a glide.
http://www.wimp.com/tacticallandings/
It’s the strobe effect. You can hear the engines on the Hercules upon touchdown. If they were windmilling they would be spinning faster.
So it could still be gliding in and the props spinning fairly quickly. These are adjustable propellers so they can be “feathered” to reduce drag as much as possible. If they were fixed pitch and brought to idle they would create drag and act as a noticeable speed brake.
One of the things that can be done with a variable pitch prop is to adjust them so that they create as much drag as possible to slow the plane down even to the point of reversing the pitch and creating forward thrust.
I’m not an aviation expert, but it’s almost certainly a strobe effect. The propellors are all turning the wrong way as the plane lands. Without power, I would think that they would be “windmilling” (turning the right way due to the plane’s motion through the air) instead.
The prop blades on the A400M (the second plane) are slightly blurry. They’re spinning at high speed, which is what causes them to be blurry in each video frame. They just happen to be spinning at the right speed, relative to the camera’s frame rate, to result in the wagonwheel effect.
FWIW, the A400 does not have reciprocating-piston engines; those are turboprops, i.e. propellers driven by gas turbine engines. Instead of creating thrust directly via a high-speed exhaust stream (as on a turbojet engine), the gas turbine’s exhaust drives a power-extracting turbine that is connected (through a gear reduction unit) to the large propeller seen on the front.
The A400M has counterrotating props, btw, to reduce turbulence, and reduce noise and wing inefficiency. #1 and #3 are CW, #2 and #4 are CCW, IIRC.
I thought this was going to be about the Gimli Glider.
And do all four engines turn at the exact same rpm? Because that’s the only explanation for a strobe effect in all four props. Otherwise, I’m thinking this plane was not under power as it landed. Engines on, of course, but essentially in neutral.
The rpm does not need to exactly match for the effect to occur, they just have to all be close to a multiple of the sampling frequency for the rotation to look very slow as it does in the video. Of course, for four propellers on the same plane, it’s not surprising that they would all be going close to the same speed.
If you watch the inside propeller on the right wing at about 0:57 it seems to change direction twice which is classic wagonwheel effect, but is hard to explain as a real propeller behaviour.
I think the number of blades, their shape and the feathering make the A400M propellers produce strange looking effects, at least compared to most prop planes.
You can see the same effect in this video from 0:40 until 7:00 as it flies around at an airshow, obviously under power. The effect doesn’t appear until it levels off from it’s steep takeoff climb. The pilots would presumably be lowering power and feathering the props for cruise at that time.
In this video at 1:15, it sure looks like the blades stop and reverse direction as it starts it’s takeoff roll, but that shouldn’t be possible. They would again probably be adjusting the RPM and feathering of the blades at this time too.
Ditto.
In general pilots & engineers want all props turning at very close to the same RPM. If they’re not you get an annoying beat frequency at a few Hertz. That slow repetitive “waah waah waaaaaaaahhhh waaaaaah waaaaah waaah waah wah wah wah wah waaaah waah waaaaaaaahhhh waaaaaah” etc. noise for hours on end is not healthy for humans. Neither is the corresponding vibration healthy for machinery.
A typical modern installation will have a synchronizer system where one engine / propeller is installed as the master and the remaining props’ RPMs are subtly fine-tuned to match exactly. The pilots’ engine controls set a gross engine power output and gross prop RPM target and then the synchronizer fine tunes from there.
In the olden dayes of piston airliners the synchronizers went so far as to be synchrophasers: they’d not only match RPMs, but would align the props so a particular master blade passed the 12 o’clock position at the same moment on all engines. I don’t know if syncrophasing is still used on modern turboprop aircraft.
And yes, all the slow-turning props you see in those videos are beat frequency artifacts of the near-match between the camera’s frame rate and the RPM x blade count of the prop(s).
There is a neat youtube of a Russian helicopter performing at an airshow with apparently almost-stationary rotors. If I can find it I’ll post the link.
Here’s the Russian helicopter video I mentioned Helicopter flying with no blades spinning ! - YouTube
And here’s one with two US helos: Helicopter gun battle, rotors stop! - YouTube
In each case the rotors are spinning at their normal RPM; it’s just the frame rate of the camera almost matching the rotor RPM & blade count.
I would think it must still be used, since
is related to the different props going in and out of phase with each other. If you locked in the same RPM on all engines at an undesirable relative phase, you could be stuck with the most intense vibrations/noise (instead of the least intense) for the duration of the flight.
On the topic of propellers and landing/takeoff approaches, I vaguely recall from a private pilots’ Ground School course I took over a decade ago that you usually want your engine running at high RPM during takeoff and landing approaches, so if you need to gain altitude or speed quickly you don’t have to wait for the engine to spool up. You adjust the pitch of the props to minimize thrust and other air surfaces (flaps, brakes, elevators) to adjust the speed as needed otherwise.
Downsides of this approach are higher fuel consumption and a greater risk of overheating the engine, natch.
This is bizarre to watch.
Love it.
Big planes can also land in an emergency with no engines on, but it’s not often too successful.
If they can actually reach the runway, they generally do OK, where “OK” mean the plane is repairable and there are no fatalities or serious injuries.
It is not good for the aircraft. I can not recall the exact AD note for the Lockheed Connstellations, but they had an issue of the tail cracking and falling off of the airplane if the engines were opperated at the wrong RPM and/or airspeed and/or not “syncronized” to each other. Vibration was the killer.
I first read about this in Ernest K Gann’s “Fate is the Hunter”. Great book and author.
The space shuttles made deadstick landings. As long as you can get to a runway, have a long enough runway, and power for the hydraulics for the flight controls, an experienced pilot should be able to land.
[QUOTE=LSLGuy]
That slow repetitive “waah waah waaaaaaaahhhh waaaaaah waaaaah waaah waah wah wah wah wah waaaah waah waaaaaaaahhhh waaaaaah”
[/quote]
If I understand LSLGuy, it goes like this. Multiple propellers can be synchronized so that they are turning at the same speed and that when one prop has a blade at the 12 o’clock position, the others do to. That should be enough to eliminate any unpleasantness or damage caused by the props going in and out of phase with one another. Synchrophased props would go a step further. Imagine one blade on each propeller is painted white, the others black. When one prop has its white blade at 12 o’clock, the others also have the white blade at 12 o’clock.
Interesting stuff. I wouldn’t even know how to begin making something like that work.