Glide Ratio

[pappas]

I found through my, likely loose, phase 1 testing on both of my Waiex's, (3300 and AV Turbo), that my best glide and minimum sink were within about 2-4 mph of each other between 78 and 82 MPH. I was usually able to get closer to 11:1 than 8:1.

I also found that in actual, coming down right now, flight conditions and with decisions and distractions, I was only able to reliably keep the craft at about 80 mph indicated. I couldn't keep it pegged at exactly 78 or 82. However, because they are so close to each other it really didn't matter for me.

I started flying in 2-stroke powered Rotax ultralights so I have quite a number of quiet engine experiences in 34 years. But only one in the Waiex.

Luckily, I have always found that whatever glide ratio I was able to achieve, it was sufficient to get me all the way to the ground!

Stay safe.

[Bryan Cotton]

Here is a better data set. I flew over the airport, and pulled to the idle stop except at 64kts and below. There I adjusted to maintain between 800-1000 rpm.

Note I used Jeff's spreadsheet so this is corrected for density altitude and true airspeed. It was nice and smooth today with an inversion. It was 4C on the ground and 8-11C during my flight. Hazy down low.

Waiex 191 glide data 11-23-22.png (16.07 KiB) Viewed 1506 times

[dbdevkc]

If I am remembering correctly, the speed at which you achieve your best glide ratio varies with the weight. So although ~11 might be the best glide ratio, since the weight of our individual planes might vary considerably from each other so our best glide speeds to achieve that ~11 will vary as well.

I think most of us understand that is the case with stall speed, but with best glide ratio speed it is not as intuitive.

[Bryan Cotton]

If I am remembering correctly, the speed at which you achieve your best glide ratio varies with the weight. So although ~11 might be the best glide ratio, since the weight of our individual planes might vary considerably from each other so our best glide speeds to achieve that ~11 will vary as well.

I think most of us understand that is the case with stall speed, but with best glide ratio speed it is not as intuitive.

From the previous page of this thread:

This brings up another point - best glide speed is a function of weight, because it is dictated by angle of attack. So at some point I need to do some light weight testing and some heavy weight testing to get a better polar and correction.

Edit: there is a lot to learn about this by flying gliders. If you are flying into a headwind, you fly faster. If you have a significant tailwind, fly slower (but not slower than minimum sink). If you are in an airmass with a lot of sink, fly faster to get out of it quickly. If you are in lift, fly slow (but not slower than minimum sink).

Based on my polar data so far, 80 would be a good speed to shoot for, and I could optimize if I had time to think about it. There is not a huge penalty for flying 80 or 90 kts at the weight I was at.

[GraemeSmith]

Just to comment on something I said previously. I stated putting the wing chordline parallel to the horizon would achieve something close to Vg.

In her column in the Feb 2023 edition of AOPA Pilot - 2022 CFI of the year Catherine Cavagnaro calls out this "rule of thumb" as being wrong. And in some cases dangerous.

I then went digging again to find out what had prompted me to this belief. I was fairly sure it was in the Handbook for Naval Aviators. And though I have completely re-read that whole piece again I can't find that reference.

Interestingly the handbook has an extensive discussion about how critical it is to maintain Vg and suggests that +/- 5% of the optimal Vg airspeed will not make a whole lot of difference to the glide ratio achieved. At least "in the heat of the moment". If all else fails the chapter repeatedly emphasizes the need to eject if things are headed south!

So - let me modify:

I have found the placing the chordline of the wing of my Sonex - parallel to the horizon will get me close to Vg. But this is not a hard and fast rule for all aircraft and not to be relied upon.