Why do best glide speed and best angle of climb speed differ?

16 Answers

1. 
   

   
   

   Kris Kortokrax on Nov 28, 2016

   Best angle of climb speed is dependent on excess thrust, which assumes that power is available.

   Best glide speed (as quoted in your POH), assumes no power is avaliable.

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2. 
   

   
   

   agronsky on Nov 28, 2016

   Kris, thanks for your answer. In my opinion this does not answer the question for the following reason:

   to be precise, excess thrust defines not the _best_ angle of climb, but just angle of climb at given AoA and speed. In other words, if I am gliding stable and trimmed at V_LD and suddenly apply full thrust, this simple rotates me (with my AoA and speed roughly preserved) up till I have no excess thrust due to added backward component of weight.

   But speed and AoA are preserved! And I’m now climbing at V_LD, having no excess thrust but still ~20 kts above V_X! That’s why I conclude it’s not excess thrust which defines things here.

   My question was more about the reason why after doing the above I still have an option to increase AoA lower speed and go to a steeper climb, while I didn’t have this reserve at idle.

   I know there’s mistake somewhere in my considerations, but I want to understand where exactly.

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3. 
   

   
   

   Kris Kortokrax on Nov 28, 2016

   The best angle of climb speed is used to clear obstacles. I cannot imagine a scenario where you would apply less than full throttle (which would result in the maximum value for excess thrust). if you are flying at the speed for best angle of climb ( i doubt that you have an angle of attack indicator in your training plane, unless you are training in the military), you will achieve the maximum angle of climb available from your airplane.

   I also cannot imagine a scenario in which one would be descending with no power at best glide speed and then would apply full power without also using the controls to manage aircraft attitude to establish the best angle of climb speed, if that is the speed and performance which is desired.

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4. 
   

   
   

   agronsky on Nov 28, 2016

   Dear Kris, thanks for your answers, I still not figuring it out, but I guess I was not clear at all in giving my question. Sorry for stealing your time.

   Best,
   Alex

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5. 
   

   
   

   JLashnits on Nov 28, 2016

   I think the difference in speeds is more likely due to the presence/absence of airflow over the wing from the prop(s).

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6. 
   

   
   

   agronsky on Nov 29, 2016

   Dear JLashnits, thanks, this looks reasonable. I also came across an opinion that full thrust setting allows lower speeds (with the same AoA) due to an increased vertical component of thrust. Can that be the main factor?

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7. 
   

   
   

   Joe Platt on Nov 29, 2016

   Hi,
   I\’m not an instructor but I believe that as you say Vx is the speed which provides the max excess power over that required for level flight, this same speed with zero power should also give the max TIME aloft. This is not the same as max distance (range) which is what you want for best glide. Max distance requires a somewhat higher speed which can be found by drawing a line tangent to the power curve. Try studying at http://www.av8n.com/how/ section7.5

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8. Best Answer
   
   

   
   

   Brian on Nov 30, 2016

   I wish I got here sooner. Go here: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80t-80.pdf

   Read pages 150-153.

   Airflow over the wing is irrelevant; consider the lake air, or any other pusher Prop aircraft, Vx will still not be the same as L/Dmax.

   Vx also has no relationship to min sink airspeed. Min sink in a Prop aircraft occurs at min Power required. Where as L/Dmax occurs at min Drag speed. Vx is a thrust problem.

   Please note my emphasis on Propeller aircraft. This is because this particular discussion will have a vastly different answer in a Jet powered aircraft. In fact, your suspicions would hold true in a Jet. The reason for the difference is because of the relationship between speed and thrust. In a Jet thrust remains nearly constant with changes in aircraft speed. This is quite different in a Propeller aircraft.

   In Propeller airplanes thrust increases as speed decreases. This makes it possible for the aircrafts theoretical best angle of climb to occur even below the aircrafts stall speed! Crazy right? More practically it means your true best angle of climb is not what your POH tells you, but instead happens right at lift off speed.

   So why does the POH give you a speed so much higher than your true best angle as Vx? Simple, legal fees would bankrupt any manufacturer who published Vs = Vx.

   Always remember the POH publishes what you need to do to meet their published performance specifications. The procedures are used to balance safety and performance. I bet you could land shorter if you approached at stall speed instead of 1.3 Vso, for instance. Then again you might also not live to tell us about it. In this later case you land really short though. 😉

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9. 
   

   
   

   agronsky on Nov 30, 2016

   Dear Max Platt,

   thanks for your answer, I see your intuition (I studied av8n sections 2, 4, 6, 7 to death these days) but I realize the difference between power and thrust kurves (see fig 4.17 and 4.18 here https://www.av8n.com/how/htm/4forces.html#sec-powers). I think I’m talking of different things.

   Let me explain more in the next comment.

   Best,
   Alex

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10. 
    

    
    

    agronsky on Nov 30, 2016

    Dear Brian, thanks for an awesome book! I’ve read thought relevant parts and understood your point. However, I think I could refine now. I know that V_X and V_LD come from the power curve, namely these are the points where the tangent going through the origin has a maximum angle.

    I would like however to bring my argumentation why _I_would_expect_ them to be the same. Pls see the picture I’ve sketched: https://www.dropbox.com/s/cw2xxvaxw5rdc3v/20161130160722411.pdf?dl=0

    I try to view things not from the point of view of power/energy, but from the point of view of forces/angles.

    1. Assumption: speed is defined by AoA (throught coeff of drag, coef of lift). It is a common knowledge.
    2. V_LD corresponds to a state where I can’t pull the stick any more (i.e. can’t increase AoA) without going steeper descent. Why it happens? See the left pic: it will produce more drag than lift — which results in rotating the path down to increase horizontal weight component (which acts like compensator of drag here).
    3. Now perform thinking experiment: remain at V_LD and just apply full thrust. My point is that NOTHING CHANGES in terms of speed, AoA, drag, lift and weight, EXCEPT that now the full thrust should be compensated by BACKWARD weight component, which results in rotating the path UP.
    My second point is that since NOTHING changed in terms of speed, AoA, drag, lift and weight, I’m perfectly balanced in my new angle of climb, and if I now pull the stick I will EXACTLY repeat the explanation of paragraph 2 above: it will increase drag faster than lift and the aircraft will rotate a bit down (meaning I was at BEST angle of climb) to reduce backward weight component.

    These are my considerations. The take away message is: the best climb angle and best glide angle (not climb rate and sink rate, which is totally different business and V_Y) are determined by the same AoA due to purely “angles-forces-terminology” explanation above.

    Thus I would expect the same speed (since AoA defines speed and vice versa!)

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11. 
    

    
    

    agronsky on Nov 30, 2016

    In the light of what I said above, my own answer to “why same AoA and different speeds?” is that full thrust contributes to lift (vertical component) and in some sense “shifts” the speeds scale to the lower speeds, as explained in Brian’s book on p. 47 (concerns stall AoA and speeds, but the same holds for any AoA and speed relation ship):

    “The typical jet aircraft does not experience
    the induced flow velocities encountered in
    propeller driven airplanes, thus the only significant
    factor is the vertical component of
    thrust. Since this vertical component contributes
    to supporting the airplane, less aerodynamic
    lift is required to hold the airplane
    in flight. If the thrust is small and the thrust
    inclination is slight at maximum lift angle,
    only negligible changes in stall speed will result.
    On the other hand, if the thrust is very
    great and is given a large inclination at maximum
    lift angle, the effect on stall speed can
    be very large.”

    Thus the AoA might be the same for V_LD and V_X actually!

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    HardLandings on Nov 30, 2016

    I think what you should be asking yourself is ‘why isn’t best glide (ld max) not the same as Vy – not Vx)

    Infact, in many of the older planes, Vy and Best Glide were quoted as the same number. That doesn’t seem to be the case these days. Best Glide, as you said, occurs at maximum Lift vs minimum Drag. Best Rate of Climb Vy is defined as the most lift per unit of time. So you can see how these would be the same, or very nearly the same from airplane to airplane. As its practically definition that the most lift over a period of time will come at the point of the least amount of drag. Now, when you are talking about Vx, your got yourself an orange in your apples to apples comparison. Here’s why: Vx is the best angle of climb, and its going to give you the most amount of lift as compared to your forward movement, not as compared to time. Your time to 100 feet or 200 feet or whatever, will be LONGER if you climb at Vx instead of Vy, but you didn’t go forward as far. At Vx you are not operating at the least drag, you have a higher induced drag because of how hard you are having those wings work, precisely because you are operating at a different angle of attack than the one that gets you L/D max.

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13. 
    

    
    

    agronsky on Nov 30, 2016

    Dear HardLandings,

    thanks for your answer. However let me assume you did not understand my question. I know about V_Y, rates and power required arguments. But I’m not interested in rates and any other “time” variables (rate of climb/sink speeds), which I totally well understand. I am interested in two analogical purely geometrical quantities. Please read my two last comments with picture. I want to identify the point of difference between to two BEST ANGLE speeds — climb and glide. Please get it: both speeds should correspond to THE SAME condition: pulling stick back (increase AoA/decrease speed) results in reverse behaviour of the aircraft — decreasing angle.

    I want to understand forces behind this behaviour. I give a very precise explanation in my two last comments. I realize I’m very deep into the stuff which usually is explained simple — by power curve and tangents to it. But I want more.

    Thanks to all once again, your answers bring me closer to the aim!

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14. 
    

    
    

    Joshua Clark on Dec 01, 2016

    Agronsky,
    I can’t give a full answer, but may be able to help you getting out of the purely angle thinking and allow the excess thrust to change the angle required.
    Consider the following scenario: You determine that L/D max for your aircraft and weight is 80 knots. However, you also have a 80 knot headwind. In that instance, holding L/D max does not actually give you your best angle of glide (straight down.) You must increase your speed to achieve any forward movement and then a better glide angle. Conversely, if you have a tailwind, a slower speed will actually achieve a better glide angle to the ground than L/D max.
    The point is not to show the definitive answer to your questions, but perhaps to get you to consider that factors outside of angle of attack (including thrust and power) will affect the angles that the aircraft flies.
    The true answer does indeed come down to the thrust produced by the engine, and how at a lower speed than L/D max, you will get more excess thrust (in propeller aircraft) and that excess thrust will affect the angle of the climb as well as the angle of attack.

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15. 
    

    
    

    Brian on Dec 03, 2016

    Agro,

    They are the same in a Jet, I mentioned that in my last post And the pages I cited for you to read confirm that. They are, however, not the same in a propeller aircraft.

    You’re trying to deal with too many variables. The first rule to any analytical analysis is to minimize the number of variables to make the problem manageable. I suggest you start looking for things to ignore, here is a small list:

    – Ignore jets. They are different, pick one.
    – Ignore climb angle. Sub 15 degrees the impact on the different forces is negligible.
    – Ignore AoA. It’s not part of the climb angle equation.
    – Ignore wind, rain, sleet, hail, lift, sink, and any other meteorological phenomena. They add useless complications for you at this stage.
    – Ignore any Vspeed except the two you care to understand.

    So what should you focus on? It sounds like you understand Vbg so put that to bed for now. Put your research into thrust and understand how it works with a propeller airplane. And I don’t mean just the pure definition or that it varies with speed. Learn where the thrust curve comes from. Why is the thrust required curve different from the power required and drag curves? How is it different? What about power/thrust available curves. Why and how are they different?

    To understand Vx in great detail requires you to understand thrust in great detail. All the rest is just white noise.

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    rmdomeni on Mar 19, 2017

    I’ll give a go at this one. Thrust is a force that contributes to work. (Word = force x distance). It has no time component like power (power = work / time). So for Vx, we are looking for the airspeed where there is the maximum distance between the thrust available and thrust required curves. We ignore the power curves. Props lose thrust as you go faster because of slippage. So the thrust available curve slants down to the right. This means that just because the drag curve (and therefore the thrust required curve) is at its lowest point, it doesn’t mean it will be at the maximum distance from thrust available. This occurs at a slower speed where there is less prop slippage. For best glide, there is no thrust available, so you shoot for the lowest point on the drag curve.

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