Performance query

[Chokdee]

Can someone confirm what happens to your MTOW and V1 with an increased clearway.
Which is more efficient in the cruise, forward or aft C of G.
Finally, are stators divergent or convergent:rolleyes:
Thanks.

[Bigmosquito]

Very good question.

I believe that an increased clearway will lead to an increased MTOW but will also lead to a lower V1.

Lets start by the definition as always...

TODA = RWY LENGTH + FULL CLEARWAY

Therefore if clearway increases, the TODA will increase as well thus allowing a higher TOW.

However, assuming that the RWY length is unchanged, a lower V1 improves the stopping distance on the RWY.
It should be appreciated that using 50 meters of clearway and lowering V1 just 1 knot is enough to raise the TOW by almost 500 kg (source taken from the book "Aerodynamics for Airline Pilots"; exemple from a DC-9).

In conclusion, if you have a long stopway, V1 can be increased but in the other hand, if you have a long clearway, V1 can be reduced.

For your other question, I don't know...
I have to go back in my library and dig into the books.
Will let you know if I find something.

Cheers.

[411A]

Aft C of G generally, but TOO far aft, not good.

Opps, forgot to add, applies to sub-sonic aeroplanes.

[Bigmosquito]

Aft of CG is correct. It lowers the drag...

Concerning the stator. I guess you are talking about engine...
They are of course convergent.
The 2 basic elements of the turbine assembly, stator and rotor.
The stator, or turbine guide vanes serve two functions.

The first function is to discharge the gas at as high speed as possible. The vanes accomplish this because the individual blades are contoured and placed at such angle so as to form what is effectively a series of small nozzles.
As the air flows through this restricted area, a portion of the gases heat and pressure energy is converted to kinetic energy that is further converted to mechanical energy when the air passes over the rotor blades.
At the same time, because the turbine guide vanes are set at a specific angle, the second function is accomplished, which is aiming the gas-flow at the rotors in the most effective manner
(source taken from "Advanced Aircraft Systems" by David Lombardo).

Hope it answers your questions...

Cheers.

[Mark 1]

There is an increase in area and a drop in static pressure as you pass through the stators, so they are divergent.

Their main purpose as I understand it, is to take out swirl from the previous stage and align the flow with the next rotor stage.
The amount of expansion is small compared to the rotor where work is extracted as the flow expands.

[lunkenheimer]

Aren't stators used in both the compressor and turbine stages of many engines? If that is the case, I'd expect a compressor stator to be convergent, and a turbine stator to be divergent.

[m&v]

Bigmosquito,don't reduce the V1 too much with the clearway credit.
Balanced(old fashioned now),used to be the same distance to Acc/stop,as go' to the 35' screen at the end of the 'dry' runway.
With the credit of the clearway,and providing one could still stop on the concrete,one was allowed to increase the Towt.
Wet'conditions introduced the 'reduced 'V1(to enable the A/craft to stop,at the same weight,on the runway.The 15' screen,was a compromise,to allow the a/craft to go!!!!.
The present quandary ,is that the 15'screen is at the end of the clearway(JAR25.111).In reality one crosses the 'end' of the concrete at about 8/9';) :rolleyes:

[Keith.Williams.]

Chokdee,

Your questions suggest that you are preparing for the JAR ATPL exams. The answers below are based on the "conventional wisdom" required for those exams.

INCREASED CLEARWAY
The effect of an increased clearway depends on what exactly is limiting your mtow. If it is the ASDA or TORA, then the extra clearway might have little or no effect. Remember you have to get from V1 to VR and VLOf within a specified fraction of the TORA, as well as reaching at least V2 by the end of the TODA. You really nead to look carefuly at the question to see what kind of situation the examiners have in mind. The clues are often in the options, rather than the question itself.


C OF G POSITION
The entire allowable C of G range is in front of the neutral point. The futher forwad we move the C of G the greater will be the tail down force required to keep the nose up. This increases the stalling speed and trim drag. So range goes down, as C oF G moves forward. It does however make the aircraft more stable.

Moving the C of G aft within the allowable range reduces the need for tailplane down force, so the trim darg and stalling speed reduce and range increases. But stability decreases.


STATORS
In an axial flow compressor both the stators and rotors form divergent ducts. The rotation of the rotors accelerates the air, while the divergent ducts between the rotor blades decelerates and compresses it. The overall efect is that airflow velocity goes up in the rotors and down again in the stators. The pressure and temperature go up both in the rotors and in the stators.

TURBINES
The shape of the ducts between the stator blades depends on the type of turbine.

In a pure inpulse turbine the ducts between the blades are convergent in the stators and parallel in the rotors. This is because impulse turbine rotors extract energy by changing the direction of the airflow.

In a pure reaction type turbine the opposite is true. The ducts in the stators are parallel and those in the rotors are convergent. This is because the rotors in reaction turbines extract energy by accelerating the gas. It most not go supersonic, so we really don't want it to be going too fast when it enters the rotors.

In a real gas turbine engine the turbines are a combination of impulse and reaction. They tend to be mainly impulse at the blade roots and mainly reaction at the tips. So the ducts between the stator blades are convergent at the roots and parallel(ish) at the tips. This is the type of turbine the examiners mean when they ask about an "impulse root turbine".