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! :laugh: 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.

Bob you have given great explanations here, but I haven't seen anyone ask this question yet...

Best glide (lift/drag) ratio as you have described first up is at about 4 deg AoA. And a heavier plane will do this at a faster airspeed.

SO, if a fully loaded 172 next to an almost completely empty (1 pilot no fuel) 172 in side-by-side formation both pulled their power all off at the same time and settled on a 4 deg AoA, they could both be expected to follow the same glide path, with the heavier plane gliding at a faster speed, and arrive at the same ground point dead ahead, with the heavier plane getting there first. OK, cool.

BUT in the 172 POH it says that best glide is 64 KIAS. Or 65, depending on who you ask. Whatever.

Thus, the KIAS airspeeds of these two planes are going to be different, right? I mean how can both the heavier and the slower plane both be achieving the advertised best glide at 64 KIAS?

AND, that being asked, what if they DID both attain 64 KIAS? Would the heavier plane then have a greater descent rate and then thus a shorter glide range?

This is an experiment that I would absolutely LOVE to do one day with two planes in the training area, and then video it, just to publish the physical results for the student pilot world to watch.

Lift and drag result from the behaviour of the passing airflow as it interacts with the SHAPE of the aircraft. The WEIGHT of the aircraft has nothing to do with it. Here is a 'thought experiment'.

Imagine we have an actual Cessna 152 and an exact replica that has been carved out of solid lead. Now we place them in a wind tunnel with each at the same angle of attack and the same airspeed. The air flowing around the two would produce exactly the same amount of lift and drag and therefore the same lift/drag ratio because their SHAPES are identical. Their vastly different weights would have nothing to do with that.

The actual [u]amount[/u] of lift and drag generated by each of these imaginary aircraft would depend upon the angle of attack and indicated air speed and nothing else.

However, if we expected the solid lead 152 to actually fly at that angle of attack, it would have to fly at a ridiculously high speed (so much for that idea!). The lift/drag ratio is also the distance/height ratio (see page 93 of the Aerodynamics book. Fig 8.14). There is only one angle of attack that will produce the best lift/drag ratio and, provided you fly at that angle of attack, you will achieve the best gliding distance in no wind.

Theoretically, every weight would have its own gliding speed for the best lift/drag ratio. However, for the average GA type aircraft, the difference between very empty and very full would be so small as to be safely ignored. To conduct the in-flight experiment you have suggested, you would actually need an angle of attack meter in the cockpit. The heavier aeroplane would have a higher rate of descent, but that is because it comes down the same slope faster.

Thanks Bob,

That's a fascinating example! I'm intrigued then to see you write that the difference between full and empty is ignorable. And I'm still a bit hazy on it. For a Cessna 172 with a wing surface area of 16.2 square metres, a single pilot empty weight (720 Kg) gives about 44 Kg per square metre, but a full weight (1100 Kg) spreads 68 Kg per SqM. That means empty weight is only 2 thirds of full weight.

So are you saying that this 50% increase in empty weight to full weight causes a speed increase down this best glide angle of attack that is negligible compared to the recommended 65 KIAS?

That's hard to imagine, considering your solid lead Cessna approaching supersonic to get the required lift. Makes me want to try that too!

I don't have your particular POH page to review. Perhaps you might post a photo of it ?

I expect that it will indicate that the speed recommended is for a specific (probably gross) weight. I checked the C172N POH and the speed nominated is for MTOW, for example.

Thanks both very much to you both, I went hunting and found this on 'Bold Method":

"There's something you need to keep in mind about best glide, though. Like most airspeeds in the POH, best glide is calculated at max gross weight. And as weight decreases, so does the speed that will maximize your distance. The change is minor, but if you're trying to get the most out of your glide and you're lighter than max gross weight, a slightly slower speed may help you out."

https://www.boldmethod.com/learn-to-fly/maneuvers/how-to-handle-a-power-off-landing-following-an-engine-failure-best-glide-or-minimum-sink-how-to-do-it/