Hi Frank,
You've made a number of interesting observations which are usually not examined in much detail.
One is that an aircraft once free of the ground becomes a part of the air mass, it no longer experiences any effect of "wind", as wind is something experienced by an observer on the ground witnessing the movement of the air mass and interpreting that as "wind".
The airplane is almost like a balloon released into the air. Almost in that the balloon has essentially no mass so with little inertia it moves almost instantly with any change in the air mass from any horizontal shear or up/down drafts, i.e. localized changes within the air mass.
The aircraft on the other hand has a significant amount of mass and thus inertia so it resists any fast change in it's flight path from horizontal shear or up/down drafts.
An illustration of this is a 747 going through localized changes within an air mass might report "light chop", while a J-3 going through that same area might report "Severe Turbulence".
The less mass an object has which is experiencing localized air mass changes, the greater the g loading on the object due to those changes.
And taking off with a tail wind can be ah "interesting". You're of course right that once the aircraft leaves the ground at it's normal lift off airspeed it flies no differently than when it lifts off at that same airspeed into a head wind.
But in order to attain that normal lift off airspeed it has to be rollin along the runway at that noraml airspeed plus the tail wind speed.
If the tail wind is just 1 or 2 knots it's not much of a problem but say 10 or 20 knots means the airplane will be on the ground roll 10-20 knots faster than normal and can get quite squirrley (difficult to control) while in contact with the ground.
The other factor with tail wind take offs (and landings) is that as the wind increases in velocity a given percentage change in the tail wind velocity component effects the aircraft exponentially.
e.g. a 20% change in the tail wind component (twc) with a 5 knot tail wind will show up for a few seconds as a 1 knot gain (if a twc decrese) or loss (if a twc increase) in airspeed until the aircraft overcomes it's inertia stabilizing at the airspeed prior to the twc chnge.
OTOH, if it's a 20 kt tail wind component there would be a 4 Kt change and the aircraft's ability to overcome a 4 Kt change would take significantly longer than the few seconds of the 1 Kt change, which could result in premature unintentional ground contact (crash or incident).
...and yes, the Vx speed(s) determined for various gross weights, CG locations, and density altitudes you determine will provide the steepest angle of climb through the air mass at those conditions in any wind conduition.
it will also be the steepest angle relative to a ground observer or a 50' obstacle;
however, if you're climbing at Vx with a 20 knot twc your climb angle relative to a ground observer and a 50' obstacle will be much less steep than under no wind conditions even though the air mass relative angle hasn't changed at all,
this because your performance perspective has changed to ground relative and not the normal in flight air mass relative.
A suggestion for recording ground relative measurements of speed and distance is to use a GPS receiver that optimally records it's measurements at a 10Hz rate.
Everyone,
Since I started this string from an admittedly poor (wrong) thesis, let me add to the endeavor to develop POH standards.
I'm afraid most comments on this thread were addressing the thread title and not it's intended subject - which is my fault, I expected the double question marks to convey too much meaning.
I should have entitled the message "Defecient POHs due to lack of specifications, particularly techniques used to obtain data."
The subject POHs are the Commercial GAMA Speciifcation No.1 required POHs, which are not exactly of much interest per se to the EAA community, but anyway something may be learned from others errors.
If I recall I attached a spread sheet (ss) in my original post which (if anyone is actually interested can be analyzed and in so doing demonstrate specifically what I'm talking about).
I'm attaching an expanded ss which may be easier to understand and a '78 C172 POH used in the ss and representative of all POHs due to GAMA specfication No. 1., and the GAMA Spec.
To simplify, or at least vebalize what's contained in the SS:
If you take the "Total distance to clear a 50" obstacle" minus the "Ground Roll Distance" (specified as such in all POHs) you would expect to have the distance from "Lift Off" to the obstacle. -
Right?
Then if you take the specified Vx KIASs converted to KCASs either specifically or averaged, convert to fpm then divide that into 50' (x 60) you have the number of seconds from lift off to 50' -
Right?
Now with number of seconds to climb to 50' you have the average climb rate from "Lift Off" to 50' -
Right?
Now take the arcsine of that climb rate (in fpm) and divide it by the previously determined fpmCAS and you have the average climb angle from "Lift Off" to 50' -
Right?
Doing the same process for Vy speeds you find the Vy angle is always greater than the Vx angle - Right? - Wrong!
So whats wrong?
1) the above logic?
2) Trigonometry?
3) the numbers specified in the POH?
4) or something NOT specified in the POH?
The attached spread sheet votes for 4) and possibly 3).
What is apparently being left out of ALL commercial POHs is the technique that was used to obtain the POH 50' obstacle clearance numbers.
The bottom line for Commercial GAMA specificatio No. 1 POHs is "trust" - BUT VERIFY.
IOW: You should flight test for about 5-50 hours depending on the A/C before using the POH specifications for any critical flight operation.
...dropping the mike
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Originally Posted by Waldo Pepper
