Glide performance for SEP(L) Pipers

[huv]

I wonder about the book glide performance figures for different PA-28 models.

From instructing on Piper Warriors (PA-28-151) I know the book glide performance of 2,1 statute miles per 1000 feet, corresponding to a best glide ratio of 11,0. I always considered this a little optimistic (glide performance figures usually are) but I have never done a flight test.

For myself I often fly a Piper Dakota (PA-28-236), which has the same wing, a slightly longer fuselage and a somewhat larger engine with a c/s prop. Until today I never checket its book glide performance although I often consider gliding distances when crossing domestic waters here in Denmark. But according to the book the Dakota glides only 1,30 NM per 1000 feet, or a best glide ratio of only 7,9.

This is a very large difference - 40% - between two very similar airplanes.

If the difference is real, I cannot think any other significant reason than the greater drag of windmilling the larger-displacement engine in the Dakota (O-540 vs. O-320).

But could it also be different ways of compiling glide data for presentation in the handbook? The Pilot's manual for the Warrior seems older than that for the Dakota; maybe the FAR 23 was changed in-between? There is no clue in the CS-23, but I do not not have the older standards. (The Archer II, PA-28-181, with an O-360 engine, is the ~same vintage as the Dakota and its glide ratio is 9,9 according the AFM. This seems to support that the difference is due to the engine.)

Yes, I do intend to test it, but not until summer with more pleasant air temperatures for the engine.

[sapperkenno]

I have a feeling that for the Dakota with the c/s prop, "best-glide" is calculated with the prop lever fully forward. I think this is due to it being the "worst case" scenario, as with a lack of oil pressure to the prop, it will fail to fully-fine (as opposed to most Multi-Engine types where it fails to a feathered position) and cause more drag when windmilling.

You will see this for yourself if you descend power off at a set airspeed with the prop lever forward and note the descent rate, and then pull the prop lever all the way back and compare at the same airspeed. Think of it that you are commanding a prop RPM from the airflow (in a power off descent) by moving the prop lever, so that the governor when trying to set a lower RPM, does this by setting a smaller blade AoA and taking a lesser bite of the air. This in turn creates less drag, and gives you the lower rate of descent.

I have done this in an Arrow, and it makes quite a marked difference. I'm sure that such a configuration was mentioned in the Arrow IV POH, but don't have access to a copy to check.

Regards.