Video here of a commercial jet aircraft undergoing a routine static engine test. The wheels are chocked, and the engines are run up to full power, one at a time. The cockpit crew are seen to be bouncing around quite a bit, although the ground speed and air speed are both pretty much zero.
Why the intense shaking? As a passenger, I don’t think I’ve ever experienced shaking like that until late in the takeoff roll, when air speed and ground speed were pretty high.
You would never as a passenger be in plane that had it’s wheels chocked while one of the engine was going full throttle. The plane is attempting to take off but is being restrained causing but it can’t so it shakes. Also it sounds like only one of the engines was being tested, which would put an unbalanced load on the plane which may cause it to shake more than if both engine were being tested.
And finally, depending on the amount of throttle given would most likely hit a natural harmonic frequency of the planes structure that would cause the plane to vibrate.
It’s the force of the engines vs. the chocks holding the aircraft in place. Two forces placed in direct opposition. If you put the front of your car up against a wall and then try to accelerate as fast as you can you will:
a) Not get anywhere
b) Vibrate a whole bunch
The air speed and ground speed are zero because the aircraft wheels are held in position by the chocks.
c) hear a loud bang as your gearbox disintegrates.
There’s some detail missing in the cause -> effect chain there. If I turn off the running engine and instead attach a cable to it that pulls the plane in a forward direction with 30,000 pounds of force (with the chocks preventing actual forward movement of the aircraft), there won’t be any vibration. What’s different about a cable pulling forward on an engine, versus jet thrust pushing forward on an engine? If you can explain what makes those two setups different, then we might actually be moving toward a useful explanation of what’s happening in the video.
This implies the existence of an oscillatory forcing function in the first place (to match with some natural resonant frequency of the test configuration). Something on the order of a few cycles per second, to match the shaking frequency seen in the cockpit. What would that forcing function be?
“Two forces placed in direct opposition” is also unsatisfying. I can imagine an infinity of scenarios in which two forces exist in direct opposition without inspiring any vibration. Example, out in my garage, gravity is pulling my car down, and the concrete floor is pushing my car up. Two forces are in direct opposition, and my car is not vibrating. What’s special about a jet engine pushing a plane forward, combined with wheel chocks holding it back?
The reason for the shaking is because the airflow in and around the engine is extremely turbulent.
Turbulence is a natural phenomenon that happens when air flows at high speeds (high Reynolds numbers) and causes random variation and instabilities at all scales.
Don’t know how to explain better. Here is a link to an article where they tried to simulate the airflow through an entire jet engine while correctly accounting for turbulence. I don’t see where in that article it says that there would be shaking but maybe someone can explain that in more detail.
From the first paragraph of that article
My bolding, there’s your shaking, I guess.
All right, I think we have our serious answers.
Now here’s the more poetic explanation: the shaking is from the wheels rolling really fast on the treadmill.
There are many videos online of drag racers sitting at a stoplight waiting to go, and revving their engines like crazy – and you can see the whole vehicle & passengers vibrating like crazy.
I’d presume the vibrations of the fast-running engines are being transmitted to the frame, either in the plane or the drag-racing car.
Yes, piston engines by their very nature deliver power in a series of violent explosions, which causes vibration. While jet’s power is a constant thing. The real answer here is the turbulence from the jets exhaust hit the ambient air that you normally don’t experience while in flight because you are leaving it behind as you travel.
And in fact, at least in a general aviation airplane (e.g., a single-engine Cessna, Piper, etc.) this is routinely done, at least briefly, before every flight. In a pre-flight procedure commonly called “run-up”, the pilot sets the parking brake and revs up the engine to full throttle, checking that the rpms thus developed are in the right ballpark. Airports generally have a wide paved area at each end of the taxiway where this is done.
The airplane definitely bucks and shakes during this procedure.
Actually, in general aviation airplanes, the run-up is rarely done to full throttle. The usual procedure in a run-up is to advance the throttle to a certain RPM, specified by the manufacturer. In the planes I’ve flown, 1700 RPM is common. That’s a little over half throttle. However, even at that power, there is a fair amount of shaking.
When you pull with a cable, the pull point is on the rigid “hull” of the airplane and the vertical distance from the pull point to the wheels is very less compared to the engine which is mounted up higher. So the moment is low. When the plane is flying, air resistance is pretty much everywhere and counteracts the thrust (it’s not there just at the wheels). If the wheel chock fails, during single engine test, the airplane will spin as well as move forward. When the airplane is flying though, the pilot can run on one engine with the other wing on high air resistance to avoid spinning.
The structural elements inside the wing, on which the engine is mounted, is designed to be not rigid but flexible. You can watch the wing flex as the plane takes off or lands and during inflight turbulence. In other words, it can be looked as a spring (cantilever). When the engine is running on the tarmac, the force from the engine is not entirely in the same geometric plane as the wing. (If the plane were flying, the lift would have lifted the wings up more compared to the centerline of the plane.) As the wing flexes, it changes the direction of the engine exhaust thereby changing the direction of the force and it becomes a feedback loop. (But well attenuated)
I didn’t explain myself clearly. Imagine the cable is attached to the spindle of one engine and pulls on the plane in a direction that matches the thrust of that engine. How would the plane “know” the difference between that cable pulling forward on the turbine spindle, and jet exhaust pushing forward on it?
I think Frankenstein Monster and snfaulkner are maybe onto it. Is it something to do with turbulent mixing of the exhaust plume propagating laterally to the empennage, thanks to basically zero slipstream velocity? Maybe when the plane is moving fast enough, the turbulent mixing zone gets swept rearward before it can propagate laterally to hit those tailfeathers.
In a static test the turbulence created by the engines is going to be more local. You are not flying away from it at several hundred miles an hour. It will also be drawing some amount of air over the wings which will want to lift, but can’t. So it is a more bumpy ride. Just imagine the amount of roiling billows of air behind the engines. Also the amount of air that is being sucked in ahead of the engines over and under the wings. But the craft is stuck within that mess.
As others have mentioned, it is not possible to have the cable pull dynamically in the direction of engine thrust.
Due the flexibility of the wing, turbulent forces, etc the centerline of the engine keeps moving during the test - something a cable can not replicate.
As some folks tumbled to above, it’s mostly the violently turbulent engine exhaust hitting the horizontal tail & thereby shaking the whole airplane.
When taxiing around we routinely see the tail feathers of the guy in front of us flapping in the exhaust turbulence of his engines.
Many engines are installed with a slight uptilt vs the airplane. So in flight the engine wake flows mostly below the tail. On the ground, and especially when stationary on the ground, that means the wake is aimed to hit the ground and bounce back up.
The next time you’re riding in back and have a window seat, watch the preceding few aircraft accelerate for take off as you’re approaching the end of the runway. Look specifically at how hard the tail is being jostled when they first power up and the airplane first starts moving. It’ll make you wonder how the thing survives 10s of thousands of hours of sitting in that kind of turbulence. The answer of course is that it’s only that bad for a few seconds of each flight.
Consider that the engines used on the A330 move roughly 2000 lbs/sec of air for each engine. Two tons of air per second. That amount of air moving past a still object will cause some vibes.