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The FAA says glide ratio doesn't change, regardless of weight, here on page 5-7.

Glide ratio is not affected by weight because, while a heavier glider sinks faster, it does so at a greater airspeed. The glider descends faster, but covers the same horizontal distance (at a higher speed) as a lighter glider with the same glide ratio and starting altitude.

However, a friend suggested to me that this is BS, and if you overload a craft above its operational limits, its glide ratio will plummet. In fact, he's confident that there's a sharp decline in glide-ratio from the point of going over the maximum weight limit. I don't know if this is true, but am interested to learn.

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Question

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The question is, quite simply, is the FAA inaccurate/wrong on this point? Can you reduce glide ratio by increasing the wingloading of a craft, as my friend suggested?

My Attempts at Research

I tried to look for polar curves that show the performance of overloaded craft, but couldn't find any' only curves within operational limits. I have no intention of testing something so dangerous myself, of course, but I do want to know the answer. I have heard anecdotes of people going well above the safety limits of their craft, for things such as world records.

I'm hoping wise people here can teach me a bit about the facts, and whether the FAA's statement is technically wrong as was suggested.

Additional Points:

He pointed out cargo planes have extremely severe limitations, where exceeding their weight limits will often be catastrophic.

There was a world record flight, I forget the plane and pilot, but I recall it was apparently loaded up to almost twice it maximum weight with fuel tanks.

Glider weight limits I saw seemed more related to landing and taking off than to concerns about performance in the air.

I've seen a couple of people state that glide ratio will only improve with a higher weight (very slightly), up till the point where A, the materials break from being overloaded, or B, the optimum speed will reach the Vne and become disabled for that fact.

Notably, even if glide ratio does remain consistent with more weight, landings, takeoffs, and manoeuvring become more difficult. It's also harder to make use of thermals.

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  • $\begingroup$ Tape a rock to a paper airplane and see how it glides. $\endgroup$
    – Jim
    Nov 15 at 6:44
  • $\begingroup$ @Jim By this you mean the FAA was wrong, and weight does have a huge impact on glide ratio? It is surprising they said it isn't affected by such, which is why I wanted to ask about this. $\endgroup$
    – user66219
    Nov 15 at 7:06
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    $\begingroup$ I don’t think the FAA is talking about “beyond operational limits” $\endgroup$
    – Jim
    Nov 15 at 8:05
  • $\begingroup$ @Jim It seems strange they make no mention of that. A couple of people on other sites have tackled the question, and believe that glide ratio will continue to remain static, and the craft will break from strain before that's likely to change. You could technically reach a point where optimum speed exceeds Vne, but they have stated that the increase in optimum speed is the square root of the change in weight, so that's very difficult to achieve. I'm hoping someone can go more deeply into this and confirm what the case is. $\endgroup$
    – user66219
    Nov 15 at 8:34
  • $\begingroup$ The glide ratio is a constant while operating within the normal operating conditions of the aircraft. Once you go outside those limits, then you likely have either a structural failure or a stall. If the FAA were interested in what happens outside the limits, they would have stated that in the question. $\endgroup$
    – jwh20
    Nov 15 at 13:16

1 Answer 1

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However, a friend suggested to me that this is BS, and if you overload a craft above its operational limits, its glide ratio will plummet. In fact, he's confident that there's a sharp decline in glide-ratio from the point of going over the maximum weight limit. I don't know if this is true, but am interested to learn.

Your friend is wrong. For that to happen, there would have to be a sharp decrease in the lift coefficient, and/ or a sharp increase in the drag coefficient, associated with any given angle-of-attack, once the maximum weight limit is exceeded. This doesn't happen.

Background info--

Glide ratio is the ratio of Lift to Drag, which is also the ratio of Lift coefficient to Drag coefficient.

In what follows we'll assume we can ignore the slight changes in Lift coefficient and Drag coefficient-- in particular the slight decrease in Drag coefficient-- associated with a slightly higher Reynold's number, as airspeed is increased at a given angle-of-attack. We'll also assume we are staying firmly in the subsonic range, so we can ignore Mach-related effects.

For any particular aircraft in a particular configuration, Lift coefficient and Drag coefficient depend entirely upon angle-of-attack. As long as angle-of-attack is constant, these coefficients are constant.

Therefore the best glide ratio relative to the airmass, which also the best glide ratio relative to the ground in still air, always occurs at the same angle-of-attack regardless of weight, for any particular given aircraft. (Optimizing the glide ratio relative to ground in various headwind or tailwind conditions appears to be beyond the scope of this question, so we'll assume that we're always talking about the still-air glide ratio, or the glide ratio relative to the airmass.)

For any given angle-of-attack, the effect of an increase in weight can be represented as follows-- to calculate the new horizontal speed and the new sink rate, take the polar curve (graph of horizontal speed versus sink rate), draw a line from the origin of the graph through the point representing flight at the best glide ratio at the lower weight, and then extend the line further in the same direction, so that the new length is increased over the original length, in proportion to the square root of the ratio of the new wing loading to the old wing loading. Both the horizontal and the vertical speed (sink rate) will be increased in proportion to the square root of the ratio of the new wing loading to the old wing loading. And so will the airspeed. So the glide ratio will remain unchanged. (Note that the airspeed is actually represented by the length of the diagonal line we've drawn, from the origin of the graph to the (horizontal speed, vertical speed) data point. For all practical purpose at reasonable glide ratios (small glide angles), this is also the same as the horizontal speed, but this approximation breaks down at extremely poor glide ratios (large glide angles).)

(Disclaimer: we're assuming that we're not breaking, or hugely deforming, the aircraft with the heavy load. It's not obvious that conventional aircraft would tend to deform in any way that significantly changes the glide ratio, before the point of structural failure. In flex-wing hang gliders, the flexible aluminum tubing comprising the leading edges of the wing does deform under heavy load in a way that changes the shape of the rest of the wing and greatly increases twist and washout and decreases glide ratio, but that's obviously beyond the intended scope of the FAA's comment that is the subject of this question.)

Glider weight limits I saw seemed more related to landing and taking off than to concerns about performance in the air.

Actually, glider weight limits are primarily dictated by concerns about the integrity of the structure under heavy load. You can imagine what would happen to the main wing spar, or the wing-fuselage attachment points, if you overload the glider severely and then "pull" several G's in a steep turn, or hit severe turbulence that imposes several extra G's of gust loading.

Links to related ASE questions:

Why is the L/D ratio numerically equal to the glide ratio?

Can we show through simple geometry rather than formulae or graphs that the best glide ratio occurs at the maximum ratio of Lift to Drag?

How are the glide polar and L/D ratio charts related?

Why would a glider have water ballast? If it is trying to stay aloft without an engine, wouldn't it be better to be as light as possible?

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