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How does weight affect our glider's performance?

Weight Vs Glide angle.

Hi Guys there seems to be some confusion regarding the effects that weight has on a glider’s angle of descent (Glide ratio) or the distance the glider will fly for a given weight at normal flying airspeeds and at minimum/maximum weights.
In fact, the weight of the glider has no effect on the glide angle or the glide distance. But it does have great effect on how fast the glider flies and how fast a heavier glider will cover the same flight distance as a lighter model.
2560px-Airfoil_lift_and_drag.svg.png


Weight variations in aircraft weight do not affect the glide angle provided that the correct airspeed is flown. Since it is the lift over drag (L/D) ratio that determines the gliding range, weight will not affect it. The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft and not the weight.
So, for example if a glider has a 50:1 glide ratio, then it travels 50 feet for every foot of altitude lost. The best glide speed is the airspeed at which, in still air, the glider achieves its best glide ratio. This is also known as the best lift/drag (L/D) speed.

To elaborate a bit, in Aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil on an aircraft wing, divided by the aerodynamic drag caused by moving through air. So, L/D can be used to describe the aerodynamic efficiency under given flight conditions. The L/D ratio for any given body will vary according to these flight conditions.

DragvsSpeed.jpg


I don’t want to get too technical here, and disappear into a load of boring math, but for reference:

The lift coefficient is defined as: CL = L/qS, where L is the lift force, S the area of the wing and q = (rU2/2) is the dynamic pressure with r the air density and U the airspeed. Similarly, the drag coefficient is written as: CD = D/qS, where D is the drag force and the other symbols have the same meaning.

OK that’s over!

For an aerofoil wing or powered aircraft, the L/D is specified when in straight and level flight. For a glider it determines the glide ratio, i.e. distance travelled against loss of height.

The term is calculated for any particular airspeed by measuring the lift generated, then dividing by the drag at that speed. These figures vary with speed, so the results are typically plotted on a 2-dimensional graph. In almost all cases the graph forms a U-shape, due to the two main components of drag. The L/D may be calculated using computational fluid dynamics or computer simulation, though at our low Reynolds numbers, the results may not be 100% accurate. It is measured empirically by testing in a wind tunnel or by a free fight test.

Coefficients_of_Drag_and_Lift_vs_AOA.jpg


The L/D ratio is affected by both the form drag of the body and by the induced drag associated with the flying surfaces creating a lifting force and it depends principally on the lift and drag coefficients, the angle of attack to the airflow and the wing aspect ratio.

The L/D ratio is inversely proportional to the energy required for a given flightpath, so that doubling the L/D ratio will require only half of the energy for the same distance travelled. This results directly in better performance for a powered plane or glider.

Lets, say you had three versions of the same glider, a light version, a medium weight version, and a heavy weight version on the top of the same hill, you would find that if you threw them all off one by one, the heavy one would reach the bottom first, the medium one second, and the light one last, all in terms of time. But you would also find that they had all flown the same distance beause the glide ratio had not changed. Logically then, if you had a light glider that could stay up in light lift, then the same glider in medium and heavy versions of it would also stay up in the same lift conditions.

You will see that Aeroic normally offers three layups. "L", "S", and "SS" Please do not think that these designtions are related to weight at all, instead they are rated as measures of stiffness and durability.

I hope that helps to explain.

Cheers,

Doc.
 
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This simple fact eluded me early on, and it seems many others as well. Once I wrapped my head around this, it was for sure a lightbulb moment.
 
Interesting topic, but Iam confused about how the same glider at different weights can "fly" or "stay up" in light conditions. We add ballast as the conditions improve (and to go faster in improved conditions). If it is not windy enough we want to keep the glider light and then add weight as it gets winder. The heavy ballasted version will not "stay up" if conditions are light. Is "staying up" and "having flown faster" and not for as long the key here or am I missing something fundamental here?
 
agarchitect said:
Interesting topic, but Iam confused about how the same glider at different weights can "fly" or "stay up" in light conditions. We add ballast as the conditions improve (and to go faster in improved conditions). If it is not windy enough we want to keep the glider light and then add weight as it gets winder. The heavy ballasted version will not "stay up" if conditions are light. Is "staying up" and "having flown faster" and not for as long the key here or am I missing something fundamental here?
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Hi Adam.

OK First, here we are not talking about ballast. We are talking about an unballasted glider, or rather three unballasted gliders under normal flying conditions.

My example was three gliders of the same type and so the same glide ratio (L/D) with different weights, but flying under normal flying conditions, where "Normal Flying Conditions" are at the L/D that provides the best glide ratio for that particular model's aerodynamic form factors - not weight.

"Lets, say you had three versions of the same glider, a light version, a medium weight version, and a heavy weight version on the top of the same hill, you would find that if you threw them all off one by one, the heavy one would reach the bottom first, the medium one second, and the light one last, all in terms of time. But you would also find that they had all flown the same distance beause the glide ratio had not changed. Logically then, if you had a light glider that could stay up in light lift, then the same glider in medium and heavy versions of it would also stay up in the same lift conditions."

Technically if your light version of the same glider can fly under certain wind/lift conditions, then so can the other heavier versions of the same glider, as they are all flying with the same glide ratio and under the same normal flying conditions.

The key points here are "FLY" under "NORMAL Flying Conditions".

I.E:

Glider 1 (the L version) is flying under normal flying conditions but is slower than gliders 2 and 3.
Glider 2 (the S version) is flying under normal flying conditions but as its heavier than Glider 1 so it will fly the same distance, faster.
Glider 3 (The SS version) is flying under flying normal conditions but as its heavier than Glider 2 so it will fly the same distance, even faster.

This has to be tempered with a little common sense as we could say OK glider 1 is 100g, Glider 2 is 1,000g and glider 3 is 10,000g, but then in that case gliders 2 and 3 would not be flying under normal conditions to attain the best L/D glide ratio.

Am I doing a good job exlpaining this, or is there an easier way?

This can be a hard concept to get your head around - especially as we all naturally, and correctly think lighter is better.

Cheers,

Doc.
 
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It is making sense. I guess I was thinking in terms of "how well" the glider would fly, that is in light conditions a light glider would typically fly "better", or stay up longer than a heavier version of the same glider. Flying better is subjective I suppose. A fast short flight may be better than a long slow flight, depending upon one’s perspective. Any flight short, long, fast or slow flight would be time better spent than being stuck in traffic :) Thanks!
 
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agarchitect said:
It is making sense. I guess I was thinking in terms of "how well" the glider would fly, that is in light conditions a light glider would typically fly "better", or stay up longer than a heavier version of the same glider. Flying better is subjective I suppose. A fast short flight may be better than a long slow flight, depending upon one’s perspective. Any flight short, long, fast or slow flight would be time better spent than being stuck in traffic :) Thanks!
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I think a simple analogy is that a "light" glider and a "heavy" glider will cover the same ground (L/D), but the lighter one will take longer than the heavier one to get to the same place. So it will "feel" like the lighter glider flies better. This can be perceived as a comfort thing.
 
Do keep in mind that time needs to be part of the overall equation. The amount of lift over a time frame needs to be added in.
 
I'll probably get tossed under the bus for this, but the lighter model would be exposed to more lift as it is flying for a longer period of time. We are talking about slope gliders here, so the longer it is in flight, it is exposed to more energy.

p.s. I probably missed it, but what is the image in your avatar?
 
Wayne said:
I'll probably get tossed under the bus for this, but the lighter model would be exposed to more lift as it is flying for a longer period of time. We are talking about slope gliders here, so the longer it is in flight, it is exposed to more energy.

p.s. I probably missed it, but what is the image in your avatar?
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Hi Wayne - the image in my Avatar is an XFLR5 CL/CD graph of my JH817 aerofoil under different Re.

Same one used in the New Typhoon MKII Toccata.

As you can see its pretty good! - If you believe computer simulations that is - but actually this one checks out in the tunnel and in actual flight. If you note the small spaces in between the lines, and the comlplete absence of a "Drag Bucket", then this is another indicator of a good glide ratio i.e: it doesn't do strange things under different conditions

Thanks for the elaboration.

Cheers,

Doc.
 
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