skydvrboy Posted November 3, 2020 Report Posted November 3, 2020 So I went out flying tonight with the sole purpose of determining the actual glide slope for my particular plane. Experiment Parameters: 105 mph indicated airspeed per POH, 500' to stabilize descent rate, time the descent for 500' vertical. First test 2500 RPM (cruise), throttle idle, mix cutoff, then "feather" prop, 500' to stabilize, 500' timed. Airplane conditions: '67 M20F, 655 lbs under gross, most 201 mods added, needs washed and waxed. Results: 2500 RPM = 33.0 seconds = glide ratio 10.2; "Feathered" = 41.1 seconds = glide ratio 12.7 POH Data: Prop windmilling 105 mph glide ratio 10.2; Prop stopped 100 mph glide ratio 12.7 So was this just dumb luck? I think the best glide speed changes with weight, so my true best glide speed should have been lower than 105 (though I don't know exactly how much or how to calculate). I also assume the 201 mods would reduce my drag from the factory airplane. So is this just an example of multiple errors compounding to cancel one another out or is the best glide ratio a relatively fixed number at a given airspeed? Quote
carusoam Posted November 3, 2020 Report Posted November 3, 2020 There is an awesome thread around here where an MSer went on a trek to learn about his glide ratio... Find Chuck Noe... @cnoe https://mooneyspace.com/search/?q=Chuck glide&updated_after=any&sortby=relevancy&search_and_or=and Best glide is a range set from MGTW at the top, to closest to empty weight at the bottom... It will be in the range of about 10ias, near 90 at the heaviest to close to 80 at the lowest... Chuck did his experiments in an M20J, which has some aerodynamic improvement over the M20F, but not a whole lot... Minimize drag... 1) gear up 2) flaps up 3) prop pulled back all the way, or stopped if enough altitude to do the math... keeping it stopped can be a challenge as well... 4) speed brakes stowed... 5) Weight and balance to the rear of the envelope... 6) controls rigged properly... PP thoughts off the top of my head... the indicated air speeds are similar for many Mooneys but there is a variation when using kias or mias... the planes that use KIAs weigh more than the Mooneys using MIAs... but their actual numbers are similar... Best regards, -a- Quote
corn_flake Posted November 3, 2020 Report Posted November 3, 2020 Did you really cut off the mixture? Meaning lights out for the engine? Quote
carusoam Posted November 3, 2020 Report Posted November 3, 2020 6 minutes ago, corn_flake said: Did you really cut off the mixture? Meaning lights out for the engine? That is a requirement for this type of work... The extra drive provided by the idling engine provides terrible data... real life is different than practice... Really shows how aerodynamic the Mooney actually is... where the idling engine provides a significant amount of power to overcome drag... A second way to look at this... if you don’t pull the throttle all the way out on landing... you float forever... A little extra MP or RPM is everything... A fun read on the same topic with a similar airfoil... search for the P51 glide ratio around here... they are amazingly similar... even though the P51 MGTW is a bit bigger.... Best regards, -a- Quote
Hank Posted November 3, 2020 Report Posted November 3, 2020 (edited) 17 minutes ago, carusoam said: There is an awesome thread around here where an MSer went on a trek to learn about his glide ratio... Find Chuck Noe... @cnoe https://mooneyspace.com/search/?q=Chuck glide&updated_after=any&sortby=relevancy&search_and_or=and Best glide is a range set from MGTW at the top, to closest to empty weight at the bottom... It will be in the range of about 10ias, near 90 at the heaviest to close to 80 at the lowest... Chuck did his experiments in an M20J, which has some aerodynamic improvement over the M20F, but not a whole lot... Minimize drag... 1) gear up 2) flaps up 3) prop pulled back all the way 4) speed brakes stowed... 5) Weight and balance to the rear of the envelope... 6) controls rigged properly... PP thoughts off the top of my head... the indicated air speeds are similar for both kias and mias... because the planes that use KIAs weigh more than the Mooneys using MIAs... Best regards, -a- Your O may have Vbg in the 90s, but for my C it's 105 mph prop turning, 100 mph prop stopped. Look at the bottom graph, which only shows Gross Weight. Any lighter, I get to figure it out . . . . Edited November 3, 2020 by Hank 1 Quote
carusoam Posted November 3, 2020 Report Posted November 3, 2020 I have to go hit the books again... -a- Quote
skydvrboy Posted November 3, 2020 Author Report Posted November 3, 2020 19 minutes ago, corn_flake said: Did you really cut off the mixture? Meaning lights out for the engine? Yes. First pulled throttle to idle, then cut the mix. When you go to restore power, you want to make sure the throttle is retarded (either from pulling the mix or running a tank dry). If you don't, it roars back to life and can over speed your prop. 9 minutes ago, Hank said: Look at the bottom graph, which only shows Gross Weight. Any lighter, I get to figure it out . . . That's basically the same graph I have. No info for less than gross weight. Quote
carusoam Posted November 3, 2020 Report Posted November 3, 2020 The O chart... 3368 91.5 KIAs 3200 89.0 KIAs 2900 84.5 KIAs 2600 80.0 KIAs Hope this helps demonstrate the affect of weight on Vbg... Sorry about the mixed old memories above... Best regards, -a- Quote
skydvrboy Posted November 3, 2020 Author Report Posted November 3, 2020 35 minutes ago, carusoam said: Find Chuck Noe... Yes! His was the thread that got me interested in doing this. I ran across it the other day when I was searching for glide info. In fact, in that thread, @Immelmanhad a great spreadsheet that made me realize all I had to do was determine how long it took to descend 500' in each configuration. I posted his results below. I didn't feel the need to do it at idle and I wasn't quite brave enough to stop the prop. http://1.bp.blogspot.com/-MTC2dOXsq7c/U4izgCOGnWI/AAAAAAAAASU/DWrLuAPKzXM/s1600/m20e_glide_test.jpg Now that I look through that thread again, a bit further down I find the glide slope chart from the J, which shows best glide speed of 105 mph at 2740 lbs (same as my F) as well as 98 mph at 2300 lbs. I guess I could have used that to extrapolate my best glide speed for my weight today should have been 94.4 mph, though I'm not sure I could have held my airspeed to the nearest 1/10 mph. 1 Quote
skydvrboy Posted November 3, 2020 Author Report Posted November 3, 2020 5 minutes ago, carusoam said: The O chart... 3368 91.5 KIAs 3200 89.0 KIAs 2900 84.5 KIAs 2600 80.0 KIAs Fascinating! Those numbers are (within rounding error) exactly the same as my F (and the J) when you look at max gross or X number of lbs below gross. I wonder if that holds true for other models like @Hank's M20C? 1 Quote
carusoam Posted November 3, 2020 Report Posted November 3, 2020 Looks like the wing design plays a huge roll in the best glide speed... from C to R... when converting kias back to mias... use very similar numbers... Oddly, My POH throws in the decimal place for speeds. But the weight is given in ranges or chunks of 300LBs... I found the graph like Hank has... Wait a minute.... What cowl flaps...? (M20Rs don’t need no stinkin’ cowl flaps...) Gliding down from 24k’ would be interesting... plenty of time to read the POH... and start a new thread on MS... using the sat phone... -a- 1 Quote
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