C172SP best glide speed question

[Guest david_eberhardt]

Hi folks ... I'm new here so hello to everyone.Been experimenting with flying the C172SP engine out ... flying best glide speed which according to a C172M Owners manual I have says it is 70 KIAS. But I found that flying at 60 KIAS gives me better distance. I experimented using the video capture feature to document my results and study them, Anyone have any idea why I could get better glide distance using 60KIAS vs 70 KIAS??? The difference was substantial. At 70 KIAS I did not make it to the runway and at 60 KIASI made it easily.Winds were set to calm. I started the eng out scenario at 3750 feet at the FAF for the ILS or LOC RWY 5 at KGON - according to the glide chart, I should get about 5.7 NM distance glide from that altitude.One note - at 60 KIAS, the prop stopped spinning completely and at 70 it spun at about 800 rpm - I actually turned of the magnetos in both scenarios.

[CoolP 283]

Which Cessna are you using there? The default a/c won't fulfill the wish for accurate performance I think. At least some air file change might be a good idea then. Maybe the Avsim library can help.

[Ken_Stallings 2]

Hi folks ... I'm new here so hello to everyone.Been experimenting with flying the C172SP engine out ... flying best glide speed which according to a C172M Owners manual I have says it is 70 KIAS. But I found that flying at 60 KIAS gives me better distance. I experimented using the video capture feature to document my results and study them, Anyone have any idea why I could get better glide distance using 60KIAS vs 70 KIAS??? The difference was substantial. At 70 KIAS I did not make it to the runway and at 60 KIASI made it easily.Winds were set to calm. I started the eng out scenario at 3750 feet at the FAF for the ILS or LOC RWY 5 at KGON - according to the glide chart, I should get about 5.7 NM distance glide from that altitude.One note - at 60 KIAS, the prop stopped spinning completely and at 70 it spun at about 800 rpm - I actually turned of the magnetos in both scenarios.

The numbers for my real world Cessna 172N for best glide speed are 65 KIAS (flaps up) and 60 KIAS (flaps down).However, in my 172N POH, Cessna uses a somewhat arcaine language to describe it. Vice best glide speed, Cessna uses the description of "Landing Without Engine Power!"So, I am not at all surprised with your observations, especially if you are applying it to the Carenado Skyhawk, which is a 172N.Now, I also have a real world POH for the C-172SP, and it lists different speeds for this same category of 70 KIAS (flaps up) and 65 KIAS (flaps down). Why the difference? Given that there is very little difference in the design of the wing, I cannot say. For the 172SP, Cessna lists a best glide range speed of 68 KIAS.Ken

[Guest david_eberhardt]

thanks guys for the replies ....I think Ken's numbers work out better for the C172SP that's in my Flt SimX ... it may be a gross weight thing that is making for 60-65 KIAS work out better than the published 70 KIAS .... I have been experimenting in the sim to see how much of a difference the different "best glide speeds" affect gliding distances. I'll go from 70, then do a run at 70+10, and then 70 - 5 or 10 .... I start from about 3800 ft agl and use the ILS or LOC Rwy 5 approach at KGON to get accurate measurements for when to start the simulations ( I start right at the FAF). My set up was based on the glide chart in my C172M Owners manual.I've noticed a few things overall: the glide distances are not greatly affected by being off speed as much as I anticipated. I wonder how it would play out in a real airplane??? (I normally fly Cherokee 140)The second thing I learned (reinforced actually) is that when faced with an engine out scenario, I will need to be ultra conservative and anticipate the the plane will not glide nearly as well as the charts might otherwise indicate. I'm thinking of making a chart based from the airplane manual and reducing all the results to expect by about 33 percent. Lastly, it is so much better to be too high than to be low and hoping for the best.Summarizing, I was at 3800 ft at the FAF for the localizer approach to rwy 15 at KGON (calm winds). The FAF is 5.7 NM from the runway according to the profile view in the published procedure. I barely made it to the runway and only when I flew (glided eng out) at about 60 KIAS. 5.7 NM equals 6.5 statute miles. The Cessan chart is a bit hard to interpolate but around 4000 ft should give you about 6-7 statute miles. I'll have to run the simulations again starting at 4000 ft and just see where I impact the runway (runway is about 5000 ft long).thanks again.