Aerodynamics For Naval Aviators, Rev. Jan. 1965, P.149. Fig. 2.19. Propeller Operation, Propeller darg contribution. This graph shows that a windmilling prop whose blade angle is greater than 22 deg. has less drag than a stationary prop, and then become equal at about 75 deg.

Paul,

 

Thanks for the reference since I was on my way there (I have the page book marked).  I delayed a bit to get actual data on my prop.

 

Here is the graph you referenced.  Please remember that the stopped prop blade is also at the referenced beta angle otherwise, the drag indication would be a straight line.

 

Now for some data to attach to this graph.

 

Prop:  The Lancair 320 equipped with the recommended Hartzell Constant Speed prop has an 84 in prop cut down to 70 inches. Its range is from 12 to 40 degrees of pitch.  Interestingly, most props peak power delivery is at about the 2/3 position from the hub.  For the 84" blade, that would be at 2/3 of 42" or at the 28" position, 7" inches shy of the end when it is cut back to 35" (70" prop).  That is exactly what I measured today - 12 degrees at the 28" chord.  Furthermore, it is a very wide prop, 6.5 inches at the longest chord and 5" at the tip.  Assuming that the blades were flat, the prop might account for 2 feet square of flat plate resistance when stopped.

 

Consider the plane - A comparably powered C172 is 675 Sq ft of wetted area and a flat plate equivalence of 6.25 Sq ft along with a similar prop.  I have read that Lancair IVs are about 2.1 Sq ft.  A Lancair 320 is 325 Sq ft of wetted area and, for these purposes, let's say it has a flat plate of 2.0.

 

Suppose I add some lines to the graph.  Vertical lines representing the 12 and 40 degree prop pitch range.  Horizontal lines  representing an overlay of airframe drag (of course, one would have to add the prop drag from the origin to the horizontal line).  Anyway:

 

 

The wind milling prop at flat pitch (say, takeoff power when the engine failed) would seem to indicate that the first thing to do (after fly-the-plane, like best glide) is pull the prop back.  I have to guess that a fixed pitch prop is somewhere in the middle of the pitch range.  In addition, the effect of a wind milling prop has a greater impact on the expected performance degradation of a 320 than on that of a C172 and were my prop to stop at fine pitch (seizure), I can expect to see the flat plate drag double - Arrrrrgh - That means going down now....

 

Should the engine loss occur at cruise, it is still better to consider pulling the prop back to coarse pitch sooner rather than later.

 

Then Tom T. wrote:

 

Another data point (perhaps not relevant to Lancairs).

A friend flying an RV-7 just recently had a total engine seizure while in flight and, thank God, dead sticked to a nearby runway without airframe damage or any injury.  Well, maybe he and wife Pam needed new shorts!

His description of the incident lists a descent rate of ~1,400 FPM during the glide.  That RV has a 180 HP Lyc IO-360 with Hartzell CS prop.  By comparison, my LNC2, with the same engine prop combination, obtains an idle power ~500 FPM descent rate in full coarse pitch and ~1,500 FPM in fine pitch.

Is it surprising to anyone that a stopped, coarse pitch prop in an RV would result in such a large descent rate?  I would have expected that the RVs stopped prop descent rate would be much closer to an LNC2 with a wind milling coarse pitch prop.  If his experience is normal for an RV, does that mean a wind milling fine pitch descent would be 3,000 FPM or more in an RV?

Any thoughts?

>>>>

 

Sure, look at the chart. A stationary propeller forced to flat pitch for lack of oil pressure (engine seizure) sure looks drag-gone-ugly when compared to that wind milling prop at 40 degrees.  Even more horrible is a wind milling prop at fine pitch, but with oil pressure still there from engine rotation (wind milling), it would help a bit to pull the prop back............

 

Something to think about.  I would not consider maneuvers to try to stop the prop as the performance loss could be too great unless I was flying with a fine pitched fixed prop (<20 degrees cruise pitch).  Know your equipment.

 

BTW Tom, I have seen similar performance numbers as you have whilst messing with the prop. Have you considered stopping the prop at fine pitch for comparison (Uh, maybe I should change my moniker to ChickenHawk)?