4 place glide ratio

[Erbman]

My performance characterization flight test is ongoing but slow (life gets in the way). The current drag polar that I'm working with shows a maximum glide ratio of 10.9 at an indicated airspeed of about 80 knots. Keep in mind this value does not include windmilling propeller drag, so the actual value may be less.

[500AGL]

Bumping this antique thread into this century.

I'm looking to see if any(one) or a number of you guys flying have collected any more data on this topic.
I'm up north at the moment but I do need to schedule some time to test this as well, for the greater good. Hard part is aligning scheduling time and weather conditions and aircraft availability.

Specifically I'm looking for :
- Best apparent glide speed (IAS) in mph
parameter 1 - No to little wind, idle rpm, no flaps, trimmed well, Temperature and Baro noted or at 29.92 and 68* (lol).
parameter 2 - Loaded Weight. (Closer to gross than empty)
parameter 3 - Close to Empty Weight

- Best apparent glide ratio.
same parameters, starting at 2000 ft, 4000 ft, 6000 ft.

Thanks,
pb

[jaredyates]

Hi Peter, I might be able to help a little with this. First, start here:

https://www.kitplanes.com/predicting-performance/

Second, I have attached my spreadsheet, with several crucial caveats:
The spreadsheet covers climbs and glides. You are only asking about glides, so the spreadsheet should be good as-is for pretty much all original-wing 4-Place Bearhawks. If you want to use the climb section of the spreadsheet, it is only applicable to the engine horsepower that you enter in cell C68. Also, more importantly, any climb performance is based on my old 76" Hartzell prop. In other words, you can't just drop 250 in cell C68 and yield numbers for a 540 Bearhawk, you have to first get the prop measurements to get the BAF (which is entered in cell M72). If you want help with this, let me know. Finally, this is only valid for the original 4-Place Bearhawk. It would not consider it valid for the Model B, Five, or Companion, and certainly not the Patrol or LSA.

Having said all of that, you can adjust the operational variables in the green cells C77 through C80, then look for the results in the blue boxes (cells C85:J92).

Answering some of your questions:
1. These numbers are going to be based on idle speed and having the prop at the coarse pitch stop. Fine pitch will make it worse. Dealing with wind is a separate subject. These numbers are all clean flaps. Temperature and PA are accounted for and adjustable in cell C78, just insert your desired Density Altitude.
2. Weight and environment make a huge difference in all of the numbers, and this is something that is often overlooked by pilots. They say "What is VX?" Well, it could be anywhere from 40 to 65 knots CAS. All performance speeds converge and become the same at the absolute ceiling. At "minimum fighting weight of 1200 pounds" and sea level, VX could be as low as 40. At 2500 pounds and 1000' DA, VX is 57 knots CAS. At 2300 and sea level DA, it is 54 knots CAS.

As you know, glide ratio does not vary with weight. It varies only slightly with DA, in fact so small that Dr. Lowry eliminated its effects in the spreadsheet. If we calculate the glide ratio as the reciprocal of the tangent of the glide angle:

Ratio = 1/tan(angle)

My data shows a glide angle of 4.9 degrees.

Ratio = 1/tan(4.9 degrees) = 11.66

I would propose that the single most influential factor on glide ratio is wind. If you have 25 knot headwind, your glide ratio is halved. I believe the more specific calculation is dividing the wind speed by the sink rate.

I would like to repeat the bootstrap testing with our current plane because it has the model B wings. There might be a little difference, but maybe not enough to measure. We'd have to test to know for sure.

[rodsmith]

Recent testing showed best glide for N3XH to be 70kt (80.5mph), glide ratio 9.5. Testing was at 2400#s, starting the glide from 8000' (elevation around here is over 6000'), baro 30.42, temp 33F.

[500AGL]

Thank you Jared & Rod. Very good info, and excellent data breakdown Jared.

Peter

[500AGL]

Funny thing about glide ratios and all the parameters that affect it.

Adding 1/3 of the headwind component to your best glide speed will help your distance, even though your speed and sink is greater. The trouble is coming out of 3000' with a loss of power and a 10-20 mph headwind, that increased glide will only gain you 220 feet more, based on AI calculations. So you have to take everything with a grain of salt. But without doubt know what the wind is doing. The handiest data I have in the cockpit is the wind direction and speed.

Know your best speed, practice engine outs in different environments, and make good decisions. Understand the value of making a decision and sticking with it unless there is time to truly consider another option. While math may tell you it will take 4 minutes to reach the ground, it will probably feel like 1 minute. Pick the landing zone, pick the approach, know what it will take to get there. I prefer to be high and long than low and short.

[OhioLSA]

Also, for any landing, remember that there's a great value in touching down with a headwind on the nose vs a tailwind pushing you. With an engine-out scenario, some folks seem to be in the mindset of "just get it on the ground." Especially for an unplanned off-airport landing, where you may hit anything from a fence to an unseen log or hole hidden by long grass, velocity is an important factor in the quality of the outcome.