Studying for written exam, L/D best glide confusion

LongRoadBob

LongRoadBob

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Title says it, the specifics.

I'm still not sure...does "best glide" change with weight? Does best glide mean angle or speed?

1) what is the primary concern for best glide? Distance (covered over the ground) or time until reaching the ground?

2) two identical airplanes where one (A) weighs less than the other (B) at best glide (path/angle) will both have covered the same distance, but B will get there ahead of A because B will have a higher airspeed to be able to maintain the correct glide angle?

3) if you have a headwind equal to the best glide speed (here we are again back at speed being the issue) your descent relative the ground will be 90 degrees. In order to compensate with a headwind you want to have a faster KIAS than "normal best glide"? If tailwind, less airspeed.

Does anyone here see where I am going off the rails? Best glide is speed (relative weight) or is it glide path?

I swear to god, I'm getting dumber and dumber as I study more and more. I thought I had a nice handle on L/D and glide path (which is a little higher speed than least total drag) but I was wrong.

Be as harsh as you want. Just need to know where I am derailing.
 
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You're on the right track -
1) Best glide is at max L/D and results in the most distance covered - the glide ratio (distance covered/altitude) is equivalent to the L/D ratio.
2) On a particular airplane in a particular configuration it happens at the same angle of attack regardless of weight, so as weight increases the speed to glide at that angle of attack will increase, resulting in the same glide ratio at a higher speed. You'll hit the ground at the same point, just moving faster when you do it.
3) Speed up in a headwind, slow down in a tailwind. Glider guys have lots of rules of thumb, I just think of it as headwind hurts distance covered, so speed up to get through it; and tailwind helps cover distance, so slow down a little to take advantage of it. Not dead-on accurate, but it helps me remember.

Nauga,
who thinks you're still on the tracks
 
LongRoadBob said:
2) two identical airplanes where one (A) weighs less than the other (B) at best glide (path/angle) will both have covered the same distance, but B will get there ahead of A because B will have a higher airspeed to be able to maintain the correct glide angle?
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Correct, as nauga already pointed out.

In theory, with no wind, the same plane made out of lead will glide exactly the same distance as one made of styrofoam. But at very different speeds.

I think of it as the styrofoam one gently wafting down ever so slowly, while the lead airplane is making like a lawn dart. But in theory they will both hit the ground at the same point from the same altitude.
 
LongRoadBob said:
I swear to god, I'm getting dumber and dumber as I study more and more. I thought I had a nice handle on L/D and glide path (which is a little higher speed than least total drag) but I was wrong
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That's normal, at least for me. Every now and then you just have one of those moments.
 
Best glide varies with weight. The number in your POH is usually the number at max gross...at lighter weight, best glide would be lower.

Best glide is how you get max range or distance after something like an engine failure. You need this speed if your goal is to get every inch of distance to make it to a runway or other safe landing spot. A good rule of thumb is that best glide is halfway between Vx and Vy.

Maximum time aloft, or max endurance, occurs at a few knots less than best glide speed. It gives you more time in the air than any other airspeed. If you were high over an airport with engine failure, you might circle down at max endurance speed. You don't need the distance - what's important is buying as much time as possible, say to restart the engine.

Both speeds are helpful outside of engine issues. If you got lost and wanted to ensure maximum range, you'd choose level flight at best glide. Want to stay in the air long as possible, choose best endurance strait and level.

These are all airspeeds, not groundspeed. As you suggest, you need to be aware of the winds to understand the range you'll have. If you're halfway over a large lake flying into a stiff headwind, flying into the wind isn't too smart...you have better odds turning around and putting the wind behind you. In your example of headwind = best glide speed, your range is zero into the wind, vs 2 x Best Glide x endurance with the wind behind you.

Hope that's some help!
 
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"Maximum time aloft, or max endurance, occurs at a few knots less than best glide speed. It gives you more time in the air than any other airspeed."

-- in gliders this is called 'minimum sink.'
 
Thanks all, all the posts here helped, a LOT.
What was tripping me up, a formula I found for converting the published gross weight/air speeds to find the speed for other than gross weight. Works for stall speeds, takeoff speeds, maneuvering speeds...but then for best glide I got unsure about if it was appropriate.

I'm getting now that is is. That as pointed out of the plane was lighter than gross, I would need a different airspeed to get best glide than. That's a neat trick too with the best glide being between Vx and Vy.

This forum is such a great resource! Thanks guys!
 
Another neat trick in an ASEL. The familiar pitch attitude for level cruise flight will approximate best glide very closely. That means no need to stare at the ASI while getting to best glide. Look out the window, put the airplane in level cruise pitch and trim for it while looking for that landing area heading there. A quick glance back to the ASI to confirm and tweak if needed.
 
midlifeflyer said:
Another neat trick in an ASEL. The familiar pitch attitude for level cruise flight will approximate best glide very closely. That means no need to stare at the ASI while getting to best glide. Look out the window, put the airplane in level cruise pitch and trim for it while looking for that landing area heading there. A quick glance back to the ASI to confirm and tweak if needed.
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My level cruise is 90-100 kts, best glide is 72 kts with full fuel and 2 person onboard. The pitch attitude for 100 and 72 are diff. What am I missing here? On a diff note, the minimum sink airspeed is lot less then 72 as noted by the AOA, but that's a diff topic altogether.

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WannFly said:
My level cruise is 90-100 kts, best glide is 72 kts with full fuel and 2 person onboard. The pitch attitude for 100 and 72 are diff. What am I missing here? On a diff note, the minimum sink airspeed is lot less then 72 as noted by the AOA, but that's a diff topic altogether.

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The pitch attitude when gliding is different than with power.
 
WannFly said:
My level cruise is 90-100 kts, best glide is 72 kts with full fuel and 2 person onboard. The pitch attitude for 100 and 72 are diff. What am I missing here? On a diff note, the minimum sink airspeed is lot less then 72 as noted by the AOA, but that's a diff topic altogether.

Sent from my SM-G935V using Tapatalk
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What you may be missing is trying it and seeing for yourself it works.

As @Salty said, one's with power, one's without. One's level flight, one's descending. Both are geared toward airfoil efficiency. Holding altitude at 72 with no power will indeed produce a dramatically different pitch attitude than level flight. Gliding just doesn't.

And yes, minimum sink will usually be less than best glide. About 25% less.
 
midlifeflyer said:
What you may be missing is trying it and seeing for yourself it works.

As @Salty said, one's with power, one's without. One's level flight, one's descending. Both are geared toward airfoil efficiency. Holding altitude at 72 with no power will indeed produce a dramatically different pitch attitude than level flight. Gliding just doesn't.

And yes, minimum sink will usually be less than best glide. About 25% less.
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Thanks. I will give this a shot next time. Minimum sink is what i think is best, but I am not as comfortable as I shud be with the AOA.

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WannFly said:
Thanks. I will give this a shot next time. Minimum sink is what i think is best, but I am not as comfortable as I shud be with the AOA.
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It depends on the goal. Best glide is about maximizing range. Minimum sink is about minimizing vertical speed. One or the other might be better. And both might be the best choice in some situations. I need max range to get the the bad but doable field; minimum sink to soften the descent and minimize the damage to me and my passengers. Both are tools in our pilot kit.

I've never done it with an AoA meter. I'd be very interested in your results.
 
midlifeflyer said:
It depends on the goal. Best glide is about maximizing range. Minimum sink is about minimizing vertical speed. One or the other might be better. And both might be the best choice in some situations. I need max range to get the the bad but doable field; minimum sink to soften the descent and minimize the damage to me and my passengers. Both are tools in our pilot kit.

I've never done it with an AoA meter. I'd be very interested in your results.
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I have never tried this close to the ground, and I am interested to learn why minimum sink doesn't give u max distance, or may be I am getting confused. The AOA manual says , if u fly the alpha angle, thats maximum lift ovet drag and goes to mention that if ur fan stops, just fly the alpha angle to the ground. I have to look up the manual, but I think it also mentions that the alpha angle is the best glide.

I have tried alpha angle at altitude, stall horn blaring, but complete aeleron authority even at 40 degree bank, ASI was at 45 without flaps. The stall speed as per POH is 49 with flaps. I will have to try more variations before I can come to any conclutions though. Rt now I don't turn it on because my CFI is not willing to explore that with me, which I don't agree to, but I respect his decision. I will have to learn that on my own, after PPL.

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WannFly said:
I have never tried this close to the ground, and I am interested to learn why minimum sink doesn't give u max distance, or may be I am getting confused.
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Think of best glide and min sink as the glide equivalent of Vy and Vx.

One gets you farther at a higher airspeed, the other lets you float a little longer but you won't get as far at the lower airspeed.
 
Depends on the plane if the attitude for best glide is close to cruise attitude. In many turbo charged planes this is not trueb or at least in the ones I have flown in.

Tim

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tspear said:
Depends on the plane if the attitude for best glide is close to cruise attitude. In many turbo charged planes this is not trueb or at least in the ones I have flown in.
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Your post reminds me to suggest that we should all be familiar with recognizing the best glide attitude visually. With an engine out, you'll have no AI or DG (assuming they're vacuum), and in an extended descent, your battery might not keep things like panel lights running all the way down, depending on the electrical load you're carrying and the condition of your battery. It's good to know how to recognize best glide just by looking out the windshield, and the faster you can get to that attitude, the better. In a 172, for example, I know that trimming all the way nose up gets me within a few knots of best glide. In any kind of engine-out situation, you'll be panicked and up to your ears in trying to make it safely to the ground - the more "automatic" you can make things like getting and holding best glide, the better your chances are.
 
Best glide is generally an airspeed, an indicator that isn't vacuum operated. Also, I'm not sure you lose vacuum if the engine is windmilling, but I honestly don't know.
 
Yes, your airspeed indicator should keep working fine in an engine outage, but there are cases where this might not be true. An example might be a glass panel where the primary airspeed indication goes out when the PFD goes out. Even if there's a backup, it's probably not in the best position and you're probably not accustomed to flying by it. Besides, that's just not where I want my attention going in an emergency.

It's a good question as to whether a windmilling engine produces enough vacuum to keep the gauges going. I'm not a mechanic, but I think this is probably something that's airplane, vacuum pump and gauge specific. In a C182 I sometimes fly, I know that with the prop at max RPM and the throttle at idle, I'll get a "Low Vacuum" warning. But I don't see this in an Arrow I also fly, although the Arrow's AI sometimes tumbles at low vacuum. It's good to know in advance what your plane does...it would be a bad thing to think you can keep the plane at max glide by using the AI to keep a level flight attitude, and then follow it into some unusual attitude as the vacuum dies and the gauge drifts. There are of course cases where an engine seizes upon failure, with the prop not moving at all...point is, it's good to work these things out in advance so that you're not trying to figure all this stuff out in the heat of the moment.
 
Just curious what written exam you are studying for that requires this in depth knowledge.
 
Although knowing your best glide airspeed is important as is being able to trim for it, to me a more important skill is being able to deadstick your plane onto a postage stamp without killing yourself. You might not always have an airport to glide to. Field selection and then putting your plane down safely probably is what will save your life.
 
SkyDog58 said:
Although knowing your best glide airspeed is important as is being able to trim for it, to me a more important skill is being able to deadstick your plane onto a postage stamp without killing yourself. You might not always have an airport to glide to. Field selection and then putting your plane down safely probably is what will save your life.
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Yeah, unless you are very high, the diff between best glide and least sink might not do very much for you. Work on the landings, because you will eventually hit the ground somewhere.
 
Salty said:
Just curious what written exam you are studying for that requires this in depth knowledge.
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Just PPL, but in Norway. I don't know how the exam is in the US. For a while I had heard that PPL in Norway was more like commercial rating in the US, but in the classroom intensive course (up til then it had been all online course with the school, books at home but quizzes online with direct help of instructors grading and answering questions) I asked about this and the instructor who had also trained in the US said they are "about the same, maybe a little more in depth in Norway" and went on to praise the US system saying that he felt you guys concentrated more (than here) on actual stick and rudder skills.

I really love learning, but it seems like it is a LOT. I've taken a full week vacation to study for Thursday's exam.
It starts at 5 p.m. And they allow six hours for going through it all. I'm trying to plug the holes and put it all together a little better. I have learned a lot but as here with my question, I feel like I get a lot of it, then a new concept or point shakes my faith as to whether I "get it" after all.
Seems like everything affects everything else :) and I've got to get through flight theory, the real tough parts for me are weather, rules and regulations (can't read up in English because we are SERA and European + Norwegian laws and refs. I'm American and the whole course is in Norwegian).

Thanks again all, for the help!
 
LongRoadBob said:
I don't know how the exam is in the US.
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I suspect your curse follows JAA (European) curriculum, in the US we follow FAA mandates. I have friends in Poland who took PPL course (JAA) and it was full of unnecessary theoretical stuff, some arcane computational problems, etc. Yes, it is harder to get PPL in Europe because there is so much emphasis on theory. FAA, however folks complain, is still a lot more pragmatic in its approach. Good luck to you.
 
Doing last day cramming now. Last night I took (about five hours, the exam is six allowed) a " sample exam" from a book that is supposed to have questions very like those on the exam.

Quite a few things we didn't touch much on in ground school. One of them (is it rare to have one?) would be whe typically in the system a carb heat sensor would be located. The test exam says (roughly, translating) it "shows" the "temperature of the gas mixture (behind the carburetor)"

Behind the carburetor? The gas mixture happens IN the carb. So I tried to find a drawing or info, but it's a haystack of "when should I use carb heat?" Kinds of things and a needle I suppose somewhere with the info about where it is and what it actually measure. Luckily, you guys are here!


The test seems to be almost obsessed with testing if I know how many times the camshaft turns, or propeller, and about the cycles of four stroke engines, etc. I could see this if it might indicate something in the real world, but I don't see it.

For some reason too, they use a different word for "nitrogen". All ground school books, we use the Norwegian "nitrogen" (just pronounces different) and oksygen (obviously oxygen) 78% and 21% respectively. It was no problem but in the test quiz they call it "kvelstoff" (literally seems to be "choking material") and "surstoff" which I knew to be oxygen. Literally "sour stuff or material"

Oh well. Checking results, the things I was most unsure of I did really ok on, navigation (which I love and did well) doesn't have actual planning, but some of the questions tripped me up. I know what to concentrate one anyway.
 
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