FlyingLapinou

...don't ask, it's a pesky French one.

Anyway, I'm still struggling with the sections involving maths. I'm getting there, but it's slow (very) and painful (very)

Could any of you maths gurus give me a hand with this one, please?

An aircraft flies at 65 m/s. Drag is 4,22 kN, power output is 36 kw. Calculate the efficiency of the propeller?
(90%, 85%, 81,6% or 50%)

ok:
Speed 65 m/s
Drag = 4220 N
Power = 36000 W

Power = Traction x Vi
36000 = T x 65
T = 36000 / 65 = 554 N

And, um, now what?

Apols - that's awfully boring, I know.

achimha

Not knowing anything at all about this but looking at this site...

Drag = thrust in steady straight flight

thrust = (shaft power * eff) / speed
eff = (thrust * speed) / shaft power

Now get the units right...

Heston

I'd work it out by comparing power.

The airplane takes a certain amount of power (lost as drag) - compare that to the power output of the engine.

But I can't get your numbers to work. Help! Are you sure you've got the numbers right?

H

FlyingLapinou

Thanks for having a stab at this Achimha and Heston I've clicked around so many internet sites trying to work it out, my mouse is about to go into meltdown... (other things, too....)

That sounds like a cunning plan.....

But I can't, can I? I mean, drag being in Newts and power output being in Watts?

I thnk the numbers are ok, but something might have been lost in translation. The original question, just in case this helps, is:

Un avion vole à 65m/s. Sa traînée est de 4,22 kN avec une puissance fournie de 36Kw. Calculer le rendement de l'hélice?

keith williams

Are you sure that all of the numbers that you have given are correct?

The useful power output is the propulsive power.

Propulsive power = Thrust x TAS

In straight and level constant TAS flight Thrust = Drag.

From your figures

We have propulsive power = 65 m/s x 4220 N = 274300 NM/s

That is 274.3 KW of propulsive power.

But you have stated that the power output is only 36 KW.

If this is the power going into the propeller then the propulsive efficiency is

Efficiency = THP/BHP = 100% x 274.3/36 = 762%.

Heston

KW has done what I did and got the same answer (I was too shy to post my workings out). The beauty of SI units is that you can see how it all works, so I'd definitely doubt the numbers.

(power is force*speed by the way, or thrust*speed, which in level steady flight, ie at constant velocity, is the same as drag*speed)

H

FlyingLapinou

Wailing and gnashing of teeth....

And a flat forehead

I'm following all you folks, thanks for the help.

Sooooooo annoying! They can't actually have got the figures wrong, can they?

Must have been thought up after a glass or two of France's finest... Tsk.

Heston

Silvaire's suggestion of the decimal point in the wrong place is sensible, but unfortunately 76% is not one of the four multiple choice answers quoted by the OP. So that can't be the error in the paper.

And yes, question papers, particularly practice papers, can be incorrect.

I'd stop worrying about this one for now as long as you've learnt something useful.

H

Heston

Incidentally there are techniques that should be understood for exams with multiple choice answers.

Usually there is not a penalty for getting an answer wrong. That means if you do not know the answer it is always worth guessing - there is a chance of randomly selecting the correct answer.

If you can weed out some of the answers as being definitely incorrect, but can't choose betwen the others, then just guess between the others - again you have a chance of getting it right.

One way of guessing is to pick answer c). Studies of exams (in all subjects) show that this is the preferred place for the correct answer. Examiners seem to be too lazy to properly randomise.

In the set of answers offered in the OP's case I'd definitely pick c) - it also has a more precise value than the others so its tempting to think that's the right one. But it could be a double bluff by the examiner, deliberately to tempt you to pick the wrong answer... (this way madness lies)

H

FlyingLapinou

Well, Heston, the answer is....



C! LOL.

So I'll certainly keep this thread in mind when I do the exam Yes, I tried bumping the decimal place and doing the sums again, but still can't get the peg to fit, as you say. But never mind, I deffo have learnt something (in the plural) useful from posting my question, so thanks to you all

If I look a bit amateur, and I know I do, what you have to remember is that I'm hitting this as overly-enthusiastic self-study student with extremely basic maths and physics, so it's a very steep learning curve for me (I have a degree in French and German, so my brain is not naturally wired to calculate propeller efficiency, but I like a challenge )

Totally agree with you, Silvaire1, and I have to say the examiners must have had their knuckles rapped as there's been more focus in recent years on general principles, with fewer brain-teasers like this one, which I dug out of a 1989 paper. But I'd like to be able to have a go at batting whatever might come my way...

There are five papers all in all. If you're interested, and have nothing better to do (I hope you have ), you can take a peek at the 2011 quezzies here:

Aerodynamics and Flight Mechanics
Aircraft Knowledge
Meteorology
Navigation-Safety-Regulations
History of Aviation

Yawn

fantom

Who cares?

OK, I only flew Chipmunk, JPs, Gnat, Hunter, Phantom, every small twin-prop, a few small twin-jets, F27, B727, F100, A320 and A330 but I cannot answer the question.

So, is it of any use?

wet wet wet

The power figure of 36kW looks way too small and is probably an error. So working back from the correct answer gives a power of 336kW, so I suspect there that a 3 got lost in the typing of the question.

Heston

Hey I like the idea of a History of Aviation paper - can we lobby the CAA to include one in the NPPL syllabus in the UK?

H