Cobalt wrote:
I always thougt that “marketing writes the POH” was a myth, clearly it wasn’t some time in the past.
No, that is acknowledged fact. The question was simply, how much marketing power would they get and where would the engineers in the company or the FAA put their collective feet down.
I have read some comments by people who were involved in the early Mooney POH’s. I have to say, those prior 1971 POH’s are quite messy and lack a lot of what I would have liked to see, yet I am surprised that the performance data they have for normal cruise regimes pretty much fit the actual data you get when you fly the airplane. Where there are differences though are in the top speeds and some rather unrealistic cruise regimes which nobody would fly. Again, range is something I would never even consider looking at without doing my own calcs.
Where I have seen quite bad figures were with Cessna, at least in the 150 manual I used to have. The range figures there were quite something.
What all of us have to remind ourselfs when we watch these figures is a simple fact: The POH’s and all the performance figures were compiled out of test pilot results. These guys know the airplanes inside out, they know how to squeeze extra knots out of them (famously with the Mooney 201 and 231) and they will achieve performance which will take the “normal” pilot quite some experience to achieve. So not everything which is optimistic is just “marketing”, quite a lot is also simply flown under heavily optimized conditions.
Still, over all, most figures I have seen are pretty much ok. If any airplane deviates massively from it’s POH, then it has likely got a different reason why which should be investigated.
Ah, yes – while many instructors frown at the idea of a “rotate” speed for light aircraft,
Rotation speed is the speed at which you rotate the axis of the aircraft, many definitions fail to say Why! It is to ensure that you acheive the screen height by the first obstacle in the Net Take Off Flight Path following failure of the critical power unit. With large aircraft this can be quite critical in terms of height and in many cases early is better than late. In a single it is irrelevant and most light twins could not get airborne with a failure at Vr.
Tumbleweed wrote:
In a single it is irrelevant
Obviously it is irrelevant in the sense of its purpose for transport-category aircraft. But in the sense of achieving the expected take-off performance in a single, I don’t think it is irrelevant at all.
It may not be called rotate speed, but it is certainly not irrelevant.
A performance take-off into a Vx climb has the aircraft accelerate up to the point it reaches Vx. You lift off at a speed between Vs and Vx. To lift off at that speed, you need to start applying back pressure before lift-off speed. If you don’t do that, you will spend more time and distance on the runway.
If you are going at 60 kts, an extra second on the runway means 30 metres. In a powerful aircraft, it also means that you will barge through Vx before you know it, and you won’t achieve the “book” climb performance.
The Acme school phraseology uses a concept of Vr for light training aircraft, which is probably harmless, although technically for Class B the term might be Vlof (lift off speed), and in the case of multi engine, the wonderfully named Vtoss (take off safety speed).
Some twins have a Vx well below Vtoss. I believe the technique for using Vx in these STOL twins is for soft field or gravel runways, where once lift off is achieved you accelerate in ground effect to Vtoss or Vyse, rather than using Vx for obstacle clearance. At least for public transport type operations.
What all of us have to remind ourselfs when we watch these figures is a simple fact: The POH’s and all the performance figures were compiled out of test pilot results. These guys know the airplanes inside out, they know how to squeeze extra knots out of them (famously with the Mooney 201 and 231) and they will achieve performance which will take the “normal” pilot quite some experience to achieve. So not everything which is optimistic is just “marketing”, quite a lot is also simply flown under heavily optimized conditions.Quote
The conditions under which the performance data is gather to support an “approved” flight manual, are closely controlled. There are complex math corrections to be applied to the data collected, to equalize it to standard conditions, and corrected weight. The data you observe while you fly, and compare to the performance data will not include these corrections, unless you’re doing a lot of math. For those interested, search “PIW CIW method”, and you’ll get a sense of it. That correction will bring all formally collected performance data to be “apples to apples” relative to other aircraft, when presented as “approved”. ALso bear in mind that if your instruments are not calibrated, you might not be getting the right data yourself. When I do performance flight testing, the ASI, Altimeter, tach and MP gauge have to be recently calibrated.
The “optimizing” which goes on to make the plane perform well would be limited to very clean, new, waxed paint, well performing engine, few antennas, and all speed fairings installed precisely. Otherwise, it’s just precise flying, which should be within the skills of all pilots.
Few flight manuals for GA single engine aircraft provide a “rotation speed”, and rightly so. Regrettably, some do seem to state “raise the nosewheel” at a stated speed. This would leave the impression that it’s okay to leave the nosewheel bearing the weight of the nose until that speed. Yes, doing so will most likely get you the takeoff distance in the performance section, but, it’s going to cost you needless wear and tear on the nose wheel assembly, and propeller. With few exceptions, GA singles do not need to be “rotated” to get airborne, they simply fly when they are ready, if the pilot maintains a suitable attitude. Keeping the nosewheel light from the beginning of the takeoff will lengthen the takeoff roll a bit (as lifting the tail of a taildragger), but unless you need that exact performance, I’d rather save maintenance cost on the third wheel!
There is this well known US instructor called Barry Schiff. He created some videos called “Proficient Flying”. In one of his videos he takes several standard aircraft such as a C172, a Twin and several others and loads the aircraft to capacity with sand bags exactly to the specs in the POH and then flies them and compares the figures in the POH to the actual figures while flying them such as the takeoff run, etc. The POH figures could never be made, not even getting close. In most cases the actual figures were 20-30 percent off (worse).
AeroPlus wrote:
The POH figures could never be made, not even getting close. In most cases the actual figures were 20-30 percent off (worse).
Which would have been a reason to take action with the FAA and have the manufacturers update their POH’s. 20-30% are way outside any tolerance. The question here arises what kind of airframes were used for this. And how airworthy they were if their performance is 20-30% off.
Which would have been a reason to take action with the FAA
Very few people would want to do that sort of thing because – in certain cases like bogus documentation – it risks an AD grounding all aircraft. This would be the same for say the KFC225 autopilot except that it does meet the certification requirements 
Well, Barry Schiff would have had the balls to do that, he is quite well listened to over there. And if he has such a claim, the FAA would listen to him too, as well as most manufacturers.
