Cruise Performance

[Crankshaft]

As we all know the book performance and the real world performance are not necessarily the same thing. Fair enough, but that’s not what I want to talk about.

I would like to know if anyone else has experienced the same differences between fixed props and constant speed’s that I have. Or if it might just be a case of those individual aircrafts that I happen to fly?


I have the impression that real cruise performance differ about 5-15% from the POH figures, (in most but the very newest aircrafts).
However, it appears that if we are talking about constant speed props we loose a quite large percent of the TAS, while fuel flow figures are quite correct. In fixed pitch, I generally notice a smaller reduction of cruise speed, but with a greater difference in fuel consumption. Once again, I do not fly enough numbers of different aircrafts to establish whether this is a general difference or not.


I have my theory for this, but let me first ask you if anyone else has experienced the same thing.

[dont overfil]

I can't give you an opinion on the different figures you have experienced between constant speed and fixed.

I do have an opinion on POH figures in general. The AC I fly now is fairly new (constant speed) and the POH figures are pretty close.

Other older AC I have flown don't get near the POH figures.
I think it is to do with age. Not so much the aircraft as the manual. There has been a change in culture. A few years ago the POH was being used as a sales aid. The figures nowadays are a lot more truthful.

D.O.

[B2N2]

A few years ago the POH was being used as a sales aid.

I think you hit the nail on the head there.
Aircraft from the 60-ies to the 80-ies in the POH tend to have some inflated numbers under more then ideal conditions or conveniently in MPH iso KTS (20% difference there).
At least some of the new aircraft that I fly have somewhat pessimistic performance figures and you do not need exceptional pilot skill to match them.

So old, unreliable numbers, new pessimistic.
Also remember that an aircraft tends to gain weight over time.
Dirt accumulates in nooks and crannies and repainting the aircraft without removing all of the old paint increases weight.
Major body panels will be replaced, cowlings, wing sheet metal, tail sections.
All depending on the amount of hangar rash or fender benders that the aircraft goes through. This will ususally lead to the aircraft being slightly out of rig which will cost a mile or two on the top end.
10 little items that will cost you a mph each add up.
A good example is the reverse, you can make a Pa28 fixed gear or an Arrow go 10-15 kts faster in cruise.
By adding so called speed mods or speed kits. The difference is usually very small per component, 2 kts for better wheel fairings, 2 kts for aileron seals, 2 kts for flap seals, 1 knot for wing fairing, etc etc.
Will cost about $1000 per knot speed increase by the way.
But you see how the process works both ways.