Query on high altitude fuel flow

[Mr_sukebe]

Out of curiosity, took my hornet to 50k feet yesterday.

At higher altitudes, max fuel flow appeared to dropping.

Would I be correct in assuming:

- the engines are simply not getting enough air to make use of more fuel, so the engine controls are deliberately restricting fuel flow?

- is this why jets (eg airliners) fly at such altitude, as I was still at high Mach numbers and ground speed, regardless of the low fuel flow?

 

Have to say, made me wonder about Concord. How did they manage to get that to fly at 60k feet and Mach 2 for 3000 miles. Just wow.

[David OC]

Quick answer

 

Low density causes low drag and the aircraft flies much faster at high altitude.;)

 

As for engines, well.....You can read these, as I'm not an engineer.:)

 

For the F/A-18, this chart below out of the A1-F18AC-NFM-200, shows you how to get the most from "this aircraft design" what weight and height.

 

 

[mcfleck]

There are a lot of difderential equations involved...

Basically you design an airplane right from the start for the best performance in a specific scenario. Being good on one end (high speed) means nearly automatically that you are bad in other situations (slow speed, landing, ...).

Additionally the equations for calculating the involved aerodynamic forces (lift, drag) change drastically in different speed bands (subsonic, transonic, supersonic).

 

Even if your plane is moving in a subsonic speed, some parts (like the wingtips) mitght be already in a transonic or even in a supersonic region. Due to the fact that the transonic speed band has the worst relative drag you should try either to fly slower (at about M0.8) or faster than your transonic speed band (>M1.1-1.2 if you are able to).

This is the reason why "supercruise" is a key design feature for modern 4.5 and 5. gen. fighters.

Taking into account that cruising with afterburners on is never the most efficient way to go, you want rather stick to the subsonic bands for maximising efficiency and minimising your fuel flow (without the supercruise capability).

As a rule of thumb you should go for a speed of M0.8 and an alpha of 4.2° (was mentioned in another thread, I hope that I remember it correctly) for max range and an alpha of 5.6° for max endurance.

 

If you are flying level at M0.8 and yor alpha is lower than 4.2 and you want max range?:

-> climb

 

Else:

-> descend

 

The higher you climb -> The "thinner" the air -> The less drag and lift -> The more alpha is needed to create the neccessary lift force to keep you in the air

 

It is always a fight between cotradicting forces and effects to find the optimal point of operation, which is also constantly changing due to changing parameters (weight, environment, and so on)

 

Sorry for the way to long post and I do not claim to be an expert on this topic.

Although all the theory behind it is very interesting it is also likely to cause headaches if you are digging deeper.

[Vilab]

 

Have to say, made me wonder about Concord. How did they manage to get that to fly at 60k feet and Mach 2 for 3000 miles. Just wow.

 

Concordes were using AB to climb to ( very ) high altitude and reach Mach 1.7, at that point they cut AB and kept accelerating on dry power

 

 

subsonic drag is low, until you reach arround 0.95 Mach, at which point there is a peak of drag which climax at Mach 1.0 and slowy decrease to supersonic drag level between Mach 1.2 ( Delta wings ) to 1.4 ( regular "Arrow" wings ).

 

ie, it doesn't cost much more power to cruise at mach 1.6 than at Mach 1.1

 

 

to get an efficient engine, you want to have a "high" Density subsonic air just before your compressor, so that you can compress it further and burn lot of fuel to produce lot of thrust.

 

 

ex : the SR-71's engine get as much air density at 60 000ft and Mach 3.0 than at sea level thanks to its inlet design, it is why its engines are very efficient there.

 

what limits Mach 2+ plane's max speed are often the inlet design which doesn't allow enough air to get faster ( Eurofighter, Rafale ), the engine which would melt due to too high incoming air temperature ( Mig-25 ) or structural limits ( F-16 canopy )