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Want to clarify about Best L/D Ratio

  • tszchun.anson
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tszchun.anson created the topic: Want to clarify about Best L/D Ratio

Dear all,

I am writing to ask about the L/D ratio. According to my understanding of the drag curve, assuming the lift remains constant, induced drag is particularly high in low airspeed (because of the high AoA required to maintain lift), while parasite drag is particularly high in high airspeed (because of the skin friction). An optimum speed, called Best Glide Speed, is where both drags meet at the lowest point, mostly achieved by angle of attack of around 4 degrees. Because of this, I thought that it is airspeed determines lift-drag ratio, instead of angle of attack. First, may I confirm that, the same airspeed does not always correspond to the same angle of attack, especially for two aircrafts with different weight?

Today I am thinking of a question: "does weight affect Best Glide Speed?". The answer is mentioned in the study guide: the heavier needs to glide faster to acheive the AoA for best L/D ratio. However, I am confused about the relationship between them. Assuming there are two identical aircraft type but with different weight, are they having the same total drag curve? Or the curve will shift to the right side if weight is increased?

Thanks very much.


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bobtait replied the topic: Want to clarify about Best L/D Ratio

Hi Anson
The key here is to remember that we are talking about level flight. The best lift/drag ratio is achieved at a particular angle of attack (about 4° for most GA aircraft). The speed to use with this angle of attack varies with weight. If your weight is increased, you will need to fly faster to obtain the increased lift required, but you will still need to fly at that particular angle of attack. The extra lift will be accompanied by extra drag but the RATIO of lift to drag will remain the same providing you fly at the best lift/drag ratio angle of attack.

Remember that in level flight lift is not negotiable - it must equal weight. However, you do have a choice about how much drag you generate. If you fly at the speed that produces minimum drag, you must be flying at the best lift drag ratio, but the AMOUNT of lift and drag will vary if weight is changed.

Note that this means that the so-called best gliding speed (best lift/drag ratio) is valid for one weight only. You would have to glide at a different speed for every weight to remain at the best lift/drag ratio angle of attack. However, for the average light GA aeroplane, that is ignored and we are usually told to glide at a particular IAS. That's because the difference between light and heavy makes very little difference to the IAS required. In any case in practice the situation is complicated by wind. A head or tail wind will also affect the speed to use.

When it comes to situations like forced landings, nothing beats basic good judgement and practice when monitoring the glide performance.

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