Assuming that your approach is at an angle of 5 degrees ... not unreasonable, I think, for a model aeroplane ... then about 115 feet of your runway becomes unavailable to you, owing to the crop. It has created a displaced threshold, of sorts.
And that's if your wheels skim the tops of the crop.
It's probably reasonable to say that the crop halves the effective length of your runway.
Last edited by bogbeagle; 09-30-2013 at 11:55 PM.