a question or two

[dynamite dean]

I am having difficulty in finding an answer to this question....
I have seacrhed for it here but I find experts conflicting answers and in the process I have confused myself!

1. On the take off roll does EPR decrease or increase...and why ?

I think it increases because a jet depends on speed, temp and pressure but reading the tech forum search threads some people say increase and others say it decreases. The best one I have heard is that it decreases but due to ram effect thrust increases Please could someone explain to this piston pilot why? As I thought EPR is a measure of a jet engines power being developed. So this would appear to me a bit conflicting.:eek:

the next few I need a little assistance....

2.What are the effects of compressibilty - how do I answer this one?

3.what does cost index mean in an aircraft - does this mean how much fuel you are allowed to burn?

4.Could anyone tell me how an INS finds true north - Gyrocompassing?

5.What effects AoA at the stall - I didn't think that AOA could be effected at the stall only the speed

I have found answers to most of my questions but these residual ones have appeared!

Thanks Dynamite

[mono]

Dynamite,

I will try to answer a couple of your questions.

EPR as you correctly state is a measure of the thrust produced by a Jet engine. It is, as its name implies, the ratio of what comes out the back over what comes in the front. The inlet pressure P1 is sensed by a single probe (like a pitot probe), which is usually inside the engine inlet cowl, but can be on the spinner (B727) or the pylon (B707). The exhaust pressure P7 is sensed by a manifold or rack of tubes with holes in the jet pipe.

In the main all EPR indicating systems will, assuming TO power is acheived before the a/c is rolling, show a decrease in EPR as the a/c accelerates down the runway. and this is quite simply because the pressure at the inlet probe increases as the ram air effect is sensed at the probe. A/c with the probe mounted in the inlet will only show a small decrease because the probe also senses the pressure of the air being sucked into the engine. The old 707 however, with its probe mounted on the pylon, away from the air being sucked into the engine used to show a marked reduction in EPR as it trundled down the runway.

Cost index is a figure provided for the a/c usually by the airline planning and performance dept. I can't give you any specifics, but it is used by the FMC to compute optimum climb and cruise performance levels. A low cost index will cause the FMC to calculate performance levels to provide a lower fuel burn. With a higher cost index, fuel burn becomes less important and speedy enroute times more important. The final figure is a compromise between fuel burn and enroute time and may change depending on the route being flown.

INS, ahh, that old chestnut. The first and most important thing to realise is that neither INS or IRS use gyros to sense north. The gyros are merely there to either electronically or mechanically maintain the INS/IRS platform level and aligned with true north.

The system senses north in the following way:- there are 3 sensitive acceleromerers each aligned 90 degrees relative to each other. We'll call them the vertical, lateral and horizontal accelerometers. During the initial alignment process, the outputs are used to calculate the vertical axis (in the old INS systems the platform was actually moved by motors so that the vertical accelerometer was physically aligned with the vertical). Once this is known then the outputs from the accelerometers are used to sense acceleration forces due to the rotation of the earth and it is the resolution of these forces that aligns the INS along the true north/south axis. Note that due to the obvious equitorial ambiguity (acceleration forces at 40 degs N and 40 degs S are the same) it is not until the a/c present position is put into the INS that true north is known.

Hope this helps

[Notso Fantastic]

Interesting questions! Maybe the EPR does change. Never been looking at it to notice. On the Boeings I've flown, autothrottle sets the required thrust and i haven't been aware of it modulating the thrust levers to maintain constant thrust. I think any change on big fan engines must be very minor.
Question 5- back in the 70s I was dragged out on standby to copilot a Certificate of Airworthiness test on a VC10 fitted with an AoA meter. Following the incidents of superstalls on Tridents and BAC 1-11s (always fatal), I was not altogether happy to find myself doing stick pushes at 15,000' over Anglia. Speed was reduced to below 100kts and AoA hovered at 15 degrees, then twitched, then jumped to 17 degrees. The stick push horns cut in and the stick was pushed forward and suddenly there was a nice view of farmland. Nice to be alive and enjoy it, but Notso couldn't help thinking 'very good theory, but what if it doesn't work?- I'd really rather be home reading the Sundays!' Back to your question- I assume if you stalled in a bank situation (increased 'g'), I think the AoA would be different?
To expand slightly on INS & True North, systems cannot sense the direction too near the Poles, so INS sets, although they can fly over the Poles, cannot be aligned preflight near the Poles. I forget the limitations, but I think if a 747 started out within about 20 degrees latitude from the Poles, the INS sets would have difficulty aligning themselves. Used to take Classic 747s about 20 minutes, the 400 version about 7.

[411A]

With manually set takeoff thrust on turbofan engines (L1011, B707 for example), the thrust must be set before a certain speed, between 40 to 80 knots for instance. To try and increase EPR to counteract the ram air EPR reduction will result in engine overboost.
INS's will align near the poles, but the time required can be rather long.
Having done full stalls in large transport jet aircraft (TriStar), the view can be, as Notso has indicated, quite exciting.:eek:

[lomapaseo]

Mono

Good answers, but the idea is to ram the air on takeoff roll not suck it. Therfore once you get moving enough to increase the pressure, there is no corresponding drop in pressure due to sucking.

[Mad (Flt) Scientist]

Originally posted by Dynamite Dean
2.What are the effects of compressibilty - how do I answer this one?
Compressibility is basically the effect of Mach number. The following are the main aerodynamic effects of Mach number.

In the low to mid Mach number range - up to say M0.6-M0.7, depending on the design of the aircraft - there are few effects and these can be safely ignored for practical considerations. One would consider an aircraft where this applied to be purely 'subsonic'.

As Mach number increases further we enter the 'transonic' region. This is where the local flow on various parts of the aircraft is approaching sonic velocity - typical areas where this will happen are at the point of maximum wing thickness on the upper surface, as the flow is accelerated over the wing, and also near the first area of increased fuselage cross section (near the cockpit for most designs) again due to flow acceleration.
As the flow in various places approaches then becomes sonic some or all of the following will be happening:
Increased drag - the so called 'drag rise'.
The drag for a given lift or angle of attack will increase pretty dramatically as the flow becomes sonic over the wings, in the case of most transport aircraft this drag rise will impose a performance-based maximum level speed limit.
Changes in handling
At transonic speeds the formation of shocks on the flying surfaces will drastically change the effectiveness of the control surfaces; trailing edge surfaces become rather less effective when located behind a shockwave. Marked reduction in roll control power from ailerons would be one example, and spoilerons or differential tail deflection (on fighters, typically) are used to provide a compensating increase in control power.
The shock system also changes somewhat the underlying aerodynamic stability characteristics of the aircraft; the static margin will increase massively as the aerodynamic centre moves aft, classically moving from about 25% chord to 50% chord or so. This makes the aircraft far more longitudinally stiff, and coupled with loss of elevator power can cause significant control problems, especially if significant pitching moment changes occur. Many early fighters were lost in transonic dives due to loss of pitch control effectiveness.
The aircraft will also become markedly less directionally stable in most cases - the fuselage destabilising effect becomes far greater and the ability of the fin to compensate does not increase correspondingly. That's why you see the following design features on some supersonic aircraft: very large fins (sized for the transonic/supersonic case), multiple fins (which are proportionally more effective than a single fin transonically/supersonically) and deployable fin surfaces (XB-70 wing tips, MiG-23/27 ventral fin)

5.What effects AoA at the stall - I didn't think that AOA could be effected at the stall only the speed

The stalling angle of attack is affected by many things, including the folllowing:

Aircraft configuration i.e. deployment of flaps and/or slats.
All other things being equal, increasing the camber of an aerofoil by deploying flaps will reduce the stalling angle of attack. Likewise, deploying slats will increase the stalling angle of attack. If you mentally picture the classical "fish-hook" CL-alpha plot, flaps cause the line to move upwards and slightly left (less alpha) while slats extend the line upwards and to the right.

Control activity
Use of wing-mounted control surfaces will also modify the angle at which the flow stalls on the local area of the wing. So trailing edge down aileron causes that section of the wing to stall at a lower angle of attack, while t/e delays the stall slightly. Spoiler deployement, by unloading that section of the wing, will slightly delay the stall AOA in a like manner. This is why it is not possible to raise the heavy wing in the 'normal' fashion close to the stall; you're actually making things worse, not better, with control activity.

Mach number
The stalling characteristics of an aerofoil are influenced by the Mach number. At higher mach the wing will stall at a lower angle of attack.
Reynolds number
The flow behaviour is very dependent upon Reynolds number, which is a measure of how easily the flow will trip from laminar to turbulent flow (crudely). This dependence cannot be simply expressed, because it can result in very dramatic changes to the type of stall experienced, depending on the actual Reynolds number.
The practical effect for an aircraft of the Mach and reynolds number variations show up in the variation of stalling behaviour with altitude, load factor, weight, etc.
Altitude affects the behaviour because at higher altitudes a (constant) stalling speed would result in a higher Mach at higher altitudes; the Mach effect would then reduce the stall AOA. The increase in altitude will also reduce the Reynolds number for a given speed. Combined, this makes the efect of increased altitude to be a reduced stall AOA.
Weight and Load Factor ('g') both work in a similar fashion. Increase in either requires the generation of more lift, and so a higher speed is required. This speed increase will result in a higher Mach and higher Reynolds number. Generally the higher Mach number effect will dominate, and a lower stalling AOA will result.

In all of the above when talking of stalling angle of attack we are referring to the actual aerodynamic stall of the wing. To further muddy the waters this may not be what is seen by the pilot to be stalling AOA.
On aircraft with stall protection systems, the aircraft may have a stall which is defined by operation of e.g. a stick pusher. The operation of this is determined by the logic built into the system, and this may not incorporate all of the above effects. For example, while use of configuration, MAch and/or altitude for SPS inputs is common, few if any manufacturers will use control wheel (aileron). The pusher firing angle is instead set to allow for variation in control activity and demonstrated to be safe for these. It will therefore appear to the pilot that stall AOA is not dependent on control activity - because the artificial system is masking that variation.

[dynamite dean]

That's a big help great;)

[boofhead]

Another reason for a different aoa at the stall is ice or other contamination on the wing.

Some airplanes (and most in certain configurations) will lose elevator effectiveness before the stall can occur, thus the wing will be at an aoa lower than normal stall aoa.