........highly pitched fixed
prop has MUCH less drag that a typical C/S prop that reverts
to flat pitch when the engine quits. My 235 has a 64x78 wood
prop, and like Paul's has a glide ratio of well over 10:1 with the
engine idling or with the mixture cutoff. I haven't measured it
with the prop stopped.

Also like Paul's aircraft, mine takes a while to stop on the runway.
This is a definite disadvantage of a highly pitched fixed prop.
It is also a challenge when descending in formation with draggier
aircraft, including LNC2s with a flatter prop.

Paul seems to be a pretty smart guy that knows a thing or three
about prop drag. I don't think he was quoting from an advertisement,
or resorting to numerical mumbo jumbo. Also, his prop is quite a bit
different. The thin tips probably reduce gliding drag relative to my
more ordinary thick  wood  prop.
He did claim 15:1 with the engine idling and not with the prop stopped.
That seems reasonable to me... and I've got a lot more hours without
an engine than with.

Bob and Paul,

 

Sorry if I was misunderstood and I haven't flown a fixed pitch prop since 1990 (C152).  Also, it was unfortunate that I hadn't brought my copy of "Aerodynamics for Naval Aviators" (AfNA) along with me while on vacation.

 

Let's look at the diagram Paul referred to:

 

Paul has mistakenly claimed that this diagram represented drag vs pitch - it does not since beta is blade angle.  Earlier, the AfNA makes two important statements:

1) Beta, the blade angle, includes the effective pitch angle, theta, plus the angle of attack.

2) Beta varies along the blade, but a characteristic value is measured at 75% of the blade length from the hub (Paul did his calculation at that point). 

 

Guys, please allow me to take back the mumbo-jumbo part.  It was a mistake.  The main purpose of my comments were to note that optimistic estimates of engine out performance will not do anyone any good when their engine fails.  Time can be lost in trouble shooting and maintaining best glide once it is reached, time that takes away from glide distance.  It would be best to know a pessimistic rule-of-thumb for evaluating what landing site might be reached.

 

Bob, I am glad to see you mention the 10:1 ratio, would you use that ratio if you lost the engine and the prop was still rotating?

 

On further thought, I would agree that a stopped fixed pitch prop would provide less drag than a stopped (at fine pitch) CS prop because the blade width and angle are probably more beneficial in the fixed pitch environment.  There also may be important differences between different fixed pitch props, too.

 

With rotating props and the CS prop at coarse pitch, I'm not sure which is best.  I will try to find the actual blade angles for my CS prop at fine and coarse settings and report back.  It does not fully feather as that adds to the complexity and weight in the hub. Twins need the feather capability to minimize drag should one engine fail and the extra one is working to lengthen the glide.  

 

Part of the problem is that prop drag is a significant drag component when our slick craft are operating at best glide without power.  Note that the AfNA chart is labeled "Propeller Drag Contribution."  Prop drag is less significant in a draggy airplane full of rivets, wings held on with struts and with wheels hanging out in the breeze.  The chart is useful in pointing out the penalty for a wind milling prop at fine pitch and the drag reduction as the pitch is coarsened, even if by only a relatively small amount.

 

Here is some data from my 320 powered LNC2 with the 70 inch Hartzell prop.  This propeller is the same design as an 84 inch prop, but with 14 inches lopped off (by Hartzell design).  Thus, the blade is very wide.  At cruise pitch (somewhat fine), power reduced to near idle and at best glide (about 107 KIAS for the load as indicated by the AOA device), the descent rate was 1500 to 1600 fpm (104 Kts is 10,836 fpm), a worse case ratio of about 6:1.  At coarse pitch, the rate was reduced to 400-500 fpm and a worse case ratio of about 21:1.  I don't know if I could get the prop to stop for a test, but I think the drag is some where between fine and coarse (the area of the chart curve where beta is less than 22 degrees).

 

BTW, I use the rotating coarse pitch 10:1 rule-of-thumb ratio because I would like to arrive at the "set down" site with plenty of reserve altitude.

 

I would say that the most important element of our LML communications is to advise the reader of the flying configuration so there is no confusion when trying to figure out the point - I think this started out that way several e-mails ago....