Is "Best Glide" misleading?

Discussion in 'Pilot Training' started by Hang 4, Sep 15, 2020 at 12:51 PM.

  1. Hang 4

    Hang 4 Line Up and Wait

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    Some recent threads on the concept of best glide have me thinking that the term itself is misleading. Glider pilots (and Hang Glider Pilots) are pretty familiar with best glide, minimum sink, MacCready theory of speed to fly and the like. In GA, people get taught to go to best glide speed if the engine goes out.

    Where I think it may go a bit too far is the word "Best". I wonder if for those who don't fully understand the concept, if "best" over emphasizes how much better best is. Here's what I mean, in an actual engine out, figuring out why it quit and figuring out where to land are two real important things to do. Getting on an EXACT airspeed much less so. It's a pretty rare scenario where making a field is a few knots difference in airspeed. Another way of thinking about it, understanding how to know if you are going to make a field or not by looking out the window and seeing how the perspective of the field is changing is a critical skill, I don't need to look at airspeed to see that.

    I'm not sure I have an actual point, other than a feeling that "best glide" might get perceived as a miracle speed and have too much focus, vs managing your glide relative to the ground and understanding what options you have once the engine has quit.

    Hypothetical, I have two fields that seem to be within glide. The closer one looks less inviting than the farther one. I fly best glide to the farther one, how do I know when to give up on that one and go for the less good, but closer one. (hint, knowing my best glide speed helps, but is not the only part of the answer)

    Discuss :)
     
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  2. MauleSkinner

    MauleSkinner Final Approach

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    I think it’s more about training pilots who need to maximize glide every day how to do it properly, vs teaching the basics to somebody who’ll probably never need it, and because of disuse has a good possibility of screwing it up anyway if they do need it.

    unfortunately the simplified version often becomes the only way it can possibly work to those who are initially taught that way.
     
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  3. dmspilot

    dmspilot En-Route

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    Will a few knots matter? No, but there has to be a standard. If airspeed control was not emphasized during pilot training, deviations of -10/+20 or more would be common. Deviations that large impact glide performance significantly.
     
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  4. Hang 4

    Hang 4 Line Up and Wait

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    True, but what about a headwind? If it's a strong one, best glide, might not provide the longest glide over the ground. Or, making 180 and going downwind might be a much better choice. I agree with your point, except for the "must be a standard". It's a dynamic situation, and a fixed speed isn't always the best answer. When I'm in my hang glider at 5k AGL and trying to make a field 20 miles downwind, I'm rarely looking at my ASI and I'm changing speed a lot. I get about 15:1 in my HG, vs 9:1 in my Cherokee, but same concept.

    Not sure there is enough time to really teach the concepts with all the other new stuff to a PP, but just seems like learning to see what's going on relative to the ground to set pitch/airspeed seems better than staring at an ASI.
     
  5. EdFred

    EdFred Taxi to Parking

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    Well if you have time to do calculus while trouble shooting the engine, go for it. :D
     
  6. Dana

    Dana Pattern Altitude

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    If your chosen landing point is moving upwards in your field of vision, you're not gonna make it.
     
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  7. Hang 4

    Hang 4 Line Up and Wait

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    Not exactly what I meant (even though I typed it). All I was trying to say is that learning how to see how your glide is progressing, and knowing how to adjust gets missed in the focus on a single number. Agree that there needs to be a starting point and best glide is the right one to start with.
     
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  8. MauleSkinner

    MauleSkinner Final Approach

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    These are the two controlling factors for most power pilots.

    Glider pilots, on the other hand, don’t have to do calculus. Most glider manuals provide a polar, and simple rules of thumb (1/2 or 1/3 of the estimated headwind) provide a good value for adjusting your Best L/D IAS for wind.
     
  9. Bill Jennings

    Bill Jennings Touchdown! Greaser! PoA Supporter

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    OK, so we know best glide supposedly gives you the best no wind range over the ground. But I've also heard that max nose up trim (and resulting slow speed) gives the longest time in the air. It's kind of like Vx vs Vy.

    So what do you need to accomplish? I'd think that once the LZ is made, transition to the slow speed to give yourself more time to setup for the landing?

    Just thinking out loud lol...
     
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  10. MauleSkinner

    MauleSkinner Final Approach

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    Longest time in the air is going to be minimum sink or best endurance speed. I don’t know of any certification basis that says that’s going to be at full nose-up trim in a glide, so be careful with “absolute” statements. ;)

    otherwise, yes, it would give you more time to “set up” for your landing, but it’s also changing your sight picture to your aiming point, which can cause more problems than it solves.
     
  11. Dana

    Dana Pattern Altitude

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    Only if full up trim gives you Vx. That may be true for some aircraft, but it's not necessarily true for all, and it will vary according to C.G.
    Stick with best glide until you're maneuvering for landing, don't give up any altitude that you might need later if the wind shifts or whatever.
     
  12. sarangan

    sarangan Line Up and Wait

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    Very good discussion points. Exact speed is not relevant because it also depends on your weight and CG. You just need to be in the ball part. Making it to a good field is important, but it is far more important to prevent a stall. The airplane won't stall unless you yank the controls. Its better to trim for a reasonable speed and sit back. Take a few deep breaths and find a field you know you can make rather than something that looks tempting at a distance. All of this is easier talk about, but may be harder to do in real life.
     
  13. Hang 4

    Hang 4 Line Up and Wait

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    Best glide does not preserve altitude, it maximises distance over the ground (assuming no wind). Minimum sink conserves altitude. Our GA planes don't have polar data, so I'm not sure if there is a meaningful difference between min sink and best glide. I'd suspect there is not. Flying really slow (or using max up trim) will definitely get you neither, as you'll be mushing. Your sink rate will go way up, speed over the ground way down. Wild guess, on an average (172 or Cherokee) GA trainer, the difference between min sink and best glide is less than 10 knots. Min sink being slower than best glide. No idea of the difference in sink rate, but guessing it's not enough to worry about.
     
  14. RussR

    RussR Pattern Altitude

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    Using max up trim in a Cherokee 180 does indeed give you best glide, as demonstrated this morning by my Flight Review applicant, at least for our weight, CG and whatever atmospheric factors affect that.
     
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  15. MauleSkinner

    MauleSkinner Final Approach

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    CG would be the most relevant factor. I assume you were close to the forward limit with just the two of you. Have him ballast the airplane to near the aft limit and see what it does.
     
  16. Tantalum

    Tantalum Final Approach

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    It's a target airspeed. You can certainly go down a rabbit hole with best glide, and the inverse(?) of Vx/Vy and all sorts of fancy math and how it all changes (or does it?) with different weights and atmospheric conditions, but I think at the end of the day having someone remember a number is the best bet. When that engine fails most people will be on the cusp of panic.. pitching for a set, known "safe" airspeed number is the best course of action

    I'm not disagreeing.. but for people who will mostly have zero experience with unpowered flight I think the simpler you can keep the engine failure scenario the better. IE, pitch for this speed, fly towards the closest suitable field, get on the radio, mags, switch tanks, pump on, mixture enrich, etc.

    Naturally, a glider pilot is much more familiar with the air column and may not descend into panic and manage a much smoother "approach" and landing in a field. But without having people familiar with unpowered flight I think the best is to keep it as simple as possible
     
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  17. Dana

    Dana Pattern Altitude

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    What I meant was that best glide will get you over a particular point higher than if you flew to it at min sink speed. But I agree that for the typical GA plane the difference probably isn't meaningful unless you're very high.
     
  18. Bill Watson

    Bill Watson En-Route

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    Minimum sink speed (which I think is typically close to Vx) has no place in your engine out planning. Once you have the field made, slowing to minimum speed is good for survivability and such but it will never get you to the right field.

    Best glide (roughly Vy?) is the number to have your head. It’s in the manuals at full gross. Best practice is simply not to go below that in an engine out situation. If you are full gross (didn’t run out of fuel and seats full) don’t go slower, better to go faster. If you ran out of fuel and alone, best glide is lower than what is in the manual but best glide speed from the manual is not a bad place to be.

    Pay attention to the wind. You may want to land into the wind, but gliding into the wind reduces you radius of action so to speak. If you are gliding into the wind, go faster than best glide. If going downwind, fly best glide because going slower won’t help much.

    How does one do realistic engine out practice? How do you simulate a windmilling prop on a truly dead engine? Does anyone simulate a stopped prop accurately? Big diff between the two.

    Best aircraft to deadstick into a field is a glider with spoilers.


    Sent from my iPad using Tapatalk Pro
     
  19. Ryanb

    Ryanb Touchdown! Greaser! PoA Supporter

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    Depends on the phase of flight. If engine power is lost under 1000agl, getting on that proper airspeed right away is quite critical. On the other hand, if you’re at altitude and experience a power loss, you may not need to be at best glide to make the desired LZ.
    Yeah, but it’s going to give you the lowest decent rate, which imo, is more important. Lower decent rate = more time to try and sort things out. There’s no way to stretch your glide unfortunately.
     
  20. Kritchlow

    Kritchlow Final Approach

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    Really?? I always taught students to, FIRST THING, go for best glide. Then pick an airport.
    Regardless where that airport is, how can you go wrong by pitching for best glide?

    Perhaps I’m missing the point.
     
  21. sarangan

    sarangan Line Up and Wait

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    Yes, trim for best glide first, and then look for a field, but not necessarily an airport. I have found it useful to glance at the NRST page of the GPS. Some grass fields are hard to spot even when you are on top of it. It would look dumb to land in a corn field when there is a runway right next to it.
     
  22. Bill Jennings

    Bill Jennings Touchdown! Greaser! PoA Supporter

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    Foreflight has a nice feature, you can set your glide ratio and best glide speed, and it constantly shows you a circle (not really a circle, more like a constantly shifting ring shape) of how far you can glide using this information along with your altitude and terrain height.

    https://blog.foreflight.com/2017/03/28/new-in-foreflight-checklist-glide-advisor-siriusxm-audio/
     
  23. Dan Thomas

    Dan Thomas Final Approach

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    If you're trying to reach the only field available, against a headwind, you need to increase the airspeed. Groundspeed matters.
     
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  24. Palmpilot

    Palmpilot Touchdown! Greaser! PoA Supporter

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    Nice feature. I'm glad to see that it's available in the Basic version of the app.
     
  25. MauleSkinner

    MauleSkinner Final Approach

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    The next obvious question becomes “how much?”

    most pilots aren’t going to be able to do the calculus in their heads during an emergency, so it comes down to the fact that you can increase range by flying faster into a headwind, but how much faster and how much more range are products of a WAG and proper use of aiming point as stated previously.
     
  26. wilkersk

    wilkersk Pattern Altitude

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    I seem to recall there being a "quick and dirty" rule-of-thumb for this. Your airspeed should change by 1/2 your wind component. So that if you have a 10kt headwind, you increase your indicated airspeed by 5 kts. Likewise, if you have a 10 knot tailwind, and you don't want to overshoot your intended touchdown, you should decrease your IAS by 5 kts.

    So, the question really is "where is the wind coming from, and how hard is it blowing?"
     
  27. MauleSkinner

    MauleSkinner Final Approach

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    That would be an incorrect application of your recalled rule of thumb. ;)
     
  28. Palmpilot

    Palmpilot Touchdown! Greaser! PoA Supporter

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    I assume you're referring to the fact that there are ways to lose excess altitude if you find you are too high when you get near the intended field or airport, but gaining more altitude is not possible short of a lucky strong updraft. It's best to keep any excess altitude until you're close enough to be sure that you won't come up short.
     
  29. MauleSkinner

    MauleSkinner Final Approach

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    No, I’m referring to attempting to increase glide distance to prevent overshooting the landing.
     
  30. geezer

    geezer Pre-takeoff checklist

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    There are two basic best glide speeds for each aircraft.

    One provides the most time before you reach the ground, with no concern with reaching a destination. This is unaffected by wind speed.

    The second provides the greatest distance traveled, ignoring time in the air. This must be adjusted up when flying into a head wind. If in doubt on that, consider flying into a head wind equal to the glide speed, which will get you no distance. The rule of thumb of half the head wind component added to best glide is a good start. Subtracting for a tail wind is not desirable, get there as quickly as possible, and deal with excess altitude after you have the field made.

    Circling, flaps, slips, and S turns can be used once you are there. A miss judged decision earlier can be a disaster.
     
  31. Hang 4

    Hang 4 Line Up and Wait

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    Strictly speaking, one is best glide; max distance over the ground, the other is minimum sink; longest time in the air. Since we don't have published polars for GA planes, we don't know what min sink speed is or what the difference in sink rate at min sink is. I suspect there is not a meaningful difference between the two and can't think of a scenario where you would benefit from min sink vs best glide. Difference in sink rate might be on the order of 100 feet/min between the two. That's not going to buy you much time and time is only useful in trying to restart.

    The reason that I stirred this pot, (and didn't do a great job of what I was trying to say) is there seemed to be a lot of discussions on the minute details of best glide; Prop spinning or not, best glide at different gross weights etc. To me this could be perceived it was a "magic" number where you went from a flying like a rock to flying like a sailplane. I totally agree that having a number is better than not, and pitching to that number should be taught by instructors and done by pilots as part of the emergency procedure. What seems to be missing is once that's done and you've done all you can to restart, what then?

    When you're flying along, how do you know what fields you could make? What direction is the wind and how strong? Is a closer field behind you if you are in a headwind. How much altitude do you need to set up an approach? What is the wind direction on the ground, how strong? Have you checked your POH glide ratio (that you've put in Foreflight for the glide ring) with real life (recognizing it will be a bit worse with the engine actually off vs idling). My Hershey Bar Cherokee claims 9:1. I've not seen that demonstrated in the tests I've made practicing engine outs.

    Also not suggesting everyone needs to understand all the archana of gliding and soaring, but rather to have a more dynamic understanding of how to reach an airport or field to make a safe landing.

    Back to which way the copper gaskets face on spark plugs. :)
     
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  32. wilkersk

    wilkersk Pattern Altitude

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    [​IMG]
     
  33. brcase

    brcase Pattern Altitude

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    I agree some over emphasize best glide speed for emergencies.
    Most glider pilots have some sort of fancy electronic computer to compute the best speed (speed to fly) based on Lift/Sink, Headwind/Tailwind, Weight of the glider, even bugs on the wing. They can often tell us what the best speed to fly is or simply tell us to speed up or slow down.

    When I simulate emergencies I often see pilots spend way to much time setting the best glide speed. There is no point in setting the best glide speed unless you are trying to get somewhere. I suspect this comes from the training which is often some form of ABC (Airspeed, Best Landing Area, Cockpit). I tell my students if you want use ABC then A = remember no stalling, don't screw up the airpeed. B = Best landing are you can find in under 3 seconds. C= attempt a restart.
    What I find from experienced pilots when simulating an emergency is they turn it around, "CBA", They attempt a restart 1st Usually change fuel tanks for verify on Both then Carb heat/Fuel pump on as appropriate. Then if they don't already have a landing area selected they take a few seconds to pick the best landing area. Then they trim for a normal approach, flaps up until they are sure they need to flaps to not overshoot the landing area. Best Glide is only used if they know they might be stretching the glide distance a bit to make a desired landing area, but often best glide is pretty close to a normal flaps up approach speed.

    Brian
    CFIIG/ASEL
     
    Last edited: Sep 17, 2020 at 9:32 AM
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  34. MauleSkinner

    MauleSkinner Final Approach

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    this should result in instruction in setting it up efficiently, IMO. There should be a known trim setting or pitch attitude that will take a few seconds to establish and be able to move on.
    If the engine can’t be restarted and your airspeed isn’t near where it needs to be, you’ve both destabilized you’re glide and need To further destabilize it to get your speed established. I suspect this is how the OP in the other thread made his determination...he got slow, and had to push the nose down so far to get his speed back that he thought his glide would be too steep.

    if you figure to land a couple thousand feet right below you, establish an approach speed. Otherwise establish something close to best glide. But establish an appropriate speed right away.
     
  35. MauleSkinner

    MauleSkinner Final Approach

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    Zackly.
     
  36. wilkersk

    wilkersk Pattern Altitude

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    That's exactly opposite of what I said. But, I've come to the realization that this thread is a waste of my time. You have a nice day there sport!
     
  37. MauleSkinner

    MauleSkinner Final Approach

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    No, by misapplying your rule of thumb, that’s exactly what you said.

    unless you were talking about a separate rule of thimb from the headwind one, but I didn’t get that impression from your post.
     
  38. dmspilot

    dmspilot En-Route

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    Both of those rules of thumb have the purpose of increasing the glide ratio above what it would be without using them.
     
  39. wilkersk

    wilkersk Pattern Altitude

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    If you increase your IAS by 1/2 the headwind component, you get back a little of the "distance" you would otherwise lose if you maintain book "best-glide" speed. Conversely, slowing your IAS by 1/2 a tailwind component, you give back a little of the extra distance you would otherwise gain if you maintain the published "best-glide" speed. ....Or so the theory goes. Personally, I think real world numbers aren't gonna change that much at the ability, altitudes and speeds most of us fly at.

    I remembered where I got that from, its in a book called "Engine Out Survival Tactics", by Nate Jaros.
     
  40. dmspilot

    dmspilot En-Route

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    No, that's not the theory. You didn't read my post or chose to ignore it.

    "In the case of a tailwind, the optimum glide speed will be reduced to maximize the benefit of the tailwind." Aerodynamics for Naval Aviators
     
    Last edited: Sep 16, 2020 at 11:07 PM