Does glide ratio change with altitude?

On Sep 19, 10:43 am, "Jim Carter" wrote:
During a recent discussion about calculating glide ratio, I began to wonder
what the effects of pressure altitude were on the glide ratio of an
aircraft. Since air is 50% as dense at FL180 as at sea level, would the
glide ratio increase (glide further) as altitude decreases? AND if there is
a difference in glide ratio as altitude changes, then what values do most
manufacturers use when they publish their numbers (if they do)?

Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the
wags can skip that part of the reply...

--
Jim Carter
Rogers, Arkansas


This is a very good question. As others have mentioned, max (L/D) and
best glide IAS remain the same regardless of altitude. But that does
not immediately lead to the conclusion that glide ratio also remains
constant. L/D and best glide speed is about indicated airspeeds. At
higher altitudes, the best glide speed (IAS) wll result in a higher
true airspeed. So one might be tempted to conclude that the airplane
may be able to glide father, which is why I suspect you asked the
question in the first place.

It is not sufficient to argue that L/D remains constant, but you also
have to show that L/D is equal to the glide ratio. Glide ratio is the
physical horizontal distance traveled over still air for a unit
vertical altitude lost. Therefore, it is the ratio between the true
airspeed and the true vertical speed (TVS). I am sure there are many
ways to do this, but I find the energy argument the most intuitive.
Consider the power lost to drag, which is (D*TAS). This should be
equal to the rate of energy given up by the airplane due to its
descent. The rate of energy lost due to the descent is W*TVS, where W
is the weight of the airplane. We also know that W=L under
unaccelarated flight. Therefore, D*TAS = L*TVS and we can TAS/TVS =
glide ratio = L/D.

This might be obvious to some, but I had to do this step by step in
order to convince myself that L/D is indeed equal to the glide ratio.
Pehaps you might find it useful.

Jim Carter wrote:

*nitpick with finer granularity*

...and if lift is linearly related to air density and drag is linearly
related to air density, then aren't lift and drag in a linear relationship
to each other?

Well, yes. Anything else wouldn't make much sense, would it? Argh. Must've
forgotten to take my mediaction or s/th.

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