Jet Engine Center of pressure

[gidge348]

Hello, I was wondering if one of the smart people out there can help me with this.

Last month a friend of mine had a big crash in his jet car.

www.facebook.com/motorplex/videos/10158362499835386/?pnref=story

Daniel was fine but as you can see... the car... Not so much

A lot of people have been trying to come up with a reason for the crash and my theory is that it was a problem with the centre of pressure (CoP) being way ahead of the centre of gravity (CoG) of the car. ie trying to fly an arrow backwards.

I am trying to tidy up the calculations for the CoG/CoP and want to take into account the centre of thrust of the engine.

Basically the point along the engine where I can say stuff being thrown out the back interacts with the engine to push it forward.

The compressor is trying to go forward and the turbine is pulling back, the burner is heating and accelerating the air and the afterburner is increasing pressure as well. The stators are all working in opposite direction??

Could I say the: Combustor? The rear face of turbine? The centre of the afterburner or the exit of the afterburner? or none of the above.

Cheers Ian...

[BFTO]

Did he released the chute before he started to steer right? Looks like he passed the finishline @ 1:02 and deployed the chute and the chute start to deploy and makes the car to steer hard right?

Good saftey gear, 440kph in to the wall and walk away..

[ingeniero]

My first guess for the forces on the compressor shaft is that the forces interact with the car though the thrust bearing. The compressor and the turbine are on same shaft and could cancel each other.

The combustor has a delta p like the compressor. The forces have to go through the frame to the kart frame.

The jet nozzle interacts at its center of area.

If the thrust and turbine forces are below the below the center of gravity of the car the forces they would tend to push the back wheels down and the front wheels up. If you hit a bump the front could continue to raise.

Tractors plowing have a similar problem when the they hit a root, the front of the tractor rises.

farmers hang wrights on the front of tractors to overcome this problem.

[ingeniero]

I looked at the film.

Could be he could not steer when the front end lost traction, because it rose or at least had less down forward force. if the front end has no traction any side forces will make the car spin about the rear axle.

As per BFTo above it probably was the chute.  My hunch, one of several, is that the chute is attached above and behind the rear axle and tends to cause the front end to rise losing steering  which is through friction, tire to road, and so force downwards on the front end.

Could have also had something to do with the slight hill at the end of the track?

Sorry for thinking while posting and not before, but as you write you think.

[gtbph]

Hi Ian,

Does he use some kind of side-thrusters to steer? On the shot from the front (0:40) it looks like little rockets are pushing the nose left and right and then a bit too much right...?

In my opinion only the line of the force matters but not the point on the line. You could attach a rope and pull from a distance or attach a rod and push, it doesn't matter as long as the direction is the same.

Cheers, Alain

[racket]

gyroscopic procession ..............it looks like the engine shit itself, sparks and crap flying out the exhaust .

[gidge348]

Thanks for all the input guys, I should have gone into more detail on how we arrived at the CoP/CoG theory for the crash.

RaceCar Engineering has a very good article on CoP/CoG that may be of interest if anyone want to go fast. www.dlra.org.au/docs/race-car-engineering.pdf

There are 2 main parts to the crash:
1. The car yaws to the right ~2-3 degrees…… “this should have been self-recoverable”.
2. The car snaps violently right and hits the wall.

For the first part of the crash, the track was cold the car was going faster than it ever had, there may have been a slight cross wind or turbulence at that point & there had some oil downs during the night but these had been cleaned and re-glued, any of these should have been a recoverable situation, what I am interested in is the second part of the crash. The car should self-correct straight, not snap violently right.

Jon, Yep Gyroscopic progression can be a big thing on shutdown but the drivers are used to that. As for the sparks, it was just the after burner can getting a bit old and rusty. The engine, turbines and stators were all fine just bashed around in the crash. Brakes, tyres, chutes, steering etc where all fine before the crash.

BFTO, the chute was pulled when he felt the car first start to yaw but the car had turned violently right (second part of the crash) before it could deploy properly.

ingeniero, What you describe with the tractor analogy is exactly the CoP/CoG situation rotating the car horizontally around the centre of gravity rather than vertically around the tractors axel.

Alain, No the car does not have side thrusters but you are right the centre of force will always act through the centre of gravity, so the net effect on yaw should be zero… thanks for that.

Cheers Ian…

[racket]

Hi Ian

At 0.09 secs into the vid theres redhot chunks of something 30 feet in the air , that ain't some rust scale , that looks like bits of "hot section" ...........LOL, "going faster than it ever had" sorta indicates an "excursion" ;-)

Theres an example of how thrust is "manufactured" within an engine in my Aircraft Gas Turbine Powerplants by Otis and it appears the greatest "thrust ??" is at the combustor outlet due to high pressure and highish velocity .

Cheers
John

[gidge348]

Hi John,

LOL yep I saw that big flash at 0.09 had me going for a while but, if you look closer I think it is some camera effect caused by the heat haze and the flashing yellow light in the background. If you follow through the side view from 0.58 to 1.02 as it passes the same area on the track there is no big flash. Check the other car, the green light behind it gets bigger & bigger during the run from the same effect.

After dissecting the engine the hot section & bearings are still ok??

Thanks for the details on the thrust manufacture position, after thinking about it unless the engine comes loose it should always create thrust through the centre of gravity so not affect the vehicle dynamics...... I think ...

Cheers Ian...

[finiteparts]

I have to side with John on this. Those are some big sparks shooting out to be bits of rust...do they store that in a brine tank? Ha! If you watch the video at 0.25 speed (got to settings, change speed and go to highest quality) you can plainly see the flame pulsating and sparks shooting out of the exhaust just before it starts to turn. Something went wrong in that engine. It surged or something. Can you share some photos of the hot section? That is really hard to believe that it is ok after all that!

www.youtube.com/watch?v=IoBpV7Lz6G4


The engine should actually stabilize the vehicle due to the gyroscopic forces produced by the relatively large rotor mass. This should create a stable line of action that should not vary much over the engine operating speeds. Art Arfons or Craig Breedlove lost a tire during one of their runs and the car remained completely stable on the remaining three tires due to the gyroscopic stabilization of the engine rotor.

But!!! If the rotor experiences a rapid change in rotational speed, there is a gyroscopic cross-coupling of forces that push sideways on the rotor. If the engine had shucked some blades, surged the compressor, etc...there could have been a rotor speed change that would manifest itself as a sideways force.

Another thought....if the tires or wheel bearings had any issues you could get a destabilizing force.

Good luck finding the cause...and I am really glad to see that he was ok.

~ Chris

[finiteparts]

Actually...if you look close at around 1:00-1:01...it looks like the tires are steering towards the wall?

~ Chris

[gidge348]

Thanks Chris I will dig out some pics of the hot end next time I am down there, it is not perfect but it's definitely not toast.

I know there could be a thousand reasons why a car gets de-stabilised, I am not looking at them at the moment.

My concern is "after" it was de-stabilised "WHY" it did not self correct and instead swapped ends and smacked the wall.


Cheers Ian...

[finiteparts]

Ian,

Just looking at the CoP and CG doesn't really give you the whole picture, it depends on all the forces acting on the car. That article basically assumes that you have minimal tire grip at the instant that the destabilizing forces push on the car. If that was the case, then you rotate around the CG. If you watch the video, this is not the case...the rear tires look pretty well planted and as such, the car will not rotate around its CG, but around the rear tire contact patch. Any force applied forward of the rear tires will act to destabilize the car.

- Chris