How, exactly, does a jet engine make a plane go?
 

Well, I read the "big red letters at the top", so here goes:

How does a jet engine make a plane go?

According to the Wikipedia.org article on "jet engine", when the gasses emerging from the engine are faster than the speed of the aircraft, forward thrust is created.

I have heard a physicist say the above concept is incorrect. Instead, he says to focus on the combustion chamber of the engine. Fuel and air burn rapidly and that combustion pushes in all directions -- that part of the combustion in the forward direction pushes on the forward wall of the chamber, which is connected, ultimately, to the airframe, and pushes the plane forward. (The gasses emerging from rear of the chamber don't have as much to push against.)

Thank you for considering my question.

Your physicist is correct.

Your physicist is wrong.

Yes part of that expansion pushes on the forward part of the combustion chamber, but part of that expansion will also push on the rear part of the combustion chamber. Newton's Law, equal and opposite and all that.

A jet engine creates an area of high pressure at the rear/exhaust end, effecting a low pressure area at the front end. Everything moves from high to low pressure (like a wing) and voila! 'Tis a simple explanation as I haven't had any caffiene yet.

Or at least he would be if your 'jet' was powered by a rocket engine!

But this doesn't explain the operation of high bypass turbo-fan engines, where a large majority of the thrust is produced by the 'fan' part. This is essentially a shrouded propellor, and has nothing whatsoever to do with the 'jet' part, other than receiving its power from the (typically 1st stage) turbine arangement.

I'd go along with your original definition:

When air is accelerated backwards (as in your description), thrust is generated. This applies equally to the fan part, and the jet part - both accelerate air backwards, and both generate forward thrust.

I think care is needed to distinguish between a 'jet' engine as used by the majority of airliners, and 'rocket' engines, which your physicist might well have been referring to.

PM

ATR72 Propeller
 

Hi
Having spent many blissful lunch breaks watching ATR72’s pull onto stand and shut down I have always been curious as to why one propeller (I can’t remember if it’s the number 1 or 2 engine/turbine) slows down and then stops very abruptly, as if a brake has been applied, whilst the other propeller windmills to a leisurely stop. Does anybody know why? Does anybody care? I’m just curious.
My guess is, if it is number 1, that that is the side the pax disembark from and it is done for safety reasons, but how is it stopped?

Cheers

CC :)

The No.2 propellor is indeed braked, by the ingeniously named 'prop. brake'. The reason is not for safety (if it was, the more likely candidate would be No.1 rather than No.2.

....and the reason is, for ATR an engine running with a braked prop. effectively becomes an APU. Quite clever really.

alot of captains will only use this as a last resort where no GPU is available. if the prop brake failed in anyway anyone stood near it would quickly paint the surroundings red... not pretty.

h73kr & jerboy

Thanks for that. I shall sleep better now!:)

Being an earthbound misfit due to hearing problems I hadn't realised the turbine was still running and just the propeller was braked. But I suppose if I had thought about it...:ugh:

Mod: Sorry if I put this in the wrong forum. I have seen & read the large red letters at the top and felt I fitted in to that category but was not sure if the question did.:O

'Good judgement comes from experience, and experience - well, that comes from poor judgement' - Anonymous:)

All the best.
CC

Full power always used on take-off?
 

As a PPLer I would always use full power on take-off but a recent discussion elsewhere suggests that this isn't always the case with commercial airliners & that 'thrust reduction' methods are employed.

Is this correct? If so, could someone explain what these measures are and why you would use them. Surely max takeoff power reduces the amount of runway required and gives the pilot more 'headroom' if an abort is required?

Cheers

Steve

Please search for 'flex' and 'reduced' and you will find lots of information.

I'll give you a start.
http://www.pprune.org/forums/showthr...highlight=flex

came from 'Questions'. Try 'Tech Log' also.

Don't forget to search properly.

Thanks - that makes sense... Will search harder next time!

Cheers

Steve

How, exactly, does a jet engine make a plane go?
 

I'd also go along with the Wikipedia explanation. Remember Isaac Newton and "Force = Mass x Acceleration"? Well, integrate that once with respect to time and you get "Force x Time = Mass x Change in Velocity" or, to use the accepted terms, "Impulse = Change in Momentum".

Thus, if you have a device, such as a pure jet engine or, even better, a big by-pass turbofan, that gives a huge mass-flow of air an increase in momentum (backwards), the force that did that (backwards) is going to have an equal and opposite reaction forwards - which is the thrust of the engine.

Measure the speed of the jet plume and a relatively simple calculation will indicate the thrust. Equally, if you measure the thrust (say on a test rig), you can calculate the speed of the jet plume.

turbofan power
 

I don't know whether this question has been asked...but how powerful are jet engines? I remember seeing where it was said that the blast is like hurricane strength winds and can cause considerable damage. Are we looking at winds of more than 120mph? Could the blast uproot trees or damage houses?

Think of it this way: the engine on large widebodies can generate 60 - 70,000 lb. of thrust. If an object were to encounter the exhaust blast at close range, it would be feeling a large fraction of that 60,000 lb. force. It could easily overturn a large truck.

I know of a guy who drove his personal car about 100 yds. behind such an engine - it rolled him over completely, fortunately without injury, but the car was a mess. :ugh:

Uncontrolled airspace - is it really?
 

Need a bit of education on this one! I live in (well under actually!) an area of what I am told is uncontrolled airspace. Over our village we have frequent visits from fast military jets (Tornados from RAF Leeming) , somewhat slower military trainers (Tucanos from RAF Linton -on-Ouse), some very high altitude commercial jets (oceanic traffic I assume) as well as an assortment of micro-lites, gliders, glider tow aircarft from a small local field and at night some turbo-props, that I assume to be cargo mail flights.

The first question is very simple - how does everybody avoid each other as it is uncontrolled..... bearing in mind the variety of speeds and sophistication of onboard equipment (do gliders have any????)

I appreciate that altitude has a bearing on this separation ...although the Tornados do cover a variety of altitudes, the others could probably all be segmented. Is this and any on-board radar, TCAS etc enough to deal with this?


My second question is that I am often intriqued by who is up there and going where - is there any website that you can access with your co-ordinates and see what is tracking thru the area?

thanks

I've no doubt that someone will give you a really good explanation... but in the meantime, "Controlled Airspace" is a particular type of airspace with defined limits both horizontally and vertically within which an air traffic control service is provided and all aircraft are subject to ATC clearances. Uncontrolled airspace is much of the space outside CAS where pilots can largely fly without ATC clearance. However, even in those areas, ATC units provide a separation service to pilots who wish to participate by warning them of other traffic andaircraft flying in the immediate vicinity of aerodromes will be "contrtolled". Controlled Airspace does not always start at ground level and the high flying commercial jets you see will be in Class A Controlled Airspace and subject to ATC clearance with separation from all traffic.

I'm not aware of any web site where you can find live information about aircraft flying over you. A gadget called SBS "Virtual radar" is available to plug into your PC and this will display aircraft using a particular type of on board radar equipment called ADS-B. However, not all aircraft transmit the required information so will not be displayed. There are web sites where flights may be viewed on a map but they rely on the information I've just mentioned and it's usually delayed by 5-10 minutes for security reasons. Try www.openatc.com but don't expect to see any military jets on there!

wheels rotating in wheel wells
 

The reason why brakes are applied before the wheels are retracted is because a spinning disc (like a wheel) causes gyroscopic forces similar to that which keeps your bicycle upright. If the wheels are rotated for retraction, these forces can cause the undercarriage to shear off - not a pleasant thought.

One might think that these forces will not be large, but all those 747 wheels are quite large and heavy, causing substantial forces.

... how powerful are jet engines? I remember seeing where it was said that the blast is like hurricane strength winds and can cause considerable damage. Are we looking at winds of more than 120mph? Could the blast uproot trees or damage houses?

On a civil large bypass turbofan engine, the core flow exit velocity at full power will be circa 200 metres per second, which is circa 450 mph. If Barit1's friend was driving 100 yards behind a big fan engine, the blast he felt would have taken only half a second to travel that distance. So, yes, they are damned powerful. Obviously, bypass flow is significantly slower.

There are a number of 'training' videos around to demonstrate the power of these engines to airside workers. They usually take the form of some car or truck passing behind the aircraft and being blown away by the blast.

At Filton, they have a traffic light system on the A38 where it passes the end of the runway to stop traffic if there's something big sitting waiting to go for this very reason.

Flap Track 6,

The nozzle velocities are much higher than that.
Locally, they are just subsonic on most 70s and 80s large turbofans i.e. about 300m/s for the bypass and maybe 450m/s for the hot nozzle.

More recent engines have reduced the velocities a bit principally for noise reasons.

The expansion cone behind the jet expands at about a 7 degree angle, so the velocity reduces considerably with distance.

However, I do recall one test where a pitot rig was put behind an engine at full power to measure the blast about 60-100m away. Confused by the lack of manometer readings, we went outside to find the rig blown into the next field.

Certainly 100mph blasts may exist at some distance, so hold your hat on.

Radio keying
 

I know this may sound very basic but can anyone let me know how the radios are keyed on commercial a/c ? are the HF and VHF radios keyed from the same button and if so how are the varuos radios selected ?

Same 'press to transmit' button/switch but selected via an audio selector panel. Have a look at http://www.b737.org.uk/ for 737 info.

clifftop
 

happened in alice springs late last year me thinks.
BA flight to sydney,
one pax had a heart attack and plane landed in alice,
couldnt take off again with all pax and enough fuel.
long story short,
relief flight from yssy took requisite no. of pax and said 747 did a short field take off and barely made it
hope that helps,
A.F

Air Force One
 

In the movie Air Force One, the 747 does some dramatic maneuvers when it lands on the airport. Can a jet do this for real? Also, it then takes off with what appears to be full flaps. Can a 747 or any other commercial plane take off with the flaps fully extended?

Whilst not exactly answering your questions, this may give you a clue

http://www.pprune.org/forums/showthread.php?t=287911

Wake vortices on approach to JFK
 

I was travelling on a Jet Blue A320 on a calm evening last week when at around 18,000ft out of nowhere we hit severe turbulence for around 15 seconds. The aircraft vibrated, dipped slightly and lurched to the right. Then - all was calm.

A short time later, the captain came on to say that we had gone over the wake of a following aircraft.

This is only the second time I have experienced this - although this time is was at a much lower altitude.

I have heard how dangerous wakes can be - are they "less" dangerous if you are at a higher altidude?

Also, what is the means for escape? Do you simply keep flying on and hope you pass through it, or is there a proper procedure?

I've had a similar experience climbing out of Heathrow when we crossed the wake of another aircraft, bit of a bump and a few sharp intakes of breath from around the cabin but no real drama. I think its only a major issue close to the ground, i.e. on take off and short finals, where the wake turbulence could potentially dump you straight into the ground...hence the spacing between a heavy and a following smaller a/c

The only advantage was many years ago on a flight to tennerife S, on approach had a member of cabin crew tidying a locker above my seat & stood on the edge of the seat, a bit of turbulance saw her sat on my knee - nice girl:D

Barking dog in hold question again!!!!
 

Probably get flamed on this one but I am aware that a regular question is asked about the PTU noise on a A320. The dog barking in the hold question.

Flown many flights in an A320 and never really noticed it. Last week flew back from Cork on Aer Lingus A320 and the bloody dog was barking all the way down to take off. Really frightened the wife who thought something was wrong with the plane. 'No problems' I said 'just a self-test on the Power Transfer Unit' as I had been taught to say on this forum.

Flew BHX-DUB-BHX yesterday on A320.Dog barked a couple of times on 2nd engine startup on both flights and then silent.

Was my experience with continual dog barking down to take off unusual?

I heard the barking dog sawing his way out of the hold last time I went on a 320...! ;)

Concorde's afterburners
 

Can anyone tell me if it was normal for concorde to use less than all 4 afterburners in the early stages after take off.

I ask because a member of another (non aviation) website I use commented that he knew when concorde was late taking off because all four A/B's were lit. Now that sounds like cobblers to me, I always assumed they were all on OR all off. Cannot see what difference it's departure time would make in a decision to rocket skywards???

Cliff

Never did see it take-off at Heathrow without all 4 afterburners on. Soon after take-off when at safe speed and height I think they were switched off for noise abatement.

Musket 90 said:
"Never did see it take-off at Heathrow without all 4 afterburners on. Soon after take-off when at safe speed and height I think they were switched off for noise abatement."


That's what I thought. Thanks for the reply.

I must have cleared Concorde for take-off a thousand times or more at Heathrow and it always used all four.

That'll do for me!:)

For supersonic acceleration (Mach 1 to about Mach 1.7), Concorde needed all four reheats (too much asymmetric thrust with three).

So if for whatever reason one of the reheats didn't light at the start of the take-off roll (which occasionally did happen), the take-off was abandoned, and the aircraft would return to the stand.

Such a rejected take-off was a pretty straightforward affair, because the aircraft would usually not have reached more than about 50mph.

I am sure I was told by a Concorde Captain that No.4 was throttled back a tad on take-off. There was a good reason for it too.

Any idea what the good reason was?:confused:

All explained at the bottom of this page on the Concorde SST website (not that I understand a word of it of course :confused:)

I think that clears that up then. A problem free take off would see all four afterburners at full whack after 60 kts, therefore all 4 flamethrowers should be clearly visible once airborne. Ipso Facto, the geezer to whom I originally referred is a muppet.

Cheers me dears!

Cliff

Cliff,
I finally cottoned on to the weird reasoning your muppet must have been using.....
Is that some strange urban legend doing the rounds?