Grass strip performance

[scorpioflyer]

Can anyone help. A mate is developing his strip and knows it has a 5% slope. There are some trees at the top.
At what point does it become an advantage to land uphill with a tailwind, and likewise take off downhill with a tailwind?
Is there a way of factoring 5% slope into take off and landing calca. I know the CAA say use 10% for a 2% slope.
Any comments from experienced strip users welcome, however untechnical!
Thanks in advance.

[DeltaTango]

I'd guess you increase the distance by 25%.....

DT

[john_tullamarine]

The normal performance model equations for slope effects are relatively straightforward and are available from a number of sources.

The main difficulty is convincing your local regulatory authority that the variations so calculated are appropriate without further flight test validation.

I am presuming that your colleague is looking at small GA aircraft in which case, if your State does not have a difficulty with operations from runways with significant slope, the exercise ought not to be difficult. You might, for instance, find that such an operation is limited to one-way takeoff and landing.

I note that you appear to be located in the UK in which case a quick email to Genghis The Engineer might give you appropriate guidance. He works in the relevant airworthiness area in that country.

[ 04 October 2001: Message edited by: john_tullamarine ]

[scorpioflyer]

Thanks John. My mate is a farmer and will be using his own field, purely for personal use, and will base his choice of aircraft (C172?) on what we can work out. I will e-mail Genghis, but meantime can you point me in the direction of a couple of sources.
Many thanks.

[john_tullamarine]

scorpioflyer,

The main problem is tracking down useful documents as the average bookshop is not well stocked with this sort of thing. Probably your best bet is to see if Genghis has any CAA-specific requirements to meet - they might well have recommended light aircraft performance methods manuals as does, for instance, the Australian CASA.

Otherwise, I would check out a local (reasonably large) library or, especially, a local university engineering (preferably aero) library and see what books are on the shelves.

The sorts of documents you might look for in the library include

(a) the Boeing Performance Engineers' Manual - I don't have my copy with me at the moment but I seem to recall that the doc number is D-1620 ?

(b) AGARD Flight Test Manual

(c) any other flight test manuals which the library has scrounged over the years.

In your local technical bookshop, you might find books such as (memory scratching now) Mair and Dunstall (Cambridge series) - a very readable engineering text on performance. Stinton's book probably has plenty of guidance material as well.

Have fun ...

[ 05 October 2001: Message edited by: john_tullamarine ]

[OzExpat]

Taking off UP a 5% slope in a C172? :eek: Yikes, how long is this strip? I'll have to go scrounging among my old airstrip info for some of the places around this part of the world, but I don't think we've got anything like that here because there wouldn't be enough length available to make it viable (and safe).

Just a word of caution here though. If it ends up being a one-way strip, your mate would do well to follow at least one example from this country. Get a thorough checkout on one-way take-off and landing from someone who knows what it's all about.

There's plenty ways for him to ruin his whole day if he doesn't learn well... :(

[Genghis the Engineer]

(1) The CAA guidelines are summarised in GA Safety Leaflet 7b, but the definitive stuff is in AIC Pink 12/1996 pink 120. However, it gives no advice about the effect of headwinds, since the CAA view (conservative but sensible) is always to use the still air figure when taking off into wind.

(2) More usefully is to find somebody who has a flight manual for an old BCAR Section K aeroplane, with similar performance and adapt the WAT chart (scale geometrically not arithmetically). This is a rather cunning graphical way of factoring slope, wind, temperature, etc. to come up with an accurate take-off distance figure.

(3) However, my gut feeling is that the only sensible approach with this sort of slope is to always take-off downhill, and land uphill, particularly with trees at the top of the hill. Landings should be straightforward, and take-offs if a bit marginal presumably you're over descending ground so long as you're airborne. What is vital in this case, is to be mentally prepared for an early go-around decision during approach !

If the available runway width is sufficient, you could do worse than leaving the grass longer on one side than the other, and land on that when the grass is dry.

G

[john_tullamarine]

Now that Genghis has made comment, the following may be useful also.

Under the old Australian ANO 101.22 rules, the local AFMs for the C172 have a graphical presentation which would be very useful - these were conservative, but not excessively so. Similarly the mathematical model upon which that style of chart (commonly known as the P-chart) was based are available via a (then) DCA published light aircraft how-to-do-it manual. The original document was a very simple analysis. Some years later a much more complex analysis was adopted by CASA - the earlier one would be far more appropriate for your buddy's requirements.

My copy is back in my office in Melbourne and I am not able, conveniently, to gain access to it from where I am working at present. You might consider approaching the Australian CASA to see if you can obtain a copy. Alternatively, when I am next in Oz, if I remember to do so, I can look the document up and send you the specific equation which was used.

[ 06 October 2001: Message edited by: john_tullamarine ]