Mooney_Driver wrote:
So even the 1965 POH’s are not that far off, as long as the sometimes optimistic top speeds and range figures are ignored :)
I still have my doubts about that. The good thing is that aircraft engines mostly make their book power thanks to the amount of maintenance that is spent on these. That is certainly not so true when it comes to classic cars from that Era. But all the dents, missing wheel spats and so on must make a difference. I believe that when you really compare apples to apples and take off at MTOW at exactly the published conditions you will measure a difference. Probably you will barely notice a difference without measuring because at 60 knots it is hard to determine the exact spot where the aircraft left the ground. But measuring it is going to make a difference.
Well with the old manuals, the bigger problem is lack of data. In many cases, you need to extrapolate as there simply is no data e.g. for take off and landing above 5000 ft. Also a lot of other stuff is missing, other is quite badly misleading (range?) .
Basically, I usually take the manuals and then fly to figure out how accurate it is for that airframe. You are very right, quite a few airframes will have massive performance issues. On the other hand, they also do not take into account mods like in my case thePowerflow Exhaust or in other cases new cowls or windshields, which turn the old cabin into a J type cabin.
In the 10 years I operate my plane now I found it pretty much makes book figures… not least because of the powerflow exhaust. For cruise, the Aspen’s TAS and the Shadin fuel flow meter have helped to determine this, for runway performance, I’ve had possibilities to get pretty accurate results too. For Samedan, 600m is the first third of the runway. You are right it won’t be totally accurate but you can easily see if you are in the ballpark or not.
Apart, being conservative is certainly a good thing. e.g. always go to the higher elevation in the manual (in my case, most airfields of Switzerland are around 1500 ft AMSL, I always use the 2500 ft tables). Likewise, one can compensate for lack of data with an adjustment of temperature. e.t.c.
In cruise you can sooner or later define a bias, a factor to correct the book figures if they are too much off. In my case, my normal cruise and long range cruise figures are pretty much spot on, while it does not quite make the marketing figures for high speed cruise. And obviously, range needs to be recalculated using the regime you are actually flying. Which btw is why a lot of folks think their manuals are wrong, they simply don’t fly the actual power setting they refer to.
I vividly recall one guy who claimed his Grumman Tiger was a 120 kt airplane. Well, when he sent a picture eventually, we found him out… he referred to IAS he was seeing on his ASI. At the altitude he was flying, TAS was 138, which is close to book. His ears are still ringing from that one I think.
