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Cessna 172 with 9:1 glide ratio Cessna 172 with 9:1 glide ratio

Was interesting to watch the explanation about glider here, especially about the glide ratio. Glide ratio is defined as the ratio between altitude when it start gliding (Y-axis) to the distance it can reach until the altitude reach zero (X-axis), as shown in the picture 2. Gliding is a descending, opposite of climbing, which is ascending. All airplanes are designed to be able to glide to anticipate the worst case when the engine is failure.

There are many discussions how to calculate an airplane when ascending, explains the relation of wing area, wing ratio, airplane load, initial speed (velocity), and the power it requires to meet the ascending rate. Then my question is, how do I bind such that information when calculating the glide ratio/distance?

Edit: I believe the weight of the airplane contributing the glide distance (ratio) it can reach. Just consider two identical airplanes (say it Cessna 172), one is fully loaded and another one is not loaded at all (except the pilot, of course). They fly at the same speed and turned off the engine at the same time.

Also consider two airplanes (also Cessna 172) with exactly same weight and load, but fly at different speed and then turned off the engine at the same time.

From both above illustrations, we know their momentums are different. Is not the momentum contributing to the glide distance (ratio)?

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    $\begingroup$ I believe the link suggested duplicate is different. If we just consider the LIFT and the DRAG ratio only, it will not be the answer. In my limited knowledge, there will be the initial velocity (Vo), the initial when an engine is off or when we start jump from a heigh if it is pure glider. Beside, I believe total weight also affecting the gliding ratio. A heavy weight however absolutely different from than light weight. We may check it from force components. So, we have to look back the answer rather than to take the answer in the provided link. So I think, better reopen this post. $\endgroup$
    – AirCraft Lover
    Sep 11, 2019 at 12:09

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One simple way is to compare thrust requirement for level flight at Vbg, where lift to drag ratio is optimal, to the weight of the plane. This ratio of pounds of thrust, which is equal to pounds of drag in steady state level flight, to the weight of the plane should give a good approximation of the glide ratio. Glide ratio is how much altitude do I give up to move how far forward. The altitude is the power source. How far forward you move is dependent on drag.

So an 1800 lb Cessna would require 200 lbs of "thrust" to glide 9:1.

Now to design for it, speed is a critical factor in selection of wing type and loading. This is the albatross vs eagle consideration. Higher speed cruisers favor the albatross.

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  • $\begingroup$ As my comment above, to determine the gliding distance or we call it gliding ratio, the initial speed (velocity) and it weight absolutely affecting. We may imagine, a heavy loaded airplane will have smaller ratio (gliding distance) compared to if that same plane unloaded. $\endgroup$
    – AirCraft Lover
    Sep 11, 2019 at 12:13
  • $\begingroup$ @AirCraft Lover this is a good one for thoughts. Obviously, a brick with tiny wings won't glide as far, but there is a range where weight does not affect (and may even improve) gliding distance. Remember, weight and altitude are your gliding energy. This is why they put ballast in gliders. The heavier glider will glide faster to get sufficient lift at optimal (lift to drag) AOA. But because it has more energy to start with, it will go as far as the lighter one. Determination of optimal gliding weight range is a useful and practical study for any design. $\endgroup$
    – Robert DiGiovanni
    Sep 11, 2019 at 12:42
  • $\begingroup$ I agree that tiny wing will not glide far. What I am saying is that beside the airplane shape (glider of course different than fighter jet in gliding), weight and initial speed or velocity also affecting the glide distance. What I am trying to argue is that L/D is not the only factor as suggested by the link above. $\endgroup$
    – AirCraft Lover
    Sep 11, 2019 at 14:22
  • $\begingroup$ The heaver glider (or airplane) will glide faster. That probably true. But the wight also will affect force factor component. Isn't it? $\endgroup$
    – AirCraft Lover
    Sep 11, 2019 at 14:25
  • $\begingroup$ @Aircraft Lover that is why I love to build and test. Within a certain range (maybe 1700 to 2300 lbs for a 172?) The glide range is the same as weight/altitude potential energy balance extra drag at higher glide speed. Too light will lessen range. Too heavy and AOA (induced drag) from lift requirement also lessens range. So we have a Weight Best Glide or Wbg graph curve too! This is also known as optimal wing loading. $\endgroup$
    – Robert DiGiovanni
    Sep 11, 2019 at 14:47

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