70/50 vs 80/50 rule

Date:         01 Dec 96 04:08:54 
From:         inet@intellisys.net (brian whatcott)
Organization: Telepath
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There is a 'rule' now percolating onto the rec.aviation groups
which supports the suggestion that at 50% of takeoff roll, you
should have 70% of required takeoff speed.

It seems to me that this must have originated with jet operations
where a constant thrust takeoff roll leads to this kind of figure.
 I have enquired of AF heavy pilots - who do not in general use this
rule. A correspondent tells me the rule is advocated by a DOT and FAA
office in Alaska, but for GA operations.

 When I work the numbers for constant speed props, I see that if I
assume constant efficiency at constant horsepower, the relationship is
much more like 80/50   to take care of the fact that a 25% of airspeed
increase sees the prop thrust drop by 20% ( for these idealized

I would like very much to learn where this rule of thumb originated-
I can only assume in airline operations?

brian whatcott <inet@intellisys.net>
Altus OK