The International Cessna 170 Association

Has anyone found or been able to calculate the 170 glide ratio?

I fly a stock "49A, with a metal prop and was looking to take advantage of the new ForeFligt tool for max glide. Our own owners manual and google were not much help. The forums seem to bounce all over with discussions of STOL kits etc.

My CFI said to climb to 4000', set up max glide speed and see what you get at 2000'! Assume that may be correct if wind conditions never change from the test day.

Something like 7/1 appears in a few places.

I used 7/1 when I set it up in Foreflight. That's just a guess on my part but I think it is a bit on the conservative side. I vaguely recall being told that 152s had a glide ratio of about 10/1 so I used that as a starting point. After I set it in FF, I then would look at the ring on the map and compare it to what I saw outside and, using my best guess, I decided that, yes, I could glide to a point in the depicted circle.

I have no idea where I got this... but from many years ago I was told the 170 (B model) had a 12:1 ratio at 67 IAS.

gahorn wrote:I have no idea where I got this... but from many years ago I was told the 170 (B model) had a 12:1 ratio at 67 IAS.

Same here, I heard this years ago and have no idea where it came from. The "glide amoeba" feature in ForeFlight was borrowed from soaring software where it has been around for a while. Just be aware that it does not include a safety height meaning that at the edge of the amoeba is where you will hit the ground leaving you no altitude to line up with a runway. Also, Foreflight uses forecast winds which can be quite different from actual, most soaring software computes actual wind. Still, a nice feature to have.

As an active glider pilot as well as 170B operator. I will say I have tested the 170B. I can confirm 67mph but it is infact 7:1 clean. If the prop is stopped it drops to 5:1 FYI. I teach cardinal speeds so 70 mph is what I teach. It is still 7:1. That is also only in the clean configuration with the propeller spinning. I use 7:1 in Foreflight as well. Doug

canav8 wrote:As an active glider pilot as well as 170B operator. I will say I have tested the 170B. I can confirm 67mph but it is infact 7:1 clean. If the prop is stopped it drops to 5:1 FYI. I teach cardinal speeds so 70 mph is what I teach. It is still 7:1. That is also only in the clean configuration with the propeller spinning. I use 7:1 in Foreflight as well. Doug

The Airplane Flying Handbook has a chart on page 12-4 that depicts the parasite drag curves for stopped and windmilling props at various blade angles. I'm not sure what degree pitch would average out to on our props, probably somewhere close to, or perhaps past, the 16° mark where they cross in the chart? When one considers the maneuvering and slow airspeed necessary to make the prop stop in a gliding 170 (assuming the engine didn't come to a stop already), is it really worth it anyway? Seems like more distraction from finding a suitable place to land. If less drag with it windmilling, then that makes it a real no-brainer to let it spin.

There is an easy way to compute glide ratios.

1) Fly the plane at idle power at a series of given airspeeds, say 65, 75, and 85 mph and note the rate of descent when stable.

2) Compute the distance flown in one minute and divide that amount by altitude lost in a minute.

EG: FLt at 75mph yields a descent rate of 860 fpm. ( I made up these numbers for purposes of illustration.)

(75 m/hr x 5280 f/m /60 min/hr ) / 860 f = 7.7 glide ratio

3) When you have completed your test runs, note the computed glide ratios and interpolate from the data to see what speed gives the best glide ratio.

Remember that although the glide ratio does not change with gross weight, the speed to achieve that glide ratio goes up as gross weight increases. If you do your test flights solo with half tanks, your best glide speed at max gross wgt will be higher.

Years ago, I did a series of test flights to determine my best glide (solo with full tanks, prop spinning) using a gps driven flight recorder (from my glider) and determined that my plane's (850s, vgs, blown side windows, external VOR antenna) best glide was at 76mph.

Is this a reasonable approximation of best glide speed for a 170? The manual doesn't really specify but the FAA says the following:

"If it’s distance you want, than you’ll need to use the speed and configuration that will get you the most distance forward for each increment of altitude lost. This is often referred to as best glide speed and, on most airplanes, it will be roughly halfway between Vx (best angle of climb speed) and Vy (best rate of climb speed).

See: https://www.faa.gov/news/safety_briefin ... _16-01.pdf

For Vx on a 170, use 62 mph which the manual calls for an intial climb over an obstacle. For Vy, use 85 mph. (my manual says best rate ranges from 85 at sl to 78 at 7,000msl.) Those numbers and the FAA's rule of thumb yield a best glide speed of 74. That same FAA publication especially estimates a best glide for a 172 at 75mph/65 kts so that's probably a reasonable number as well for a 170.

As a consequence, I think the actual best glide speed is probably somewhere in the range of 75mph when light and somewhat higher when heavier.

I do not agree that stopped prop decreases glide range. On the contrary, the drag of the spinning prop disk is greater than a stopped prop and reduces the glide ratio. As an fyi, you need to get a 170 very slow, close to stall speed, to stop the prop in flight. Low down, IMHO, its not worth the effort and the risk of stalling.

If only we had a theodolite so we could gauge the exact distance to the field we plan to glide to..... we'd then find a use for determining glide-ratio.

Going back to Basics: Head for the field and hold a steady speed (which equates to a steady pitch-attitude). If your nose/cowl begins to cover up the field... you're gonna make it with altitude to spare. If your nose/cowl is dropping below the field (i.e., the field appears to rise in the windshield)... you ain't gonna make it and need to select another spot.

<edit while pdb was typing>
Waaay back when I taught primary and we used castor-oil in engines.... I taught students that in the event of an engine failure in no-wind conditions, ...run the engine-failure checklist memory-items, achieve best glide-speed (usually Vx) and one could generally make it to any field within an inch above the cowl or anywhere beneath the cowl and/or within half-way-up the wing-strut.

It was a generous margin but gave the inexperienced (or out-of-practiced) a successful solution.

George is absolutely right but knowing and flying the best glide speed is crucial to optimizing the glide.

I think the 7:1 number might come fron Sportsman STOL where we read, "Flight test figures have shown a Cessna 170B at gross weight increase its glide distance from 7-1 to 13-1."

Gents,

Great discussion. As a new owner this is exactly what I needed to know also. Thanks!