Gliding Distance

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Re: Gliding Distance

Postby chornedsnorkack » Fri Jan 15, 2010 4:33 pm

Suppose that you, say, have both engines wrecked at the top of the climb by features like ash, vultures or overheating the engines... and find yourself at 40 000 feet with no prospect of relighting and 170 tons of fuel with absolutely no use except to power the postcrash fire.

When and where would you rather dump it?

L/D depends only slightly on weight.

If it did not depend on weight at all then the distance you could glide would be independent on the weight. Jets at cruise altitude are decent gliders... Azores Glider covered 140 km, so if the engines had failed for other reasons, and the tanks had been full, the same distance would have been covered, only faster.

But it does depend on weight.

For one, Mach number depends on true air speed. If the best glide speed for a fully loaded, 340 t airplane at altitude were, say, 0,84 M then after dumping the fuel, at 170 t empty weight, the best glide would turn out to be just 0,6 M.

Actually, lowering M improves L/D, at least in subsonic range (Not sure how to pick best glide speed and weight at the range 1...1,5 M. If you have 90 tons of fuel you do not want powering post-crash fire, would you start dumping right away at FL600 and M2,0, or after you have successfully gone to FL300 and M0,9, or at the end of the trip below FL100?).

Now, how about Reynolds number? At small Mach numbers, how does decreasing the weight and thus TAS affect L/D?
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Re: Gliding Distance

Postby beaky » Fri Jan 15, 2010 7:31 pm

Here's another question that I asked the same professor today, and he didn't exactly give me a straight answer. The thing with him is that he's quite a bit older of a fellow and he's got tonnes of experience, he's got every sort of license under the sun including an aerobatic's instructor rating so I asked him how to recover from a tail stall, the only thing he really told me was to pull back on the stick instead of forward, which didn't make a whole lot of sense, I then asked him about power settings for such a recovery and he wasn't too sure as the last time he had looked at such a procedure was at least three years ago... and the likelihood of this scenario ever happening at my school is slim to none as we are not allowed to fly in known icing conditions, both single engines and multi-engines owned by the school simply aren't rated for such conditions. Being a curious person who wants to know everything about everything, what is the procedure for such a scenario?

I dunno... tail stalls are more of a jet thing; I'm not sure. But it sort of makes sense (pitching up)- most horizontal stabs are angled relative to the fuselage so that they provide a downward force ("negative lift"). If that surface has stalled, that probably means its leading edge is too low (or ice accumulation is having the equivalent effect). So pitching up should reduce that "negative angle of attack" so that the stab starts doing its thing again.

Another indication that this is why is the fact that if that stab is providing down force, if it stalls, it stops producing down force... so the nose of the plane will try to pitch down, requiring you to pull back anyway.


Ask him if that's why... I'm curious myself, and probably wrong, because it seems so simple...   ;D
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Re: Gliding Distance

Postby SaultFresh » Fri Jan 15, 2010 10:03 pm

I'll have to ask him next week when I have his Human Factors class again... I'm sure he knows a bit more than he leads on to. The whole reason why it came up though was because he said he suspects that's what happened to that plane that went down last year in Buffalo, where they couldn't recover. He did however say that it's rare, the conditions have to be right, and it's hard to tell, because he did say the nose dips, so it could look like just a normal stall, in either case, it's kind of a scary thing, and I really have no idea what else could be done to recover other than that pulling back on the stick, which does make sense somewhat now. I have a feeling though he's not going to say much else about it, there might be a few others that I could ask about it though.
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Re: Gliding Distance

Postby beaky » Fri Jan 15, 2010 10:38 pm

Getting back to the heavier plane climbing scenario:

Let's forget the obvious problems associated with weight and its effect on takeoff roll and breaking out of ground effect... let's assume each pilot is already established at the desired climb rate to go from Point A to Point B, with Point B is at
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Re: Gliding Distance

Postby beaky » Fri Jan 15, 2010 10:59 pm


Now, how about Reynolds number? At small Mach numbers, how does decreasing the weight and thus TAS affect L/D?


I need to look up "Reynolds number", LOL, but my "horseback guesstimate" is that TAS, like any airspeed, will have to increase with weight to yield the desired L/D (not the L/D speed, but the actual lift-to-drag ratio). Look at it this way: weight, like load factor, has to be considered, even though they don't call it "L/D/W".  I know odd things happen in the transsonic zone, but I'll wager my two cents that the same rule about weight applies in that case.
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Re: Gliding Distance

Postby Brett_Henderson » Fri Jan 15, 2010 11:02 pm

We're gonna have to discuss this stuff over a beer sometime.
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Re: Gliding Distance

Postby SaultFresh » Sat Jan 16, 2010 2:07 am

I kind of have a sneaking suspicion that those two climbing planes would make it relatively closer to the same time. My reasoning is that, while both airplanes will have to increase their airspeed as height increases to maintain the rate of climb they want (assuming they're both climbing at the same rate), then the heavier airplane will have to be traveling at a faster velocity than the lighter one, simply because of drag I think. I think it's very possible to get the two planes at point B at the same time.

Reynolds number on the other hand has more to do with airflow, and can be used to find where the transition point between laminar and turbulent airflow over a wing.  I can't really make some of these symbols... so bare with me... Reynolds number can be expressed as Re= (VL)/v, where V is the velocity in ft/s, L is the exposed m.a.c., and v is the kinematic viscosity, which I had to pull that one off of wikipediahttp://en.wikipedia.org/wiki/Reynolds_number#Transition_Reynolds_number... it's not even listed in my own notes. The higher V is, the higher the Reynold's number. V is comprised of two variables, Mach number, and the speed of sound "a". Both variables can change, so if you're mach number is higher, and "a" has remained constant, than you're Reynolds number will be higher, meaning you will have more turbulent flow over the wings than laminar flow, which means more drag. How's that for a whole lot of late night information just being thrown into the wing... haha, anyhow, for the dumping of the fuel, there's probably a checklist to follow that would include when an appropriate time for that would be. If it were me, McGyvering my way through such a scenario, I would probably dump it sooner than later, just to give me more time to set up, and less stuff to worry about later. I mean, if you dump it after giving up hope on the engines, than you'll have time to set up the attitude you want (because it will change), and it's one less thing to worry about later on. Now that could be the wrong thing to do, I certainly don't know, I mean, traveling at such a quick speed brings so many different variables like possibly compressible flow which depends how fast you really are traveling and all sorts of drag. Kind of makes sense to me to get light as quick as possible to shed the drag, but again, I don't know, I've never traveled anywhere near as fast, or as high, as the speeds and altitudes. Also, please correct me if I made any mistakes in attempting to explain Reynolds number, not that I need this information for the slow flying in small planes that I do, but it never hurts to store some of this stuff away in the back of the brain.
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Re: Gliding Distance

Postby Brett_Henderson » Sat Jan 16, 2010 6:16 am

I kind of have a sneaking suspicion that those two climbing planes would make it relatively closer to the same time. My reasoning is that, while both airplanes will have to increase their airspeed as height increases to maintain the rate of climb they want (assuming they're both climbing at the same rate), then the heavier airplane will have to be traveling at a faster velocity than the lighter one, simply because of drag I think. I think it's very possible to get the two planes at point B at the same time.


If point B is distance from takeoff, AND an altitude.. the heavier airplane would need more thrust than the lighter.. AND bigger wings to get to the same point, at the same time.
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Re: Gliding Distance

Postby DaveSims » Sat Jan 16, 2010 7:46 am

Here's another question that I asked the same professor today, and he didn't exactly give me a straight answer. The thing with him is that he's quite a bit older of a fellow and he's got tonnes of experience, he's got every sort of license under the sun including an aerobatic's instructor rating so I asked him how to recover from a tail stall, the only thing he really told me was to pull back on the stick instead of forward, which didn't make a whole lot of sense, I then asked him about power settings for such a recovery and he wasn't too sure as the last time he had looked at such a procedure was at least three years ago... and the likelihood of this scenario ever happening at my school is slim to none as we are not allowed to fly in known icing conditions, both single engines and multi-engines owned by the school simply aren't rated for such conditions. Being a curious person who wants to know everything about everything, what is the procedure for such a scenario?

I dunno... tail stalls are more of a jet thing; I'm not sure. But it sort of makes sense (pitching up)- most horizontal stabs are angled relative to the fuselage so that they provide a downward force ("negative lift"). If that surface has stalled, that probably means its leading edge is too low (or ice accumulation is having the equivalent effect). So pitching up should reduce that "negative angle of attack" so that the stab starts doing its thing again.

Another indication that this is why is the fact that if that stab is providing down force, if it stalls, it stops producing down force... so the nose of the plane will try to pitch down, requiring you to pull back anyway.


Ask him if that's why... I'm curious myself, and probably wrong, because it seems so simple...
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Re: Gliding Distance

Postby beaky » Sat Jan 16, 2010 1:34 pm

I kind of have a sneaking suspicion that those two climbing planes would make it relatively closer to the same time. My reasoning is that, while both airplanes will have to increase their airspeed as height increases to maintain the rate of climb they want (assuming they're both climbing at the same rate), then the heavier airplane will have to be traveling at a faster velocity than the lighter one, simply because of drag I think. I think it's very possible to get the two planes at point B at the same time.


If point B is distance from takeoff, AND an altitude.. the heavier airplane would need more thrust than the lighter.. AND bigger wings to get to the same point, at the same time.

But what if Point A was not takeoff, but some random point when climb is firmly established?
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Re: Gliding Distance

Postby Brett_Henderson » Sat Jan 16, 2010 3:15 pm

I kind of have a sneaking suspicion that those two climbing planes would make it relatively closer to the same time. My reasoning is that, while both airplanes will have to increase their airspeed as height increases to maintain the rate of climb they want (assuming they're both climbing at the same rate), then the heavier airplane will have to be traveling at a faster velocity than the lighter one, simply because of drag I think. I think it's very possible to get the two planes at point B at the same time.


If point B is distance from takeoff, AND an altitude.. the heavier airplane would need more thrust than the lighter.. AND bigger wings to get to the same point, at the same time.

But what if Point A was not takeoff, but some random point when climb is firmly established?



I think the same would apply.

An
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Re: Gliding Distance

Postby beaky » Sat Jan 16, 2010 7:31 pm

[quote]

A quick glance at a C172 takeoff chart shows a 35% higher FPM climb-rate at
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Re: Gliding Distance

Postby Brett_Henderson » Sat Jan 16, 2010 8:27 pm

The assumption is that the heavier airplane will need to fly at a higher IAS just to climb fast enough to manage it... but will it get there faster?


That's moot, in our discussion.. we're assuming a full power-climb, so the heavier airplane doesn't have the luxury that the heavier "glider" has (available energy that varies by load).

You might be able to get to point B at  Vx, but that throws another variable in there. At Vy, the heavier airplane will have to fly a big "S" (lengthening the hypotenuse) to reach  B  without having to bouble back (an that introduces the lost lift during the turns.. :D  )

Yeah.. my head hurts too.. :P
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Re: Gliding Distance

Postby SaultFresh » Sat Jan 16, 2010 8:55 pm

[quote][quote]
this is what happens when you don't get to fly often enough...
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Re: Gliding Distance

Postby olderndirt » Sun Jan 17, 2010 8:57 pm

making my head hurt. :D
Been reading along - nodded off a couple of times and my head, teeth and butt hurt.
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