I often wondered what those markings were for 
John S. Denker lists the 50/70 as the wrong rule
http://www.av8n.com/how/htm/takeoff.html#sec-monitoring-takeoff-wrong
Interesting and good read.
But he misquotes the rule (vs i have been told it) and then says it is wrong….
If you have NOT reached 70% at 50% runway, then you should ABORT take/off.
…because it is not a safe assumption that you gain the necessary speed on the remaining runway while taking away useful decelerating distance.
The rule is not a positive predictor, but a warning sign. Fully in the spirit of his article ;-)
ch.ess wrote:
But he misquotes the rule (vs i have been told it) and then says it is wrong….If you have NOT reached 70% at 50% runway, then you should ABORT take/off.
Yes he does. But his point remains. The warning sign is, seemingly at least, overly optimistic, and if you DO reach 70% speed, well … I have never heard of this rule, but it has a merit IMO. It should however be derived from the physics of a propeller driven aircraft, both fixed pitch and constant speed.
I don’t use the 70% rule at the half way point on the runway, I use it as a measure of performance at the half way point of my expected ground roll. So my airport has a 5500 foot runway but has precision markings at 500, 1000, and 1500 feet. In my Bonanza, I expect to be at lift off speed of 70 Kts around 1000 feet, so at the 500 foot marker, I verify I am at 50 Kts.
LeSving wrote:
It should however be derived from the physics of a propeller driven aircraft, both fixed pitch and constant speed.
It is derived from physics with the assumption that the acceleration is constant during the takeoff run and as long as that assumption holds it does not matter the type of engine or propeller. With a constant acceleration, the ratio from V1 to V2 where V1 is the lift off speed and V2 is the speed at the 1/2 distance point is V2 = V1/√2 .
NCYankee wrote:
and as long as that assumption holds it does not matter the type of engine or propeller
That’s the problem. This is nonphysical for a propeller, and generally nonphysical for an airplane. Constant acceleration requires constant thrust (vs speed). It also requires zero effect from friction. Drag is typically a function of v² and rolling resistance is a function of v. With a constant speed prop we have constant power (not thrust). Power equals thrust times v. So the thrust is proportional to the inverse of v. This means the assumption is all wrong to start with for a constant speed prop.
A fixed pitch prop is probably better off because the power increases with speed, at least up to some velocity where it will equal a constant speed prop, and then it will drop again.
I agree with NCYankee’s interpretation of this “rule”, but in practice I think it is of little use for short and improvised runways.
1. The ASI may not even register, let alone be accurate, at 70% of lift-off speed (typically 20 to 25 knots)
2. During a typical ground roll of five or six seconds, the driver’s attention is most usefully focused outside the cockpit. Heavily laden or with a glider in tow it may be as much as ten seconds, but in the latter case we have to look in the mirror as well as everywhere else.
3. If the runway seems gruesomely short, we probably shouldn’t be there in that particular aircraft.
The alternative “potato count” method in this article is more practical, and safer. If the tail doesn’t want to come up after counting the requisite number of potatoes (typically two or three), something is probably wrong and that’s the time to stop. Of course, that requires a bit of familiarity with the airplane, but we can do fifty or sixty stop & go landings in a couple of hours, and until then it’s probably best to stick to situations in which take-off performance is irrelevant.
LeSving wrote:
thrust is proportional to the inverse of v
Not in the take-off roll. All of the above written about prop and acceleration is a lot more complicated than the simple “props are constant power so thrust decreases with speed”.
For a fixed pitch prop there are two opposite effects
The second of the two is a result of propeller efficiency, which is low at low speeds (to the order of 20%,IIRC), then increases to reach the peak at the speed the prop is designed for (to the order of 85%), and then decreases again.
At the beginning of the take-off roll, the effect of prop efficiency is quite large and typically outweighs the effect of speed, and later in the take off roll this is typically the other way round. And all of that depends on the exact prop and aircraft, as quite correctly airframe drag etc. also enters into the equation
Cobalt wrote:
And all of that depends on the exact prop and aircraft,
Indeed.
The good thing about this rule is that it incorporate all parameters for aircraft, engine, weather, wheels and airfield state…unlike many of the POH numbers
First, I suppose I will follow the POH guidlines then test it on the day condition with 1/2 of the runway using this rule and hope for the best on the other half of the runway. The rule literraly makes zero assumptions appart from assuming that the engine delivers the same horse power when trottle is fully open (of course it will fail if you blow a tire, lose and engine or you start runing on grass on tje second half of the runway).
