Soft field data

[JTrain]

Hey,

Airplane Flight Manuals typically give data for normal and short field takeoffs. Is there any way to somewhat accurately predict soft field takeoff distance (from real grass fields). Obviously the nature of the soft fields makes uniform calculations and slide rules impossible since they vary so much with drag factor/field conditions.

Also - from real world soft field experience - how does soft field takeoff distance vary versus the AFM computed numbers?

Thanks in advance,

jt

 

[Tonala2k]

Do this,

On a given day, calculate short field TO on hard, dry surface. Go to a soft field of your chosing(DRY, please). Have someone mark where you apply full-thottle (as on soft field you can't stop at the edge of the runway) and where you lift off. Do it on hot days and cold day. Low pressure and high pressure. Low alt and high alt. Max weight, min weight. Aft C.G.. Fwd C.G.. Compare your numbers to the books.

That sounds like a lot more fun to me than just finding the formula.

 

[JTrain]

Do this,

On a given day, calculate short field TO on hard, dry surface. Go to a soft field of your chosing(DRY, please). Have someone mark where you apply full-thottle (as on soft field you can't stop at the edge of the runway) and where you lift off. Do it on hot days and cold day. Low pressure and high pressure. Low alt and high alt. Max weight, min weight. Aft C.G.. Fwd C.G.. Compare your numbers to the books.

That sounds like a lot more fun to me than just finding the formula.

That does sound like fun. Unfortunately I'm in a situation where I'm doing (real) soft field t/o's on a daily basis. The AFM takeoff charts do include some adjustment for soft fields (15% penalty), unfortunately I'm just not sure how accurate these numbers are. In practice, soft field takeoffs really seem to drag out a lot longer than were the surface say dirt, gravel, or paved. More than the AFM would have you believe.

I'm just interested in other perspectives and thoughts from people who've been in similar flying environments.

Thanks in advance,

jt

 

[MauleSkinner]

Hey,

Airplane Flight Manuals typically give data for normal and short field takeoffs. Is there any way to somewhat accurately predict soft field takeoff distance (from real grass fields). Obviously the nature of the soft fields makes uniform calculations and slide rules impossible since they vary so much with drag factor/field conditions.

Also - from real world soft field experience - how does soft field takeoff distance vary versus the AFM computed numbers?

Thanks in advance,

jt

First, let's get our terminology straight...is it a grass field or a soft field? A grass field is a solid field with grass on it. Basically normal takeoff technique (edit: short field technique, since I'm being terminologically anal), with the extra 15% in the AFM for the additional drag of the grass.

A soft field, on the other hand, has you sinking into the surface, and requires the soft field technique. The AFM can't cover all the possible permutations of a soft field, and so doesn't give you information for them. Plan on LOTS of extra runway.

My guess is that you are using a soft-field technique on a solid grass runway, and the extra drag of the soft-field technique is compounding 15% penalty for grass, and giving you the extra takeoff distance versus the book numbers.

Fly safe!

David

p.s. Sporty's has a TOP (Take Off Performance) computer that I've found to be very accurate once you calibrate it...Last time I used it with my Maule, liftoff was within 50 feet of my calculations (fortunately, I still had about 200 feet of runway left after that )

For those of you who may be interested, I had my TOP computer in my flight bag, so here are some rough figures that it comes up with...

For "turf", it adds about 8%.

for "grass", it adds about 13%.

for "long grass", it adds about 25%.

The softest setting is actually a range...labeled "soft field, mud, snow", it adds anywhere from 30 to 100%.

 

[A Squared]

You're asking for something which doesn't exist, and would be impossible to provide in any meaningful way. "Soft" could range from damp gravel that you barely leave a tire imprint in ... to mucky sand which is so gooey and deep you can't taxi with full power, let alone take off.

Any "soft field" takeoff data would have to be for a certain level of softness, and would only be meaningful for that specific set of conditions, and would be completely useless to you unless you had some way of determining that your conditions were the same as the conditions in the book.

 

[JTrain]

You're asking for something which doesn't exist, and would be impossible to provide in any meaningful way. "Soft" could range from damp gravel that you barely leave a tire imprint in ... to mucky sand which is so gooey and deep you can't taxi with full power, let alone take off.

Any "soft field" takeoff data would have to be for a certain level of softness, and would only be meaningful for that specific set of conditions, and would be completely useless to you unless you had some way of determining that your conditions were the same as the conditions in the book.

That is sort of what I figured. I was just a little annoyed after doing a t/o that even by bumping up the takeoff roll by a nice margin (similar to what Maule suggested) still seemed a little more than what I expected. Thanks for sharing your perspectives - it is quite something to be flying in/out of soft & grass fields on a daily basis.

jt