JET BLAST - How strong?
 

Possibly a funny question:

How long is the thrust (or jet beam) at T/O for a BAe146.

Would anything be damaged behind the runway, if it is 60m away?

Any sources?

Thanks!

Good question. It's called jet blast. Sorry, BAe146 data not to hand right now. You need the manufacturer's data. Quite possible to get damage at 60m. Depends on the aircraft thrust setting (was it 'stand on brakes and open throttle' OR a more gentle rolling start?).

See this for the 737-800 jet blast envelope:
737-800 jet blast. Possibly the BAe146 blast is 50% of the 737-800. Above 80 kph is when damage occurs.

Thanks.
Jet blast - that was the missing word!

What kind of "anything" are you referring to? Something with a lot of surface area and very lightweight might be damaged in a 20kt wind. Something small & dense might withstand 100kt easily.

I refer to trees, cars etc.

I know a test manager whose car got blown off into a tree by a widebody engine jet blast (about 80 meters behind the engine).

He was in the car at the time. :p

When I used to run Tristar engines in BAH for Gulf Air, we didn't have a run bay but parked the aircraft on the apron, and blocked off the road behind with signs and cones. One day we blew over a car load of airport police. The reason was, they saw the sign but because they were police they decided it didn't apply to them. Nearly wrote off a Land Rover, and they didn't look too good either. Their boss gave them a telling off.
One day the wind was wrong and the tower let us park on the taxiway. The VASI's disappeared over the runway.
There is a lot of power in a jet blast.

For wide-body Airbus aircraft exhaust velocity at idle thrust is 70 knots if you are “standing” abeam the tailplane. At takeoff thrust, exhaust velocity has reduced to 70 knots by about 1000 feet (300 m) behind the aircraft. Airbus uses the 70 knot criteria as a potentially damaging wind velocity.

For the BAe146 at takeoff thrust, standing directly behind the aircraft is thought to be similar to holding your girlfriend's hairdryer in your face but with an oily smell...

Never saw the video of the little airport van being blown over and away by a 747 engine? I don't know if the video is 100% real but a 747 engine at full thrust would pack some punch.

There's a reason to check the undershoot when lining up for departure.

NASA/ASRS link to some statistics.

That link provided by OverRun raises a big question in my mind. I hope this isn't digressing too much.

If the jet blast is coming out the back of the engine at somwhere around 350 km/h (190 kt) at say 98% N1 (typical number for our 800's) how do the engines produce any thrust when the aircraft is flying at or above 190 kt? Surly if the aircraft is passing through the air at say 250 kt or even 300 kt, it would be imposible for the engines to be doing any work on the air. Or another way of looking at it, if the jet blast comes out the back of the engine at 190 kt, and the jet engine has a forward speed of 300 kt, the air passing through the engine, enters the front of the engine with zero velocity, and comes out the back with a FORWARD velocity of 110 kt (300 kt - 190 kt). That doesn't make any sense.

Or another analogy. While a boat is tied to a warf, it's outboard motor can push water out the back at 20 kt. There is no way that boat can reach speeds of 30 kt!!. The closer the boat gets to 20 kts across the water, the closer the propeller gets to achieving zero work on the water (that is, applying force over a distance).

Can anyone enlighten me?

Thanks.

Hi Blip,
I'm not a professional pilot, but just got a degree in aero eng, so here's my 2 pence:

Gas turbines all work on pressure ratios. Sitting still on the runway the engine has to 'suck' ambient presure air in to the fan, it then compresses it, burns it, takes some work out and throws the air out of the back. How quickly it comes out will depend what the engine was designed to do, but for your question it comes out at 190kts.

At 300kts at runway level (i know... but it removes changing altitude pressures from confusing the situation) the air entering the engine is at a higher pressure than stationary due to the relative velocity of the aircraft. Assuming the engine is working at the same operating point (to keep component efficiencies the same), when you apply the pressure ratios through the engine cycle you'll find the jet velocity is a lot higher than 300kts.

Hope this explains it and i'm sorry if any of the above patronises you... but don't know what you already know!!

It's quite a long time since my Aero Eng degree but I can remember a few details. As jhorton said, I think you'll find that the exhaust gas leaves the engine at rather more than 190kts in most flight regimes.

There are actually two factors affecting engine thrust. These are the change in momentum of air passing through the engine, which you have already considered and also the stagnation pressure differential across the engine. If the exhaust gas is at a higher pressure than the ambient air, further thrust will be generated.

On most engines exhaust gas expands to equal the ambient pressure and accelerates so the pressure effect is zero. However on many military turbojets the exhaust nozzle becomes choked. This means that the exhaust has accelerated to Mach 1 (bear in mind that Mach 1 for the hot exhaust gas will be significantly faster than Mach 1 for the ambient air) and can accelerate no more. In this case, exhaust will leave the engine nozzle at its local speed of sound and at a higher than ambient pressure. In this situation the engine generates thrust both from momentum change and from pressure differential.

Geeky enough for you?

Frank Whittle is credited with the invention of the jet engine purely on the basis of his identification of "acoustic" thrust technology! Some reports attribute him as stating "as static thrust disipates acoustic thrust increases and the increase is sufficent to conterract the total loss of static thrust at higher air speeds"

Purists of this theory often cite the Concord is the pinnacle of acoustic thrust development however others are adamant the BN Islander should hold this unique place is history.

Stage 3 noise restrictions resulted in a decline in this field of engineering however it is rumoured that RR had a secret development program in operation. Of course that may explain some of the engine problems the A380 has faced.

VTIC

I don't understand the last reply. Thrust is the reaction to momentum change through the nozzle - jet noise is due to the turbulence and vorticity of the shear layer between the exhaust and surrounding air.

At high powers, the exhausts are just below sonic velocities (supersonic nozzles being mostly a military preserve); that is about 600 knots for the fan nozzle and 700-900 knots for the hot core exhaust.

Typically this will expand to about 3 times the diameter or 10 times the area over about 30m, so the relative velocities should be about 60-90 knots at that distance.

VTIC = very tongue in cheek

Those of you in the UK may have seen the Top Gear episode where they got a Virgin Atlantic 747 at Prestwick, ran two engines at take-off power and rolled a Ford Mondeo then a Citroen 2CV 50 metres behind the jet. The subsequent destruction of both cars was quite spectacular:E

Watch UKTV people on Sky for the repeats.

2CV v 747. :}

Video . . .

http://www.flurl.com/uploaded/Fordje_blazen_10257.html

Now that is funny!:
Interestingly, a large structure flies off the Mondeo. I suspect it must be the rear bumper. I have observed that the Mondeo rear bumper is possibly the weakest and worst design on the roads today (usually lying beside the road). I was shocked to see what a broken one was made of- it appears to be polystyrene foam with a thin vinyl plastic skin. Beautifully colour matched to the car, but as much use as a chocolate ashtay on a motorbike. No wonder they come off so easily, and no wonder I will never buy a Ford!

<killjoy hat on>Is it really a good idea to roll cars roughly along a runway/taxiway? All that FOD and gougy bits wouldn't seem to be entirely compatible with normal ops... </hat>

If it had been Clarkson himself driving the 2CV, the experiment would have been entirely justified.

http://img401.imageshack.us/img401/6...a3205pd.th.jpg http://img401.imageshack.us/img401/9...a3214rt.th.jpg