Q's for you all

[SixDemonBag]

Just for interest:

-Does a jet have 'critical' engines?

-Would you rather lose the upwind or downwind engine on a jet?

-Why do Turboprops like the old Herc's and the P-3C orions, even the Electras have square tip props? What is the reasoning?

That last one's really bugging me. Thanks!

[Zeke]

If the tips were any further out they would break the sound barrier and be very noisy and not very efficent.

Q tips just add an aerodynamic fence to make the aerofoil more like a 2D wing and reduce the induced drag.

Cheers

Z

[DEFCON]

Imagine a 4 jet (engs on wings) taking-off near to max cross wind limits. If you have lots of rudder already put in for the take-off roll and you lose either outboard eng, then one of the engines failing will require a lesser rudder input and the other outboard eng failiure more rudder input (with nearly if not max rudder input already in to combat the crosswind). Therefore that outboard engine, requiring more rudder than that available on the take-off roll, might have you coming off the runway on the take-off roll unless you reduced the thrust that is dragging you off the runway.

EVEN I DON'T UNDERSTAND THAT SO EXPL BELOW.
Ie; taking off to the N with wind from the E means that you would have left rudder in on the roll as the tail is pushed to the W and nose to the E. If the No 4 eng (1234 L to R) fails pulling nose to R with 1+2 eng pushing L wing to R means you need even more L rudder, which is fully applied with max crosswind anyway = going off runway to the R unless power down ENG 1+2. THERFORE NO 4, OR INTO WIND OUTBOARD ENG IS THE CRITICAL ONE ON A JET.

[SixDemonBag]

Thanks you two.

So are there any factors that would create a critical engine on a jet in NIL wind? And does the airflow into the upwind/downwind engines on take-off factor into this as well?

Also, the square tip props, I would have thought tapered props would have produced the least induced drag (like the design of modern wings at the moment). If Square tips are more efficient and reduce noise, why aren't they on everything?

More questions....i know, sorry.

[Ceppo]

I'd imagine the props are sufficiently short as to allow the tips to remain subsonic. Maybe it's an aerodynamic improvement on the propellers themselves. Rounded tips cause less induced drag than square ones so it's a bonus really.

Also Flying INTO wind is better on take off because it increases the indicated airspeed.

In other words. if the aircraft is travelling up a north pointing runway at 80 knots and the wind speed is 80 knots in a southerly direction, the indicated air speed (IAS) will be 160 knots. If it can take off at 130 knots, then the aircraft could actually take off at the 80 knots it is actually travelling at (Ground speed)

Clearly if the opposite happens..i.e northerly wind of 80K and the aircraft is heading down a northerly runway, then it would have to take off at 210K (ground speed) which would be 130K (indicated air speed) I don't know how the engines react in this situation. I can't imagine them haing many problems with either.

(I don't know if that's what you were asking or whether you wanted information only on the engines.)

[This message has been edited by Ceppo (edited 02 August 2000).]

[SixDemonBag]

I mean't up/downwind is a lateral situation, Where there is shielding from the fuse. Thanks anyway...

Still, are there any real facts about those square tips???

[Two Bars]

In a jet aircraft in very strong crosswinds, it is possible to get compressor stalling (quite spectacular sometimes) on the upwind engines because of inlet airflow disruption. The downwind side is not affected as much since the fuselage shields the downwind engines from the full brunt of the crosswind.

[SixDemonBag]

O.K...Thanks for all the info, but let's start this question again.

I understand that airborne, the relitive airflow is not affected by wind (As the aircraft it travelling with the wind in terms of drift...obviously). However, on the ground, in the take-off roll, where the effect of a crosswind is evident...which engine will create the most asymmetric thrust is it were to fail? I already understand the fact you may be holding rudder into wind (therefore reducing you travel available) and it may limit your controlability initially. But any effect to engine perfomance in a crosswind?

[Gaucho]

Q for you.
When is the Cathay interview?
:rolleyes:

[deltahotel]

no effect on thrust. it's all to do with controllability(?), rudder authority and swing due to the failed engine

[SixDemonBag]

Cathay??? Haven't even got the application in there buddy. Maybe next time.

[moon_towers]

Gaucho,
In a few days,this month.
see ya.

Wish you luck.

[mustafagander]

Two Bars,
Never seen the upwind engs surge/stall in xwind, but I've seen and heard some ball-tearing surges on downwind engines. Anyone who operated B747s with JT9D-3A engs will know what I mean, especially in reverse!!!

6DB,
As I understand the term, a jet does not have a critical engine.
If I must lose an engine on a xwind t/o I'd prefer the downwind one because I wouldn't be likely to run out of rudder. Reversing the rudder input can be mighty tricky though!!!

[Gaucho]

moon_towers, sorry been there (a few years ago)even the cocktail party, but wasn't good enough!
Good luck to you. :)

[This message has been edited by Gaucho (edited 04 August 2000).]

[unusualAttitude]

I've enjoyed this thread. Fascinating. However, it's got my juices running (groans can be heard from far away!).

Information I recently received from Tor regarding the crash of the Tu-144 at an airshow in France mentioned that the a/c suffered 'compressor stall'. Others on this thread have alluded to this phenomenon.

Give me some BGT here - explain c/stall to me. Seems to me that the eng's compressor disc(s)--being driven by the turbine--will always be able to 'suck' in its air requirement (it must do when the a/c is stationary on the ground).

Come on back!

[Zeke]

Compressor stall is a reduction or disturbance to the airflow in the compressor resulting in a reduced axial flow component.

As the axial flow component reduces the angle of attack of the compressor blades increases. Compressor stall is more prevalent in axial compressors.

Once a compressor stage stalls this will normally have downstream effects of subsequent stages, and cause abnormal pressure gradients (the pressure gradient should try and put air towards the exhaust). The reduction in the pressure gradient can lead to a reduction in the axial flow to a point where the flow can stagnate or even reverse in very severe cases.

After the engine is stable other causes of compressor stall are :

Poor air flow coming into the inlet :

Turbulence
High angle of attack
High sideslip angles

Poor fuel or fuel control management

Excessive fuel flow
Rapid opening of the throttles
Malfunctioning fuel control unit or governor

High altitude flight
At very high altitudes the air can become thin that at a given RPM the compressor blades will stall

What you see/hear

At the lower end you may get some slight vibration and little if any cockpit instrumentation changes. More spectacular stalls (and generally more expensive) will show up as fluctuation in RPM, EGT, or loud bangs or metal coming out the rear.

How to fix it …

Reduce throttle, angle of attack, sideslip (may not be possible in a cross wind), or height.

Other ways to over come stalling include …

Multiple spool design so that each spool is operating near design RPM, the RB-211 is an example of a triple spool engine

Bleed valves to reduce any excessive pressure build up.

On a PT-6 the compressor consists of axial and a centrifugal compressor stages, and during a start at low RPM the axial compressor is more efficient than the centrifugal stage and can cause a back pressure on the axial stage. To relieve this they have a compressor bleed valve which bleeds off P2.5 air (air after the axial compressor) until the engine RPM increases and slowly closes between 70-90% RPM.

Airflow conditioning devices like variable stator or guide vanes to improve the airflow direction onto the compressor blades.

Cheers

Z

[silkhands]

Yes, there's a critical engine in a Jet and it depends only on its spin.
Lets say that the engines on a specific aircraft spin clockwise from behind.
The left engine would be the critical because the right engine will have the biggest momentum.
That's because on the right engine the center of momentum will be slightly to the right of its axe due to the spin direction.

[Checkboard]

With regard to square tip props:

Square tips are less efficient than eliptical tips, and weigh more. They are, however easier to produce, and have plenty of waste material that can be removed (filed) when the prop experiences leading edge nicks - probably a factor on the C-130 as it is designed for low grade strips. As the thrust produced at the tips is minimal, the efficiency loss is low.

Silkhands, your comment applies to props ( "P-factor" ), but not jets (although the bit about momentum is out of left field). Sorry.

[This message has been edited by Checkboard (edited 07 August 2000).]

[Big jugs]

Checkboard,

I thought the factors that effect the ability of props to absorb power are:
1. Increasing the number of blades
2. Increasing the chord
3. Increasing the camber
4. Increasing the blade angle
5. Increasing the RPM
6. Increasing the length



[This message has been edited by Big jugs (edited 07 August 2000).]

[Wouldya blowme]

I also believe that square tip props have more "solidity" and therefore can produce much more torque. So even though the P-3 and C-130 have the exact same engine, the c-130 produce more torque. The loss of efficiency with the square tips at cruise speed (300 Kts) in the c-130 means that the p-3 with its rounded props can crz a lot faster.

[Weary]

Silkhands - you post sounds like something that came out of the Monty Python school of logic. Whatever those pills are that you are taking, stop it. ;) ;)

SixDemonBag - In reference to square tip vrs round tips. Yes, they produce more drag, BUT, if you are trying to put the maximum amount of power into a propeller, for a given rpm and pitch, and assuming a constant number of blades, you can only then increase the area of blade. If you have already reached the limiting diameter (i.e. trans-sonic tips), that only leaves increasing the blade's chord, or it's camber. Square tips are an improvement in the former. Square tips = "wider" chord = more area = more thrust (just like a helicopter blade). For a given blade diameter, a round tip prop cannot produce the same thrust as a square tip, unless it has a wider chord elsewhere. It is however, like everything, a compromise.
The peculiarities of the engine it is mated to is a big factor in chosing the design, as is it's anticipated operating environment. A given aircraft might perform better using square tips instead of round tips, because of the power curve characteristics of the engines and the flight envelope and role of the aircraft itself. As Checkboard points out, a big avantage of a square tip is it's resilience to stone chips (perhaps SFTOL operations), and ease/economy of manufacture.
It's clearly a case of horses for courses.


[This message has been edited by Weary (edited 07 September 2000).]

[This message has been edited by Weary (edited 07 September 2000).]

[Bullethead]

The square tipped blades on a Herc prop are for high thrust at low speed, useful for dragging the aircraft out of short unimproved strips. Problems of transonic flow at the tip are avoided primarily because the shape and therefore the drag of a Herc prohibits a really high speed cruise. Hercs cruise at around 300KTAS.

The Orion however, only operates out of longer hard surfaced runways and spends the majority of its time in high speed cruise and so has a rounded tipped props.

If you compare the two aircraft you will notice that the Orion has a much thinner wing and smaller diameter fuselage than the Herc, also the Orions MAUW at 135,000lbs is 20,000lbs lighter than the Herc.

:): :):