The International Cessna^® 170 Association

gahorn wrote:Grass extends takeoff ground-roll by 20% if it's less than half-wheel-height and dry. (I don't recall where I got this bit of info, but it's been with me for about 40+ years and seems to work. I think it was from an old-geezer...perhaps he got it from old-geezer-school....maybe bluElder attended that school and can pitch-in..??)

Taller grass (than half wheel-height) really adds to the takeoff distance due to rolling-resistance.

Wet grass will make that worse and upon landing, will behave as if it's greased....(as it will if you abort that takeoff....which I HAVE experienced with shaking knees.)

Makes sense, but I'd like to understand it better. Is this due to the rough surface disrupting Ground Effect or because the airflow into the pitot tube is somehow disrupted, again due to the rough surface affecting static pressure? Without accurate airspeed indications, rotation speed ends up indicated later than normal thus extending the takeoff roll. Or is it really the difference in friction between the ground and gear? I'm trying to figure out where the difference between grass and pavement exists here: L = (1/2) d v2 s CL... (WARNING, NERD ALERT). I have experienced this phenomenon but I never really gave it much thought at the time. I was just thankful that we made it off the ground without hitting the trees and immediately filed the experience in the archives under the "Forget that crap!" tab.

I have a grass strip setting @ 3400' msl with 800' useable. On a 58 deg. day I used the majority of it (750' +) with myself & 115 lb grandson, (310#), full fuel, cruise prop, etc. That was the deciding factor in going to eng & prop upgrade. I am still working with the new equipment, but have cut the distance by 200', & I don't have it dialed in good yet. Very happy so far!

Does anybody have any specs with climb props on the O300? If you are not worried about cruise, could be the way to go, especially with 1450' of strip.

How close is the nearest NPIAS airport to your aircraft location?

Many Cessna 170/172 pilots try to think of the a/c as a two passenger airplane with mathematical challenges in two rear seats. You must not skip the computation every time
you load up passengers.Pencil down.

I have rarely loaded my 170 with with four adults. One occasion I required a couple to unpack their beautiful luggage suitcases, we used brown grocery bags to repack. I was unpopular
but saved 15 lbs.

For the last thirty years large commercial jets have sensors (ACARS) in landing gear that reports W.O.W. Every time a plane moves some one is doing math.

Can you lengthen your grass strip?.

nehringer wrote:

gahorn wrote:Grass extends takeoff ground-roll by 20% if it's less than half-wheel-height and dry. (I don't recall where I got this bit of info, but it's been with me for about 40+ years and seems to work. I think it was from an old-geezer...perhaps he got it from old-geezer-school....maybe bluElder attended that school and can pitch-in..??)

Taller grass (than half wheel-height) really adds to the takeoff distance due to rolling-resistance.

Wet grass will make that worse and upon landing, will behave as if it's greased....(as it will if you abort that takeoff....which I HAVE experienced with shaking knees.)

Makes sense, but I'd like to understand it better. Is this due to the rough surface disrupting Ground Effect or because the airflow into the pitot tube is somehow disrupted, again due to the rough surface affecting static pressure? Without accurate airspeed indications, rotation speed ends up indicated later than normal thus extending the takeoff roll. Or is it really the difference in friction between the ground and gear? I'm trying to figure out where the difference between grass and pavement exists here: L = (1/2) d v2 s CL... (WARNING, NERD ALERT). I have experienced this phenomenon but I never really gave it much thought at the time. I was just thankful that we made it off the ground without hitting the trees and immediately filed the experience in the archives under the "Forget that crap!" tab.

Sorry to be so slow in response...just realized no one else had jumped in with the answer to your question.
It's really very simple matter. It's the increased rolling-resistance of grass over a "dry, level, hard-surfaced runway" that aircraft performance was based upon when published/certified. Don't let your imagination get too actively involved about a rough-surface "interaction" with "ground effect" or indicated airspeed, etc.