Hey everyone, long time reader, first time poster.

I have finished all 7 of the CPL subjects as well as my IREX exam and I'm coming up against some issues with revision prior to my CPL test flight.

I am going through my Aerodynamics KDR's and one of the questions I got wrong was regarding the section in the Part 61 MOS where it is asking about "changes in weight and altitude (height) on; glide range and endurance.

I am having a tough time finding and understanding it. I cannot remember the questions I got wrong so instead I'm trying to improve my knowledge so i can explain it to the testing officer when he asks. If anyone can help explain it to me that would be greatly appreciated.

Thanks,
Matt

PS - Don't ask me why this has posted in the general enquiries section, it was intended for the CPL Aerodynamics section... for some reason has ended up here.

It can be simply proved that maximum glide range in still air is achieved by gliding at the best lift/drag ratio angle of attack [about 4°] for most GA type wings - page 8.5 (92) of my Aerodynamics book. It is the angle of attack alone that decides the lift/drag ratio but the speed required to achieve that angle of attack varies with aircraft weight. A heavy aircraft needs to glide faster than a light one to have the same angle of attack.

Weight has no effect on the still air glide distance possible it is only the angle of attack that decides that. Therefore, since weight has no affect on the DISTANCE you can glide in still air, the heavy aircraft and the light aircraft glide down the same sloping surface in the sky. However, since the heavy aircraft is gliding faster it reaches the bottom sooner and spends less TIME in the air. So weight does not affect the gliding RANGE but it will affect the gliding ENDURANCE.

The speed we are talking about here is the INDICATED AIR SPEED and that requires a higher TRUE AIR SPEED with increased height, so it is true to say that, for the same aircraft weight, the gliding TRUE AIR SPEED will be higher at high altitude. Once again, increased altitude will have no effect on the gliding distance providing you use the correct INDICATED AIR SPEED to achieve the best lift/drag ratio angle of attack. At high altitude you will be gliding down the same sloping surface at a higher true speed. It will take you less time to descend through a given height but the distance covered will be the same [assuming no wind]. Height, like weight, has no effect on the gliding distance but it will affect the gliding endurance.

Bob

Hi Bob,

Thats perfect, I have been trying to understand that for a while and now I wish I had posted here earlier! I appreciate your response.

Regards,
Matt

Hey everyone,

Can anybody help me with this question? I`ve been trying to get my head around this so any clarification will be appreciated.
The effect of a headwind on glide range and glide endurance if an aeroplane is flown at its best lift/drag ratio speed is -

A)range and endurance will both be reduced
B)range will be unaffected but endurance will increase
C)range will be reduced but endurance will be unaffected
D)range will be unaffected but endurance will be reduce

The answer is [c]. Once an aircraft has become airborne, it becomes part and parcel of the air. If the air is moving (wind) the aircraft moves with it. So as far as the aircraft is concerned, there is no wind, it is simply flying within a moving 'box' of air. The only time the aircraft will will be affected by the wind is when there is a SUDDEN change of wind velocity (wind shear). An aircraft gliding in a headwind is gliding in a moving 'box' of air.

If you hold a constant speed in a constant wind, the TIME it takes to pass through 1000 feet will always be the same - wind or no wind. However, the wind will decide where the aircraft will be when it gets to the ground. Wind affects the ANGLE of descent but not the RATE of descent. If you were gliding at 70 knots into a 70 knot headwind, and observer on the ground would see you descending vertically and therefore your gliding RANGE would be zero. However, that would have no effect on the TIME it took to come down.

Thank you very much Mr Tait. for the clarification.
Kind regards

OK with head wind the gliding range will decrease but what about tailwind? will it increase?

By a similar argument, the wind will not affect the TIME it takes to lose the height, but a tailwind will produce a higher ground speed. If you are in the air for the same time with a higher ground speed, you will cover more ground - the RATE of descent will not be affected. but the ANGLE of descent will be shallower in a tailwind.

Gliding rate increases in a tailwind and decreases in a headwind but rate of descent is not affected by wind.

Hi,

A flight journey is divided into 3 stages, one with a tail wind, second with no tail or head wind, and 3rd is with a tail wind. my questions what is the method to calculate the total range of the flight and will the specific fuel consumption change in each part of the journey ?

I'm not to sure what you mean by the total range of the flight. That would be the sum of the distances for each stage and wind would have no effect. Wind doesn't change the distance between two aerodromes.The specific fuel consumption on each stage could be found by dividing the ground speed on that stage by the fuel flow (miles or gallons per hour). That will give you miles per gallon or per litre. Alternatively you could divide the fuel flow (gallons or litres per hour) by the ground speed. That will give you litres or gallons per mile.