Questions Questions Questions

chris

Well-Known Member
Hey guys,

I have some Qs that I would like some insight on. These come after I have been reading Kreshner's Advanced Pilot Manual.

1. I realize that if I am flying an a/c at 65 KIAS at sea level, and then I climb to 10,000 and maintain 65 KIAS, the only difference is the TAS. However, will the AOA of the 2 a/c be the exact same? I believe they should based on the lift equation, but I am not sure.

2. In the chapter on Glides, Kreshner mentions that weight has NO effect on the maximum distance that an aircraft can glide. ie. an a/c will glide the same distance at max gross weight than at a lighter weight, assuming you maintain the proper IAS (i.e. the correct AOA to fly at in a best glide situation). I just find it odd... I figure more weight=more drag, which should lower the glide ratio.

3. Assume 2 of the exact same make, model and configuration (including weight). Both are at 10,000 ft, and both lose their engine and pitch for the max glide distance AOA. Aircraft A is experiencing standard conditions, while a/c B is gliding in standard+ 20 degrees Celsius conditions. Will they glide the same distance? Kreshner seems to hint that you must use the same IAS for any given altitude or temperature (the glide IAS should only be changed with weight) and that your range will not be affected.
ie) a/c B will have a higher TAS, but it will also be burning more fuel, as more power is required to maintain a specific IAS in a warmer air mass. However, the ratio of nm/fuel burned will be approximately the same for both a/c.

Well, that's all *for now*


Thanks for any responses.
 

Ophir

Well-Known Member
boy, let's see if I can tackle a bit of this. I will need help though.

[ QUOTE ]
1. I realize that if I am flying an a/c at 65 KIAS at sea level, and then I climb to 10,000 and maintain 65 KIAS, the only difference is the TAS. However, will the AOA of the 2 a/c be the exact same? I believe they should based on the lift equation, but I am not sure.

[/ QUOTE ]

This is a bit ambiguous but here we go. Both a/c will be indicating 65 knots but indicating is the opperative word here. To break it down, they both will be getting the same amount of air molecules in the pitot tube per second to register on the ASI. But, we all know that at 10,000ft there is considerably less pressure in the atmosphere and therefore fewer molecules of air in a cubic meter. So, as result, the higher aircraft will be flying at a higher true airspeed. So why don't we fly really high all the time? (sorry that sounds bad) If we go faster the higher we are souldn't we fly as high as we can all the time? Why is it that at a certain altitude the plane will stop climbing? Yes, we can take into account the engine and the decrease in power because the fuel isn't burning as well. But more importantly is the fact that air gets so thin that the plane is no longer producing the same amount of lift as the weight of the plane.

So if we are to disect this you will see that for each altitude the plane will have to fly at a higher and higher angle of attack to produce the needed lift to counteract the weight of the plane. Higher you go=higher AOA. At some point the plane reaches its critical AOA and stalls. so you are right, it is based on the lift equation.

[ QUOTE ]
2. In the chapter on Glides, Kreshner mentions that weight has NO effect on the maximum distance that an aircraft can glide. ie. an a/c will glide the same distance at max gross weight than at a lighter weight, assuming you maintain the proper IAS (i.e. the correct AOA to fly at in a best glide situation). I just find it odd... I figure more weight=more drag, which should lower the glide ratio.

[/ QUOTE ]
I am not sure I get that one.

[ QUOTE ]
3. Assume 2 of the exact same make, model and configuration (including weight). Both are at 10,000 ft, and both lose their engine and pitch for the max glide distance AOA. Aircraft A is experiencing standard conditions, while a/c B is gliding in standard+ 20 degrees Celsius conditions. Will they glide the same distance? Kreshner seems to hint that you must use the same IAS for any given altitude or temperature (the glide IAS should only be changed with weight) and that your range will not be affected.
ie) a/c B will have a higher TAS, but it will also be burning more fuel, as more power is required to maintain a specific IAS in a warmer air mass. However, the ratio of nm/fuel burned will be approximately the same for both a/c.

[/ QUOTE ]

Basically you have created a scenario that can be put into density altitude there. As the temp goes up for a non-standard atmosphere, solve for standard atmosphere and then temp and you will come out with a given density altitude therefore refer back to question one. But the range thing I don't get in this one. I will not say he is incorrect but it would appear to me that higher weight will require a higher AOA for all situations. As a result the net drag will always be greater and therefore your range would not the same.

Bueller?
 

E_Dawg

Moderator
1) yes

2) the only difference is the heavier a/c is faster and will reach the ground sooner... but will glide the same distance - gliders race with water in the wings for this reason.

3) IAS, pitch attitude, and glide distance would be the same in both cases, but the guy on the hot day will get to the ground faster because of a higher TAS and sink rate.
 

chris

Well-Known Member
Thanks for the responses.

Ed,

Do you know why the glide distance stays the same (aerodynamically speaking?) I tried to ask this to some instructors today, and many disagreed with Kershner. It seems odd that a heavier a/c can glide the same distance than a lighter one.

Also, in regards to my 3rd Q (the range one)... initally I thought it odd that both a/c could be at the same AOA, despite the fact of the non-standard conditions. Is it that a/c B (ISA+20) is flying at a higher TAS in order to be able to maintain the same AOA?

If that is the case, it seems then that since density altitude is higher for a/c B, it will require more power in order to be able to maintain the 65 IAS. This implies higher fuel burn, right? Does the range stay the same because the ratio of nm:fuel burned stays the same for both a/c... ie (A= lower fuel burn, but lower TAS.... B= higher fuel burn, but higher TAS... thus, nm/fuel burned can theoretically be the same for both a/c and the range will not differ).

This makes sense for a condition when the power is on, but when you lose your engine, fuel burn is completely irrelevant (perhaps this is why you lost the engine!). Thus, it seems the one with the higher TAS should go further... but it doesn't, and I can't seem to understand why.

I would really appreciate some more insight into this!


I think the problem is that I think too much... less time in the flying books and more time with girls would probably do me better
 

JayB

New Member
[ QUOTE ]
Do you know why the glide distance stays the same (aerodynamically speaking?) I tried to ask this to some instructors today, and many disagreed with Kershner. It seems odd that a heavier a/c can glide the same distance than a lighter one.

[/ QUOTE ]

Chirs,

Kershner is correct in that weight has no effect on the distance an aircraft will glide. The only thing that changes is the best glide IAS. The reason for this is quite simple.

The most efficient AOA an airplane can achieve is attained at (L/D)max, which is the AOA that gives you the least amount of drag, and therefore the greatest range. Therefore, (L/D)max is equal to the glide ratio. The AOA for (L/D)max does not change regardless of weight or altitude, however the airspeed for (L/D)max does vary with weight.

In summary, as your weight increases, your glide speed also must increase in order to obtain the best glide ratio; and when you do that, you are actually obtaining (L/D)max, which gives you the best range for your glide. Since the AOA for (L/D)max does not change with weight, the glide distance will be the same for both light and heavy aircraft of the same type. The only difference is that the heavier aircraft will reach the ground a lot faster, due to the higher speed required to maintain (L/D)max.

Now that you're armed with knowledge you can go teach those instructors of yours a thing or two.


--Jay
 

Nick

Well-Known Member
Seagull,
Could you give the name of those books? That link you posted didn't work.
 

CAVOK

New Member
"Kershner is correct in that weight has no effect on the distance an aircraft will glide. The only thing that changes is the best glide IAS. The reason for this is quite simple.

The most efficient AOA an airplane can achieve is attained at (L/D)max, which is the AOA that gives you the least amount of drag, and therefore the greatest range. Therefore, (L/D)max is equal to the glide ratio. The AOA for (L/D)max does not change regardless of weight or altitude, however the airspeed for (L/D)max does vary with weight.

In summary, as your weight increases, your glide speed also must increase in order to obtain the best glide ratio; and when you do that, you are actually obtaining (L/D)max, which gives you the best range for your glide. Since the AOA for (L/D)max does not change with weight, the glide distance will be the same for both light and heavy aircraft of the same type. The only difference is that the heavier aircraft will reach the ground a lot faster, due to the higher speed required to maintain (L/D)max."

VERY well put! Nice job with that explanation. I have been asked this question millions of times, and after I give my students the answer of "glide distance stays the same as long as the IAS is increased with the increase in weight". I always get the "why...why...why?" and after the thrid or fourth why, we end up pulling out the books.
 
Top