The relationship between Thrust, Fuel flow, Airspeed, RPM, Altitude, ambient pressure

[manucordier]

Hi,

Does anyone have a clear explanation to understand the logic behind the answers to the 3 following questions (JAA ATPL exam, FlyAround):

Question 1 :

At constant thrust and constant altitude the fuel flow of a jet engine:

Answer: increases slightly with increasing airspeed.

Question 2 :

At a constant Mach number the thrust and the fuel flow of a jet engine:

Answer: decreases in proportion to the ambient pressure at constant Temperature.

Question 3 :

The thrust of a jet engine at constant RPM :

Answer: increases in proportion to airspeed

Thank you for your help,
Emmanuel Cordier.

[Keith.Williams.]

Question 1 :

At constant thrust and constant altitude the fuel flow of a jet engine:

Answer: increases slightly with increasing airspeed.

Increasing airspeed increases the mass flowrate of air going through the engine, so it requires more fuel. The Fuel Control Unit will make the necessary adjustments without pilot intervention.

The "constant thrust" part of the question is a little bit questionable in view to Question 3 below. Use of the words "At constant throttle setting..." would have been better.

Question 2 :

At a constant Mach number the thrust and the fuel flow of a jet engine:

Answer: decreases in proportion to the ambient pressure at constant Temperature.

Flying at constant Mach and constant temperature means that the TAS will also be constant. If the air pressure decreases, the air density will also decrease. This will reduce the air mass flow rate, so the fuel flow will also reduce.


Question 3 :

The thrust of a jet engine at constant RPM :

Answer: increases in proportion to airspeed

This question is a little bit dubious.

If you look at the graph of thrust against airspeed for a pure (no by-pass) turbojet engine you will see that thrust decreases gradually up to about 250 knots then increase gradually at higher airspeeds. This question would be oK had it specified (a non by-pass turbojet engine in the higher airspeed range).

[manucordier]

That's another very clear answer Keith.
Thanks a lot.

EmmanueL.

[kaptn]

The common factor of these questions is fuel flow which is directly related to the air density. Air density decreases with altitude, requiring less fuel. A combination that can be used to find out about thrust, airspeed,RPM and ambiant pressure :

1-Constant altitude => constant density (approximatly constant) + constant thrust (fluel flow constant) ==> If Airspeed increases => engine will need to regulate its fuel consumption as more air is entering the engine making the mixture less rich in Fuel.

2-MAch number is directly related to the Airspeed .The airspeed is measured by the pressure diffrence in the pitot tube, and the pressure is impacted by the variation in Temperature/density...If the pilot makes a descente/climb with a constant airspeed (constant mach), then air temperature/density will vary with altitude....And the same as the first question, air entering the engine creates the need to increase Thrust/Fuel Flow to meet the mixture requirment for the Airspeed set in the altitude set.

3-Constant RPM does not mean constant airspeed neither constant thrust normally, because if you are flying with an engine at 3000 RPM in an area where air density in not constant, then more or less air is needed to the amount of fuel injected..hence thrust increases with more air and decreases with less air.


Engines have a control system that control a lot of parameters (fuel flow, Fuel T°....) When the pilot sets up the autopilote ,autothrottle...in a certain altitude, the control system will make the necessary variations in airspeed, thrust... to keep the required values (set by the pilot) constant.
Hope that help.s