Glide performance and gross weight
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But it does. At a higher wing loading the same lift drag ratio (glide angle) is obtained at a higher airspeed than for a lower gross weight.
Think about the vectors involving thrust, weight, lift and drag. At a steady glide angle all vectors form an equilibrium. Without any thrust, the forward force to overcome drag is provided by gravity and the aircraft is moving downwards with respect to the air around it. The forward motion also provides the lift to overcome the weight. If the gross weight is increased then the amount of lift needed can only be obtained at a higher airspeed. This higher airspeed can only be obtained by descending faster, and results in more drag. Interestingly, for most low drag aircraft, the best Lift:drag ratio remains the same, but is obtained at a higher airspeed for a higher gross weight. This is why gliders carry water ballast in strong conditions. The best Lift:drag ratio is obtained at a higher speed and hence gives better penetration into the wind.
Think about the vectors involving thrust, weight, lift and drag. At a steady glide angle all vectors form an equilibrium. Without any thrust, the forward force to overcome drag is provided by gravity and the aircraft is moving downwards with respect to the air around it. The forward motion also provides the lift to overcome the weight. If the gross weight is increased then the amount of lift needed can only be obtained at a higher airspeed. This higher airspeed can only be obtained by descending faster, and results in more drag. Interestingly, for most low drag aircraft, the best Lift:drag ratio remains the same, but is obtained at a higher airspeed for a higher gross weight. This is why gliders carry water ballast in strong conditions. The best Lift:drag ratio is obtained at a higher speed and hence gives better penetration into the wind.
Last edited by Ka6crpe; 17th Aug 2013 at 12:16.
Yes that's my recollection from 30 years ago.
Weight does not change the glide distance, only the time.
A heavy Aircraft will hit the earth in a shorter time but at around the same place as a light version......
Weight does not change the glide distance, only the time.
A heavy Aircraft will hit the earth in a shorter time but at around the same place as a light version......
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Mr.Buzzy and Nitpicker330, you are both correct. The glide distance remains the same because the glide ratio remains the same, as long as the speed is increased. If you keep the speed the same as for the lighter weight then the glide distance will decrease.
I wish I could post my glider's polar curve up here to demonstrate.
The glide distance remains the same because the glide ratio remains the same, as long as the speed is increased. If you keep the speed the same as for the lighter weight then the glide distance will decrease. I wish I could post my glider's polar curve up here to demonstrate.
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Spot on. In still air, both will travel the same distance along the same angle of descent (provided they are at the same angle of attack: assuming this AoA is the one that provides best L/D, then they will glide the greatest distance possible). The heavier a/c however will "slide down the slope" faster, as any given AoA will be obtained at a higher speed for a heavier aircraft. So in still air they will travel the same distance but the heavier one will reach the ground first.
Once wind kicks in it's a different story though: if it's a headwind, the light aircraft (which is taking longer to descend) will be exposed to the effects of the headwind for longer and travel a shorter distance. The heavier one will travel further (hence the ballast in gliders as they battle their way through headwinds towards the next updraft). The opposite occurs for a tailwind, with the light aircraft traveling further as it enjoys the benefit of the tailwind for longer.
Once wind kicks in it's a different story though: if it's a headwind, the light aircraft (which is taking longer to descend) will be exposed to the effects of the headwind for longer and travel a shorter distance. The heavier one will travel further (hence the ballast in gliders as they battle their way through headwinds towards the next updraft). The opposite occurs for a tailwind, with the light aircraft traveling further as it enjoys the benefit of the tailwind for longer.
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But have you considered that the speed for best glide angle is higher than
the speed for best L/D in a headwind
the speed for best L/D in a headwind
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Originally Posted by Q94H
Does compressibility drag have a higher effect on the heavier aircraft than the lighter aircraft
P.S.
Based on the above graph, when a heavier and a lighter aircraft descend side-by-side each at its max L/D from the same cruise altitude, the lighter aircraft will have a slight advantage until they get to lower altitudes where the heavier aircraft has the advantage.
Last edited by HazelNuts39; 19th Aug 2013 at 16:35. Reason: graph edited
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When a heavier and a lighter aircraft descend side-by-side from the same cruise altitude, the lighter aircraft will have a slight advantage until they get to lower altitudes where the heavier aircraft has the advantage.
Secondly, I am amazed that over the same range of mach numbers, .2 to .8, the aoa for max L/D will vary from an initial value of 6 degrees (I'm assuming that degrees is the units), to a value of about 2.5 degrees, all while keeping a near constant max L/D of 22. Are you able to comment on this?
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Originally Posted by hawk37
HazelNuts, does the above explain the very slight variation of speed for max L/D between mach .2 and mach .8 shown in your graph?
I cannot explain why the effect of Mach is as shown, except that I would expect the max. L/D to reduce near the aerodynamic ceiling beyond M.8. Compressibility changes the pressure distribution around the airplane. I would be surprised if the effect on drag would be identical to the effect on lift at any given angle of attack.
Last edited by HazelNuts39; 21st Aug 2013 at 12:07.
Mr.Buzzy and Nitpicker330, you are both correct. The glide distance remains the same because the glide ratio remains the same, as long as the speed is increased. If you keep the speed the same as for the lighter weight then the glide distance will decrease.
I wish I could post my glider's polar curve up here to demonstrate.
The glide distance remains the same because the glide ratio remains the same, as long as the speed is increased. If you keep the speed the same as for the lighter weight then the glide distance will decrease. I wish I could post my glider's polar curve up here to demonstrate.
Suffices to say that L/D is primarily a function of AoA. Weight itself is not a factor influencing the max L/D value. As Long as you keep Alpha at max L/D the Lift/Drag Ratio (=Glide Ratio) and thus the distance achieved from a given altitude will remain unchanged.
The only things which we ignore in this case are the Re- Number effect on L/D when the Speed changes due to different weight and as has been mentioned compressibility effects once >M0,8.
In reality these two factors can be ignored since the relatively small speed difference will have no appreciable effect on L/D and thus distance achieved.
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hawk37
As you say, HN39's chart is type specific, and I am not sure the small increase in L/D between 0.2M and 0.5 is at all typical - certainly for the aircraft with which I am familiar the drag and L/D are virtually constant up to about 0.6M where compressibility starts to kick in.
I can offer an explanation for the AoA effect though. When compressibility starts to be significant it increases the effective lift dependent drag contribution so the profile/lift dependent drag balance is shifted, pushing the lift coefficient for L/Dmax down to a slightly lower value - say from 0.65 down to about 0.6. This by itself cannot explain the reduction in AoA for best L/D.
The main reason for the AoA reduction is that the lift curve slope of the wing increases dramatically with increasing Mach No. - typically from about 5.0 at 0.2M up to 8.0 or higher at 0.8M. The lift coefficient at zero AoA will be about 0.15, so the AoA for L/Dmax would typically be about 5.7deg at 0.2M falling to about 3.2deg at 0.8M. I'm using a different aircraft to HN39 of course, but the numbers are not very different.
As you say, HN39's chart is type specific, and I am not sure the small increase in L/D between 0.2M and 0.5 is at all typical - certainly for the aircraft with which I am familiar the drag and L/D are virtually constant up to about 0.6M where compressibility starts to kick in.
I can offer an explanation for the AoA effect though. When compressibility starts to be significant it increases the effective lift dependent drag contribution so the profile/lift dependent drag balance is shifted, pushing the lift coefficient for L/Dmax down to a slightly lower value - say from 0.65 down to about 0.6. This by itself cannot explain the reduction in AoA for best L/D.
The main reason for the AoA reduction is that the lift curve slope of the wing increases dramatically with increasing Mach No. - typically from about 5.0 at 0.2M up to 8.0 or higher at 0.8M. The lift coefficient at zero AoA will be about 0.15, so the AoA for L/Dmax would typically be about 5.7deg at 0.2M falling to about 3.2deg at 0.8M. I'm using a different aircraft to HN39 of course, but the numbers are not very different.
Last edited by Owain Glyndwr; 19th Aug 2013 at 07:25.
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An interesting discussion.
On a more practical level with regards to everyday descent performance, I find new jet trainees have a little trouble explaining the effect weight on a typical jet descent. ie. Why does a heavier jet require more track miles for descent compared to the same jet at a lighter weight, assuming a flight idle descent at typical jet descent speeds (280kt/250kt below 10,000)?
The answer of course lies in how close an aircraft’s descent speed is to it’s current best L/D (driftdown) speed for the current weight.
Example;
B737 at 41,000’ – track miles required for an idle descent in nil wind;
70,000kg -142nm Best L/D speed: 247kts
50,000kg - 119nm Best L/D speed: 209kts
Descending at 280kts or 250kts below 10,000’, the heavier 737 will be descending at a speed closer to it’s current best L/D speed compared to the lighter example. At 250kts, the heavy 737 is descending at almost it’s best L/D speed and the lighter version is descending much faster than best L/D. At 280kts the lighter jet is even further from best L/D.
So the bottom line to remember? If you are held high on descent (and unable to increase your descent speed), a heavy jet is going to have more trouble getting down compared to a lighter one. At lighter weights and the same descent speed, you will have a much steeper descent path (higher rate of descent) compared to a descent at a heavier weight.
On a more practical level with regards to everyday descent performance, I find new jet trainees have a little trouble explaining the effect weight on a typical jet descent. ie. Why does a heavier jet require more track miles for descent compared to the same jet at a lighter weight, assuming a flight idle descent at typical jet descent speeds (280kt/250kt below 10,000)?
The answer of course lies in how close an aircraft’s descent speed is to it’s current best L/D (driftdown) speed for the current weight.
Example;
B737 at 41,000’ – track miles required for an idle descent in nil wind;
70,000kg -142nm Best L/D speed: 247kts
50,000kg - 119nm Best L/D speed: 209kts
Descending at 280kts or 250kts below 10,000’, the heavier 737 will be descending at a speed closer to it’s current best L/D speed compared to the lighter example. At 250kts, the heavy 737 is descending at almost it’s best L/D speed and the lighter version is descending much faster than best L/D. At 280kts the lighter jet is even further from best L/D.
So the bottom line to remember? If you are held high on descent (and unable to increase your descent speed), a heavy jet is going to have more trouble getting down compared to a lighter one. At lighter weights and the same descent speed, you will have a much steeper descent path (higher rate of descent) compared to a descent at a heavier weight.
Last edited by Warped Wings; 19th Aug 2013 at 08:36.
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I've edited the graph in post #11 to show more clearly what it represents.
I removed the AoA curve because it is not relevant to the question being addressed, and also because the AoA's seem to be less well defined than the L/D values.
The two curves in red show the variation of L/D vs Mach at a particular weight at two altitudes, FL150 for the left curve and FL350 for that on the right. Many of those curves could be drawn to cover all altitudes and weights. The blue line connects the maxima of all the 'red' curves that could be drawn. So any of those curves, for whatever weight and altitude, will have a maximum L/D that lies on the blue line.
I removed the AoA curve because it is not relevant to the question being addressed, and also because the AoA's seem to be less well defined than the L/D values.
The two curves in red show the variation of L/D vs Mach at a particular weight at two altitudes, FL150 for the left curve and FL350 for that on the right. Many of those curves could be drawn to cover all altitudes and weights. The blue line connects the maxima of all the 'red' curves that could be drawn. So any of those curves, for whatever weight and altitude, will have a maximum L/D that lies on the blue line.
Last edited by HazelNuts39; 19th Aug 2013 at 16:16.
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Warped Wing's explanation is a good one.
With 747 actual landing weights anywhere from 380,000 to 630,000 (-200), descent planning involved beginning descent late (light) or early (heavy) when descending at the same Mach/IAS profile. With that weight range, the effect is very noticeable.
With 747 actual landing weights anywhere from 380,000 to 630,000 (-200), descent planning involved beginning descent late (light) or early (heavy) when descending at the same Mach/IAS profile. With that weight range, the effect is very noticeable.
hawk37
As you say, HN39's chart is type specific, and I am not sure the small increase in L/D between 0.2M and 0.5 is at all typical - certainly for the aircraft with which I am familiar the drag and L/D are virtually constant up to about 0.6M where compressibility starts to kick in.
As you say, HN39's chart is type specific, and I am not sure the small increase in L/D between 0.2M and 0.5 is at all typical - certainly for the aircraft with which I am familiar the drag and L/D are virtually constant up to about 0.6M where compressibility starts to kick in.
Last edited by henra; 19th Aug 2013 at 20:00.