Thoughts on "Best Glide"

455 Bravo Uniform said:
Good stuff. Besides being fun and interesting, some glider training is going to get on my bucket list.
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It will make you humble....

I never had a single inadvertent stall in a powered aircraft, even in slow flight below the white arc. Plenty of intentional stalls. I can't say the same thing about a glider. I have barely an hour in gliders and I've stalled unintentionally dozens of times. And slipped the crap out of them unintentionally a few times, too (damn yaw string...).
 
MAKG1 said:
Best glide is basically max L/D (actually, that's best endurance, but it's numerically close)...
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Best glide *is* at L/Dmax. Best endurance is at L/Dmax for a prop (oops) jet. Best *range* is LDmax for a prop. See dmspilot's post #45 and any basic airplane performance textbook.

Nauga,
threaded
 
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MAKG1 said:
I think that's true of most small singles. And very much on purpose. Best glide is basically max L/D (actually, that's best endurance, but it's numerically close), which is also where the peak in the power curve is. Slower than that, and you're "behind the curve." You can land that way, but not without power.

Don't confuse approach and landing speeds, though. Landing at 65 will be very bad news for your nosegear and prop. It's a LITTLE fast for approach, but will do fine on a runway that isn't really short.
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nauga said:
Best glide *is* at L/Dmax. Best endurance is at L/Dmax for a prop.

Nauga,
threaded
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Not quite.

"Performance conditions which occur at (L/D)max are:
  • maximum endurance of jet powered airplanes
  • maximum range of propeller driven airplanes
  • maximum climb angle for jet powered airplanes
  • maximum power-off glide range, jet or prop"
—Aerodynamics for Naval Aviators, p. 32

As nauga said, best glide IS L/D max. Not sure where MAKG gets "basically" and "numerically close." Additionally, MAKG is incorrect because L/D max does not coincide with the peak of the power curve. This information is in Chapter 3 of the AFH and Chapter 10 of the PHAK.

Furthermore, for propeller airplanes, best endurance is slower than L/D max because, at lower speeds less power is required to produce a unit of thrust as at higher speeds. This more than offsets the increase in induced drag to a certain point. Explained in Aerodynamics for Naval Aviators pp. 96-98.
 
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dmspilot said:
Not quite.

"Performance conditions which occur at (L/D)max are:
  • maximum endurance of jet powered airplanes
  • maximum range of propeller driven airplanes
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Oops...you are of course correct. That'll teach me to proofread :rolleyes:
In my defense, I had it right in another thread: How to get 15 hours one one tank :D

Nauga,
and the zone of confusion
 
455 Bravo Uniform said:
Good stuff. Besides being fun and interesting, some glider training is going to get on my bucket list.
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As a glider pilot myself, I can definitely recommend getting a glider rating. It made me MUCH more comfortable in the powered craft that I fly. I am also in the process of reading:
Engine Out Survival Tactics: Fighter Pilot Tactics for General Aviation Engine Loss Emergencies
Nate S. Jaros

The author points out that the USAF teaches that your aiming point should always start out about 1/3 the way down the landing spot (runway, field, etc0. That way, you come in a little high, and you have that extra altitude in the "bank" to make final adjustments.

In my opinion, Rykymus has a really good idea of practicing engine out regularly as well as teaching yourself how to spot a good landing spot. The engine out author emphasizes regular practice as well. In fact, he advocates using fixed positions on the plane to regularly, and accurately, be able to spot the limits of a 6 degree glide slope (or "wire" as he calls it). This glideslope is the minimum sink, and is almost universal in small, single-engine, light planes. He says that you can use a spot on the wing (leading or trailing edge), or something in the cockpit dash or cowling. He advocates physically marking these spots with paint or tape so that, in an emergency, you know where to look to spot the outer limits of a landable piece of real-estate.
 
@Ozone that "wire" makes perfect sense, having read and remembering that same "glide line" concept from Langewiesche's "Stick & Rudder". In fact, I went back to look at the book again and I had dog-eared the book on that page. Page 272
 
455 Bravo Uniform said:
@Ozone that "wire" makes perfect sense, having read and remembering that same "glide line" concept from Langewiesche's "Stick & Rudder". In fact, I went back to look at the book again and I had dog-eared the book on that page. Page 272
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Student here, I was trying to figure out, that mark...if it was on the wing you mean it would show physically where the limit of your range would be to the left or right? If on the cowling this mark would always be pretty accurate? Whether you were 1000' AGL or 3000' or whatever?

Would it aid in the approach as well at all, or not (thinking because then you are not using L/D but approach speed?) how far down towards landing would it be useful?

Hope this isn't a total dumbass question.
 
LongRoadBob said:
Student here, I was trying to figure out, that mark...if it was on the wing you mean it would show physically where the limit of your range would be to the left or right? If on the cowling this mark would always be pretty accurate? Whether you were 1000' AGL or 3000' or whatever?

Would it aid in the approach as well at all, or not (thinking because then you are not using L/D but approach speed?) how far down towards landing would it be useful?

Hope this isn't a total dumbass question.
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There are some rules of thumb along those lines, but they start breaking down if there is any wind, and especially if you put the flaps down. Engine-out glides are always clean until they can't be anymore or you need to lose altitude. Approaches are usually done with flaps, and full flaps result in a lower pitch angle than clean, for a given airspeed. On some planes, the difference is rather large.

It's better to place the aimpoint on the windscreen (mentally) and adjust throttle to keep it in the same place. It will move with each configuration change, but should stay there in between if you're stabilized.
 
If you want to look up the stuff that glider guys use look up the term polar curve...
 
Just find a decent thermal, and climb back up. Not that hard really :D
 
I have put orographic lift to good use in powered airplanes at times.
 
I've gotten some "free performance" from mother nature, but nothing I'd be able to soar on yet.
 
LongRoadBob said:
Student here, I was trying to figure out, that mark...if it was on the wing you mean it would show physically where the limit of your range would be to the left or right? If on the cowling this mark would always be pretty accurate? Whether you were 1000' AGL or 3000' or whatever?

Would it aid in the approach as well at all, or not (thinking because then you are not using L/D but approach speed?) how far down towards landing would it be useful?

Hope this isn't a total dumbass question.
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If the mark was on the wing, obviously you'd have to be wings level to use it as a guide, and you'd have to have a low wing plane. On a high wing I guess you could mark the strut. I would assume you'd make the mark on the cowling to be correct at the best glide attitude and therefore it wouldn't be correct at other attitudes(flaps down for example, or other speeds). Wind would also be a factor, as would your seat height, but I can see how it would be helpful to quickly rule out what you can't make. I would also assume after you had practice using these marks you'd get a feel for what 6deg(or whatever angle works for you) looks like and just kind of see it without the visual aid. There is an astronomer's trick where you can use the stars(known degrees of separation) to calibrate your hand at arms length to figure out various angles that way.
 
vontresc said:
Just find a decent thermal, and climb back up. Not that hard really :D
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Last week I got into an amazing wave in my single engine Cessna while on altitude hold. I was a bit busy looking outside (surveying terrain for future glider flights), and by the time I glanced at my altimeter I noticed I had climbed hundreds of feet, eventually over a thousand! This was all on the A/P "altitude hold", which normally doesn't deviate by more than 100 feet even in a strong updraft (and typically stays within +/-10'). Eventually I had to pull the throttle to near-idle, pull carb heat and push the nose down to descend to my pre-set altitude. I suspect I could have soared in that wave for hours, had I needed to.
 
Palmpilot said:
I have put orographic lift to good use in powered airplanes at times.
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So have I. Sometimes on purpose.

Once, locally, I got a nice smooth Vno climb in a 172. It was thankfully at full power or else I wouldn't have been able to avoid a Class B bust. I had to pull it to idle to level off, but it did, 500 below the floor.
 
RotorDude said:
Last week I got into an amazing wave in my single engine Cessna while on altitude hold. I was a bit busy looking outside (surveying terrain for future glider flights), and by the time I glanced at my altimeter I noticed I had climbed hundreds of feet, eventually over a thousand! This was all on the A/P "altitude hold", which normally doesn't deviate by more than 100 feet even in a strong updraft (and typically stays within +/-10'). Eventually I had to pull the throttle to near-idle, pull carb heat and push the nose down to descend to my pre-set altitude. I suspect I could have soared in that wave for hours, had I needed to.
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Unless you also got very fast while this was going on, your alt hold is not working properly. It sounds much more like pitch hold.
 
MAKG1 said:
Unless you also got very fast while this was going on, your alt hold is not working properly. It sounds much more like pitch hold.
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No, it's definitely altitude hold (not aware of an A/P with "pitch hold", though they might exist, for all I know). IOW, normally after a deviation, it starts gradually correcting back to the "hold" altitude, but when it got over 1000' and kept climbing, I gave up on it and took over manually.
BTW, I captured lots of data from that flight, including hi-res GPS-derived track and POV video, so afterwards I analyzed that flight to ensure that it was wave, including getting an opinion from an expert, who confirmed it for me.
 
RotorDude said:
No, it's definitely altitude hold (not aware of an A/P with "pitch hold", though they might exist, for all I know). IOW, normally after a deviation, it starts gradually correcting back to the "hold" altitude, but when it got over 1000' and kept climbing, I gave up on it and took over manually.
BTW, I captured lots of data from that flight, including hi-res GPS-derived track and POV video, so afterwards I analyzed that flight to ensure that it was wave, including getting an opinion from an expert, who confirmed it for me.
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Every two axis autopilot I've used aside from really crappy old STECs has a pitch hold mode. Read your manual. It's what they do when you turn "off" the alt hold. KAP140s will show "PIT" on the LCD, GFC700s will do the same on the PFD.

It's also the recommended setting in mountain wave conditions, to avoid speed excursions.

Alt hold in wave conditions, when working properly, will result in large airspeed excursions. During the updraft phase, it will get very fast, and during the downdraft phase, very slow. This is generally the opposite of what you want to do in the wave, as it extends your stay in the downdraft and shortens it in the updraft.
 
MAKG1 said:
Every two axis autopilot I've used aside from really crappy old STECs has a pitch hold mode. Read your manual. It's what they do when you turn "off" the alt hold. KAP140s will show "PIT" on the LCD, GFC700s will do the same on the PFD.

It's also the recommended setting in mountain wave conditions, to avoid speed excursions.

Alt hold in wave conditions, when working properly, will result in large airspeed excursions. During the updraft phase, it will get very fast, and during the downdraft phase, very slow. This is generally the opposite of what you want to do in the wave, as it extends your stay in the downdraft and shortens it in the updraft.
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MAKG, this is the unit I have, which definitely has an "Altitude Hold" function (and no "pitch hold"), which is what I use it for exclusively (I do only manual approaches, so I don't use the glide slope mode, and I find the vertical speed mode to be next to useless, so virtually never use that). The unit allows you to capture an altitude and hold it. If needed, you can modify that selected altitude (within +/- 200') by pressing buttons.
What is nice about it, is that unlike other units, the pitch and airspeed excursions are never excessive. It flies very smoothly, just like a good pilot. Which means I can fly it worry-free in any kind of turbulence or up/downdrafts, because its corrections are always very gentle, essentially trying to asymptotically recapture the selected altitude without excessive airspeed excursions (on either side).
And as I mentioned above, during this weird wave encounter, unlike my previous experience (over decades and thousands of hours), the updraft was so great that the A/P was unable to maintain altitude, climbing over 1000 feet before I took over.
I presume that other units that don't have that smooth touch built in might push the nose over hard and exceed airspeed limits (or stall in the other direction), but as mentioned this unit doesn't do that, hence the altitude gain, albeit the first time in memory that it exceeded more than a couple of hundred feet.
 
vontresc said:
Pitch mode????? I can tell you for a fact that Pitch Mode is NOT present in the KAP140. It is purely rate of climb based. Now if the climb rate exceeds the range of motion of the pitch servo, you will be climbing, ant that's more likely to have happened.

http://www.bendixking.com/HWL/media/PDF/PilotsGuides/PG-KAP140.pdf
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My own Altitude Hold A/P (to which I linked above) has both VS hold and Alt hold. You could theoretically hold "0 VS", but I am pretty sure that would start drifting immediately. In any case, I never use the VS mode, as I noted above. OTOH, my Altitude Hold is excellent, with very gentle corrections, usable in any kind of air.
Like you, I have never heard of a "Pitch Hold" function in an A/P (at least not in a small GA type), but that doesn't mean it doesn't exist somewhere.
 
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MAKG1 said:
So have I. Sometimes on purpose.

Once, locally, I got a nice smooth Vno climb in a 172. It was thankfully at full power or else I wouldn't have been able to avoid a Class B bust. I had to pull it to idle to level off, but it did, 500 below the floor.
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There was one flight where it wasn't clear that I would be able to get across Tehachapi Pass without retreating and finding out where the lift was. The effect was so strong that I opted to spiral up to 14,500 before crossing.
 
MAKG1 said:
There are some rules of thumb along those lines, but they start breaking down if there is any wind, and especially if you put the flaps down. Engine-out glides are always clean until they can't be anymore or you need to lose altitude. Approaches are usually done with flaps, and full flaps result in a lower pitch angle than clean, for a given airspeed. On some planes, the difference is rather large.

It's better to place the aimpoint on the windscreen (mentally) and adjust throttle to keep it in the same place. It will move with each configuration change, but should stay there in between if you're stabilized.
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Thanks, just curious but would flaps 10-15 degrees lengthen glide, and full flaps because of drag shorten and give a more steep glide path? I'm pretty sure about the second part being right, but am not sure about if light flaps would lengthen it.
 
LongRoadBob said:
Thanks, just curious but would flaps 10-15 degrees lengthen glide, and full flaps because of drag shorten and give a more steep glide path? I'm pretty sure about the second part being right, but am not sure about if light flaps would lengthen it.
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Flaps always increase drag, so no. Light flaps increase lift, but slow you down.
 
LongRoadBob said:
Thanks, just curious but would flaps 10-15 degrees lengthen glide, and full flaps because of drag shorten and give a more steep glide path? I'm pretty sure about the second part being right, but am not sure about if light flaps would lengthen it.
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I like to think about flaps as steepening the approach. So all else being equal, it would shorten the glide.
 
LongRoadBob said:
Thanks, just curious but would flaps 10-15 degrees lengthen glide, and full flaps because of drag shorten and give a more steep glide path? I'm pretty sure about the second part being right, but am not sure about if light flaps would lengthen it.
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Remember the four forces are in equilibrium most of the time, meaning lift = weight. We usually teach students that flaps increase lift, but it is more precise to say that flaps allow you to create the same amount of lift at a lower speed (or lower angle of attack). After extending flaps, even a small amount, the airplane returns to equilibrium with the same amount of lift but now more drag.
 
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dmspilot said:
Remember the four forces are in equilibrium most of the time, meaning lift = weight. We usually teach students that flaps increase lift, but it is more precise to say that flaps allow you to create the same amount of lift at a lower speed (or lower angle of attack). After extending flaps, even a small amount, the airplane returns to equilibrium with the same amount of lift but now more drag.
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Thanks for the extra explanation, and that is a great point. I actually had learned that, but have been studying so much and concepts and points kind of come and go. What you point out here is an important distinction and I will remember that point about same lift at lower speed. Thanks!
 
dmspilot said:
Remember the four forces are in equilibrium most of the time, meaning lift = weight.
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This is an mportant point.

In any steady state - level flight, climb, descent, whatever, lift=weight - or more exactly total up forces equal total down forces.

Any time the plane is not in a steady state, you will have acceleration in one plane or another until equilibrium is regained. For instance, in a constant rate climb, lift=weight - the plane climbs due to excess thrust, not excess lift. Increase the lift and the plane will accelerate upwards, with an increasing vertical speed, but then slow as lift again becomes equal to weight at the new, slower speed.

Anyway, looking at it that way helps me.
 
FastEddieB said:
This is an mportant point.

In any steady state - level flight, climb, descent, whatever, lift=weight - or more exactly total up forces equal total down forces.

Any time the plane is not in a steady state, you will have acceleration in one plane or another until equilibrium is regained. For instance, in a constant rate climb, lift=weight - the plane climbs due to excess thrust, not excess lift. Increase the lift and the plane will accelerate upwards, with an increasing vertical speed, but then slow as lift again becomes equal to weight at the new, slower speed.

Anyway, looking at it that way helps me.
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A bit of correction.

In any steady state configuration -- constant rate climbs, turns, descents, and so on, the sum of all four forces is zero. This does NOT mean lift = weight and thrust = drag separately. It's easier perhaps to see this in a glide, where thrust is zero, but the forces still balance. Drag is NOT zero.
 
MAKG1 said:
A bit of correction.

In any steady state configuration -- constant rate climbs, turns, descents, and so on, the sum of all four forces is zero. This does NOT mean lift = weight and thrust = drag separately. It's easier perhaps to see this in a glide, where thrust is zero, but the forces still balance. Drag is NOT zero.
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I think that's what I was going for with my qualifier, "...or more exactly total up forces equal total down forces."

But I can see even that fell slightly short of being exactly correct. Works as a mental construct most of the time.
 
exncsurfer said:
......There is an astronomer's trick where you can use the stars(known degrees of separation) to calibrate your hand at arms length to figure out various angles that way.
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The book I mentioned describes exactly that hand technique. I can see how the technique would help for anything in front of you, but I cant see how it would help for anything off to the side since one cannot stick their hand out the cockpit window. Here's a link, for those interested: http://oneminuteastronomer.com/860/measuring-sky/
 
I just steered the airplane to my desired track and picked a star that was prominent straight ahead and used that to fly towards. Way cool way to navigate!
 
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