ATPL Perf and Load Question

[Hailstop3]

Hi

I'm in the process of self studying my Australian ATPLs and have my performance and loading exam on the 13th. I think ive got a grip on it all except for one thing that is really bugging me. My current study notes don't seem to cover turboprop aircraft too well which is half the problem.

Here are 2 questions.

A turboprop aircraft's max L/D speed (best lift drag ratio speed) is at Y. At what speed do you need to fly to achieve max rate of climb?

A turbojet aircraft's max L/D speed is at Y. At what speed do you need to fly to achieve max angle of climb?

Both questions have the options Y, less than Y, greater than Y.

My problem is that the correct answer for both is Y.

I understand that obviously there is a difference in relation to best angle and rate of climb from the power required/power available curves, and drag curves, that is due to their propulsion methods, but could someone please explain it all relating to the 2 aircraft as i can't seem to nut it out.

Thanks in advance.

[training wheels]

I understand that obviously there is a difference in relation to best angle and rate of climb from the power required/power available curves, and drag curves, that is due to their propulsion methods, but could someone please explain it all relating to the 2 aircraft as i can't seem to nut it out.
You mean you actually wanna get the correct answer and understand why as well? My hats off to you, matey! ;) (..joking ..)

I don't know whether this helps but here are my notes on climbs;

The angle of climb depends directly on the amount of excess THRUST available and the weight of the aircraft.
The lower the weight, the greater the angle of climb
The greater the thrust, the greater the angle of climb
The lower the drag, the greater the angle of climb
The rate of climb depends directly on the amount of excess POWER available and the weight of the aircraft
The lower the weight, the greater the rate of climb
The greater the power, the greater the rate of climb
The lower the drag, the greater the rate of climb

So you can achieve both Vx and Vy at the max L/D ratio since the difference between Vx and Vy is to do with excess Thrust and Power and not Lift and Drag.

[Hailstop3]

Quote:
The angle of climb depends directly on the amount of excess THRUST available and the weight of the aircraft.
The lower the weight, the greater the angle of climb
The greater the thrust, the greater the angle of climb
The lower the drag, the greater the angle of climb

Quote:
The rate of climb depends directly on the amount of excess POWER available and the weight of the aircraft
The lower the weight, the greater the rate of climb
The greater the power, the greater the rate of climb
The lower the drag, the greater the rate of climb

So you can achieve both Vx and Vy at the max L/D ratio since the difference between Vx and Vy is to do with excess Thrust and Power and not Lift and Drag.







That does help a fair bit, but in relation to the speed for L/D max, what i figure is that a turbo prop aircraft achieves max excess power at the point of L/D max, and a turbojet aircraft achieves max excess thrust at the point of L/D max. Is this correct, and if anyone knows, why does it occur?

Yeah i am a bit of a person for understanding why as well as knowing the correct answer, so any insight would be great.

Cheers

[Muffinman]

Mate here's my 2 bobs worth - caution I do not have a text book with me at the moment and this is from memory.

In the jet example, the thrust available curve is often presented as 'flat' as the speed increases.

Think of the thrust required curve as a drag curve. So as this curve decreases towards a minimum value then starts increasing again, the minimum value of thrust(drag) will occur at the max excess thrust or the L/D max position. This is best angle speed. Tangent this minimum curve and you get Vy - best rate - always higher in value.

In the prop case the power available curve continues moving up (increasing) before it sarts to reach a maximum value then starts to decrease over the increasing speed value range.

Consider the same power required curve underneath - again read this as a drag line and you have max angle again at the bottom of this curve BUT L/D max occuring at the speed where the greatest distance exists between the Power Aavail and power required curves - same logic with the jet - but this is best rate and is also the tangent point of the curve - again just like in the jet case.

I hope this helps:ok:

[ITCZ]

marcuste747, great question.

The problem lies in the fact that many pilots do not differentiate between THRUST, which is a FORCE, and POWER, which is RATE of doing WORK (and Work is the amount of energy transferred by a Force -- sometimes Work is defined as the product of force and distance).

Pilots often use the terms thrust and power interchangeably, but in aerodynamics we are in the world of physics and mechanical descriptions.

Muffinman relates the Vy answer back to thrust curves and power curves -- which is the correct thing to do. The reader has to remember that thrust curves are not power curves!

Broadly speaking, a turbojet engine is a thrust producing device, with output given as lb or kN. Propellors, whether gas turbine or piston, are power producing devices, output given as hp etc.

Aerodynamics for Naval Aviators has a more thorough explanation.

[Hailstop3]

thanks very much for the replies people. Very informative. And i do see the difference between the thrust and power. I will go through all this again tomorrow when im not tired and drinking beer and hopefully it will all fall into place. Cheers

Marcus

[Hailstop3]

Thanks all for the help. Ended up there not being any question relating to this on the exam. Not too worry, at least i know now.

Oh and i passed, 82% :cool:

Thanks all

[Muffinman]

Well done :D
Good luck with any of the others:ok:

muffin