crusing on heavy and low weight

[Captfahad]

Hi guyz plz solve this question if for example an aircraft of same type and same altitude lets suppose fL340 and one aircraft is heavy in weight and other is low weight which aircraft will consume more fuel i know the heavy weight aircraft has to go at high AOA then light but doesnt it increases Mcrit at high AOA plz do rep it wud be an appreciable act thanks...

[flight_mode]

The heavier aircraft will consume more fuel

[OhNoCB]

Higher weight means higher angle of attack to generate the same amount of lift if all other conditions remain equal. However higher angle of attack means higher drag which means you will need more thrust to maintain the same speed.

Mcrit does not change with angle of attack, it is the speed at which somewhere on the airframe the local flow will go sonic.

[Captfahad]

I read this one one forum it says...

At FL200 ,Assume Aerofoil 'A' 's airflow reaches M1.0 on the upper surface at forward speed of M 0.85, This defines Mcrit of A as Mcrit=M0.85, which means the upper surface of the wing has accelerated the air by M0.15

[OhNoCB]

Yes that is Mcrit as I defined in my previous post.

... it is the speed at which somewhere on the airframe the local flow will go sonic.

To explain further if required (apologies if it's not), The acceleration you describe is one of the basic theories of how a wing produces lift. It is shaped in such a way that the air going over the upper side is accelerated. According to Bernoulli's principle, whenever a fluid (air in our case) is accelerated, there is a drop in pressure. When you have a lower pressure on top of the wing compared to a higher pressure below the wing, the wing will want to move into the area of lower pressure thus 'lifting'.

The problem of Mcrit is that the airspeed (or Mach no.) of the airframe is not equal to the speed of the air everywhere on/around it. It doesn't take into account this acceleration over the top of the wing. This means that at some Mach no., you are going to have the speed over the wing reaching the speed of sound / going sonic / Mach 1.0.

When this happens the properties of the air change, and you get energy losses and extra drag, which is NOT a desired thing, thus you avoid letting the air over the upper surface reach a sonic speed, and the only way that you can do this from the cockpit is to not exceed Mcrit, which is the speed that the clever people have worked out above which a sonic flow will happen somewhere (for example, M0.85).

[keith williams]

In the statement that:

Mcrit does not change with angle of attack, it is the speed at which somewhere on the airframe the local flow will go sonic.

The part that says:

it is the speed at which somewhere on the airframe the local flow will go sonic.

Is correct.

But the part that says:

Mcrit does not change with angle of attack,

Is not correct.

When we are flying at Mcrit the fastest airflow at any point over the airframe is Mach 1.

This (Mach 1) is the sum of Mcrit + the greatest acceleration over the airframe.

Rearranging this equation gives us Mcrit = Mach 1 - the greatest acceleration.

If we increase the angle of attack we will increase the acceleration over the aircraft. This means that we do not need to go so fast to achieve Mach 1 airflow at some point over the airframe.

So increasing angle of attack will DECREASE Mcrit.

[Captfahad]

soo if we are flying at Vmo the maximum operating speed of aircraft and increase the maximum AOA the airflow wont increase up to Mcrit?

i was wondering that as heavy aircraft fly at its maximum cruise altitude where an aircraft reaches its maximum speed where speed cannot be increases if it were to happen the aircraft will reach to the mach and where there is a problem to be tuck under , as it reaches to the point where he can no longer increases the airspeed he will increases the AOA to compensate for the lift to weight ratio, so when he increases AOA wont the wing increases airflow to 1 mach ?

[keith williams]

With modern airliners nothing particularly exciting happens at Mcrit. The vast majority of cruise flight for such aircraft occurs at speeds slightly higher than Mcrit, but lower than Mmo/Vmo. In suitable conditions the passengers may see the shock waves on the wings. But these shock waves are not intense enough to cause any serious problems.

The factor that really places a practical limit on cruising speed is the large drag increase that occurs at Mcdr (Critical Drag Rise Mach Number). This makes it uneconomical to cruise at higher speeds. Mcdr is higher than Mcrit.

When told about Mcrit, shock waves and tuck under, many students imagine that that aircraft will enter uncontrollable dives and the world will end whenever Mcrit is reached. This simply is not true.


Your comments are very confused, but I will deal with each of them in turn.

soo if we are flying at Vmo the maximum operating speed of aircraft and increase the maximum AOA the airflow wont increase up to Mcrit?

We would not be cruising at Vmo. But if we were to increase angle of attack this would slow down, decrease Mcrit and also increase the acceleration of the airflow. If we were to increase thrust to maintain our airspeed and if our initial speed were close enough to Mcrit, then yes we would eventually reach Mcrit. And the airflow over some point on the aircraft would reach Mach 1.

i was wondering that as heavy aircraft fly at its maximum cruise altitude where an aircraft reaches its maximum speed where speed cannot be increases

In normal cruise flight the aircraft is not flying at its maximum possible speed. Cruise speed will be slightly less that the speed at which high-speed buffet occurs.

if it were to happen the aircraft will reach to the mach and where there is a problem to be tuck under ,

In order to get into a tuck under situation the pilot would need to ignore the high-speed buffet and keep on accelerating. He/she would need to be a fool to do this.

as it reaches to the point where he can no longer increases the airspeed he will increases the AOA to compensate for the lift to weight ratio,

This statement makes no sense. In cruise flight the weight gradually decreases as fuel is burned. So why should the lift to weight ratio require an increase in speed or angle of attack?

[Captfahad]

Thank you soooo much williams i hav got it now :-) thanks for the detailed answer and urime appriciate it bro