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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. |
#12
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Does glide ratio change with altitude?
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. Ad. -- The mail address works, but please notify me via usenet of any mail you send to it, as it has a retention period of just a few hours. |
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