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Glide Range and Endurance

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Mzahr replied the topic: Glide Range and Endurance

Hi,

Q5) SEE Attached image
identical a/c in every respect except Gross weight to achieve maximum range in nil wind conditions

Answer is B - Heavier Aircraft Should Fly Faster than the light one .

This Makes sense as the Heavier Aircraft Requires More Lift to INC WEIGHT in Level Flight therefore this is achieved by what you choose to use and in this scenario you need to use a HIGHER IAS to achieve the extra lift required to Fly at the Best L/D AOA . understand you have also used more Power For Thrust to achieve the higher speed and which increases parasite Drag Not induced Drag, AND THIS IS WHAT WE NEED TO DO FOR RANGE TO FLY AT THE BEST L/D AOA NOT SPEED "IM ASSUMING".

THEN THIS QUESTION CONTRADICTING ANSWER ABOVE

Q7 ) Refers you to Total Drag Graph with Speed S being the Best L/D speed and question asks to use least amount of fuel in a given distance nil wind a/c must be flown at ..

answers:
A) At speed S for any weight (ANSWER IS A)
b) faster than speed S if weight increased
c) slower than speed s if weight increased
d) faster than speed s if weight is reduced

so the answer now being A above, contradicts answer to q 5 having to fly at a higher speed to achieve the L/D AOA at increased weight so my understanding and the confusion here is what question will BE asked about weight in relation to range,

SO WHEN WEIGHT IS INCREASED, FOR MAX RANGE.... IN NIL WIND
DO WE USE A HIGHER AOA AND USE THE Best L/D Speed OR....

USE AN INCREASED IAS AND THE BEST L/D AOA That we need To consider For Range ?

So it all comes down to what is the higher penalty with increased weight, the power thrust required for slight increase
in induced drag or parasite drag ?

am i on the right track or am i missing something ?

Thank You For your time :)
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smith replied the topic: Glide Range and Endurance

Understanding how changes in weight and altitude affect glide range and endurance is crucial for flight planning. Consider these factors when explaining the concept during your CPL test flight. Good luck!

best,
Leena Smith
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  • John.Heddles
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John.Heddles replied the topic: Glide Range and Endurance

First, it is absolutely great to see that you are thinking more deeply about this aspect of the story rather than just skimming the texts and trying to pass the exams. Well done.

Turning to your questions -

This Makes sense as the Heavier Aircraft Requires More Lift to INC WEIGHT in Level Flight therefore this is achieved by what you choose to use and in this scenario you need to use a HIGHER IAS to achieve the extra lift required to Fly at the Best L/D AOA .
.
Keep in mind that the actual L/D is going to change as the weight increases. You are getting just a little confused, I think, by presuming that only weight is changing. You still will need to increase speed to peg the relevant L/D.

THEN THIS QUESTION CONTRADICTING ANSWER ABOVE

How does it contradict ? Looks fine to me. Note that Bob makes the same observation that drag (ie L/D) varies if the speed varies.

SO WHEN WEIGHT IS INCREASED, FOR MAX RANGE.... IN NIL WIND
DO WE USE A HIGHER AOA AND USE THE Best L/D Speed OR....


You are still looking for the speed for optimum L/D but that will vary with weight

Stick with it. We all learn by tossing the ideas around and eventually sorting out what's what.

Engineering specialist in aircraft performance and weight control.
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Mzahr replied the topic: Glide Range and Endurance

Thanks John ,

When you say the “Actual L/D changes when weight increases ! You are referring to the increase in lift required due to the increase in weight therefore as we are aiming to achieve the Best L/D AOA we need to adjust speed to achieve this . As the BEST L/D AOA is not affected by weight , so understand to peg the relevant BEST L/D AOA speed needs to Be adjusted for different weights.

So my question now to put this into reality based on the Graph presented The speed shown on the Graph For Best L/D speed to Achieve this best L/D AOA that has been computed on the Aerofool test Graph and now presented on the Actual aircraft Graph showing speeds not AOA, is this speed shown worked out by engineers on this A/c at the MAX GROSS WEIGHT for the aircraft ?

So subsequently when looking at the graph and noting the Best L/D speed this is for Max Gross weight, you will achieve the maximum range for fuel burnt /nm if flown at that AOA and speed for that max weight . So this means that weight cannot be increased as it’s at its MAXIMUM GROSS WEIGHT “ ! Therefore at a reduced weight the speed needs to be Slower to be able to peg the best L/D AOA , ( and understand that the lift and drag values will now be lower for that lighter weight and this lighter weight will have increased RANGE as it is using less power for thrust to be able to maintain straight and level flight in equilibrium flying at that Slower IAS to produce the BEST L/D AOA . So this is what you meant by the L/D values that change it’s based on the speed and the power req’d , not on the actual Graph for where this BEST L/D AOA occurs?

And now noted, Bob makes mention for light aircraft, The effect of increased weight is usually ignored for RANGE . And to fly at the MAX GROSS WEIGHT L/D speed shown on graph for all weights .
(THE SQUEEZE AINT WORTH THE JUICE )

However for ENDURANCE it’s quite obvious as it’s min power S/L Speed req’d and you will have to use more power for heavier A/c therefore higher RPM = more fuel !

Than you for your time in reading and responding :) .
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  • John.Heddles
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John.Heddles replied the topic: Glide Range and Endurance

When you say the “Actual L/D changes when weight increases ! You are referring to the increase in lift required due to the increase in weight therefore as we are aiming to achieve the Best L/D AOA we need to adjust speed to achieve this . As the BEST L/D AOA is not affected by weight , so understand to peg the relevant BEST L/D AOA speed needs to Be adjusted for different weights.

Not quite. If the weight changes, the drag changes and you need to redo the sums for the curve to establish the "new" best L/D position,

So my question now to put this into reality based on the Graph presented The speed shown on the Graph For Best L/D speed to Achieve this best L/D AOA that has been computed on the Aerofool test Graph and now presented on the Actual aircraft Graph showing speeds not AOA, is this speed shown worked out by engineers on this A/c at the MAX GROSS WEIGHT for the aircraft ?


First, what is .. the Aerofool test Graph ?

Presenting the data in terms of speed will correlate with alpha for steady conditions.

not on the actual Graph for where this BEST L/D AOA occurs?

The graph comes from both theory and flight test and will vary with the parameters involved. For any particular L/D chart, there will be a maximum value point.

The following graph provides an idea of the variation in L/D with weight (wing loading) for a specific aircraft.





The effect of increased weight is usually ignored for RANGE . And to fly at the MAX GROSS WEIGHT L/D speed shown on graph for all weights .

True enough. Depends on the particular aircraft's numbers and how finely you want to do the sums.

Engineering specialist in aircraft performance and weight control.
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Mzahr replied the topic: Glide Range and Endurance

Typo it was meant to be Aerofoil Test , The theory test you refer to , Testing Aoa at constant airspeed .

Just note that Bob makes mention to weight not changing the Best L/D ,
Refer to attached highlighted
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John.Heddles replied the topic: Glide Range and Endurance

[color=]Just note that Bob makes mention to weight not changing the Best L/D ,[/color]

And that is a reasonable approximation. The previous DG-1000 charts show, quite clearly for that machine, that the variation in L/D max is pretty small.

Keep in mind that we generally talk in simplified terms in pilot training texts as the aim is to get the basics of the story across. The detailed stuff is secondary for routine pilot use. Where it seems reasonable to get a little into the nitty gritty, I see no reason why we shouldn't add a bit more of the engineering side of the story.

Engineering specialist in aircraft performance and weight control.
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Mzahr replied the topic: Glide Range and Endurance

Now I’m really confused ! as per Bobs explanation how I have comprehended this all made sense to me through his text and explanations was that the Best L/D AOA that occurs in the Aerofoil Test , AOA and constant airspeed gives us figures where the lift will be the greatest compared to drag at constant airspeeds and associated AOA , he goes on to explain that this cannot be tested in the real aircraft for obvious reasons which I also completely comprehend and understand , So this is what brings me to the understanding that The best L/D AOA will always be the greatest LiFt to Drag irrespective of weight and speed because if you increase lift by using a faster speed you also increase drag but it will be proportional to the Aerofoil test that produced the Best L/D ? So although you are heavier and using a Higher speed at the required best L/D AOA , arnt you going to be producing still the Best L/D for that weight ( understand the actual figures when you calculate will change , but withought going into these calculations, and changing the graph by adjusting speed is this the right concept ?
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Mzahr replied the topic: Glide Range and Endurance


OK , THANK YOU
I DID NOTICE THIS SMALL CHANGE AND I UNDERSTAND NOW HOW IT DOES CHANGE AND WHY WE ARE JUST AT THE SURFACE WITHOUT GETTING INTO THE NITTY GRITTY AND THEN USING A CERTAIN SPEED EITHER INCREASING A LITTLE OR DECREASING TO BE ABLE TO TACKLE THE DIFFERENCES THAT ARE A GOOD RULE OF THUMB FOR THAT SPECIFIC A/C . Thank you again for your time it has been really helpful !

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John.Heddles replied the topic: Glide Range and Endurance

Hang in there. It progressively makes more and more sense as you keep reading and thinking it through. Very few of the pilot trainee folks are able to just pick it all up with a quick flick through the books.

.. he goes on to explain that this cannot be tested in the real aircraft for obvious reasons

Again, you need to keep in mind that pilot textbooks are intended for pilot training needs. The pilot doesn't need to be an engineer or physicist but should have enough of an idea of what's going on to do the pushing and pulling things in a manner not inconsistent with engineering reality.

Actually, you can measure all this lift and drag stuff in flight - that's one of the things which we pay TPs and FTEs to do in their day jobs. But it isn't really necessary for the line pilot to be up to those details. The end results (in a broad brush sense) are what the line pilot needs to understand.

One of the things Bob does well is talking the talk in a style and at a level appropriate to what the student pilot needs

Engineering specialist in aircraft performance and weight control.
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