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Glide Speeds

sof!ail09

I'm New Here
First posting.

I need some info to complete ForeFlight performance data for my RV7A. These are not included in my POH.

1. Endurance (Holding) speed

2. Best Glide Speed ( Max Distance)

3. Min Sink Rate Glide Speed

4. Glide Ratio.

Is there a chart that shows Best Glide Speed for various altitudes?

Thanks, [email protected]
 
First posting.
Is there a chart that shows Best Glide Speed for various altitudes?

Thanks, [email protected]

Best glide speed, expressed as an indicated airspeed, is independent of altitude. However, it does vary as the square root of gross weight.
Curiously glide ratio, flown at best glide speed, is independent of weight (no wind). But if you need to glide into the wind, you’ll cover more distance over the ground if you’re heavy rather than light.
 
An approach to getting the answers

First posting.

I need some info to complete ForeFlight performance data for my RV7A. These are not included in my POH.

1. Endurance (Holding) speed

2. Best Glide Speed ( Max Distance)

3. Min Sink Rate Glide Speed

4. Glide Ratio.

Is there a chart that shows Best Glide Speed for various altitudes?

Thanks, [email protected]

I struggled with this when I first started flying my 7A. First, you have to define you KIND of speed. Specifically, you want to know those speeds with the prop doing what? Stopped? Windmilling? Producing power? Pure as in zero thrust? This is harder than you might think at first.

Next is easier. While there are some glitches in the curves, it's basically true for an RV or any non-linear conventional aircraft that the BE (best endurance), BG (Best Glide/Best Range) and GL (Glide ratio) are mathematically related. I recently posted some information on how to get Jack Norris's books for free. The one about economical flying is very rich in this subject.

But the BE and drag-minimum speeds are in a fixed ratio. In other words, your speed for least drag is almost exactly 1.32X the speed for BE. The speed for best range is essentially the same as the speed for best drag. At first it is counter-intuitive that the best endurance actually has a lot more drag but that is simply because it's a slower speed and power is drag x speed.

The minimum sink speed and the best endurance speed should be the same because the minimum sink must necessarily be the one that loses the least potential energy.

OK, now for the fun part. Ignoring issues like the engine's/prop's inconsistent efficiency at those low speeds, you can easily find the speed for BE or least-drag experimentally with a little careful flying. I won't go into detail here, but I have verified with a few cases that this works.

Fly the airplane in the calmest, stillest air you can, of course. For an airplane such as a 7, use either full rich or a very accurate power measurement. Fly the airplane as slow as you can without adding power and maintaining altitude. That is your best endurance and your best range speed is 1.32X that speed. BTW, your Carson's speed is then 1.32 x 1.32 x your best endurance speed. if you do this accurately you will probably be within 1 knot of the correct, actual, in-flight values. I also suggest temporarily switching your glass panel to KM instead of M or Kt. That's because a KM is only .62 as large as a M and that gives you better precision.

Lastly, if you would like a copy of a spreadsheet that lays all this out interactively with known, accepted equations, just PM me. I have one already for my 7A and the the CAFE 6A among other examples.
 
Stick something around 90kt, measure your descent rate and figure the glide angle - I would guess around 9:1. Weight does impact the speed but not that much (heavier = faster). When you need these numbers your brain will be thinking about many other issues so your ability to hold speed within +/- 5kt will be limited! As mentioned above altitude doesn't change the IAS.
 
Some good references

First posting.

I need some info to complete ForeFlight performance data for my RV7A. These are not included in my POH.

1. Endurance (Holding) speed

2. Best Glide Speed ( Max Distance)

3. Min Sink Rate Glide Speed

4. Glide Ratio.

Is there a chart that shows Best Glide Speed for various altitudes?

Thanks, [email protected]

Here are two CAFE reports that some may find very useful.View attachment CAFE-report-RV6A.pdf

I have responded to two requests for the spreadsheet and this is to help those members.
 

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  • CAFERV-9A.pdf
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