Takeoff Speed - 2-blade vs 3-blade prop

[AspiringOwner]

There is a significant difference in "takeoff speed" (=rotation?) between 2-blade and the 3-blade McCauley props in the Mooney Eagle: 67 kts vs. 76 kts. I have been going by the 67 kts number that applies to my Eagle 2, but do find myself accelerating in ground effect with the stall warning blaring quite often. Curious what others are doing. Does adding some more speed before rotation hurt in any way?

Flight Supplement, M20S with McCauley three blade prop:

 

POH Mooney Eagle, 2-blade McCauley prop:

 

 

Edited Saturday at 11:26 PM by AspiringOwner

[Slick Nick]

I don’t see why the prop would have any effect on rotation speed. It’s the same wing, isn’t it? The wing only cares about how much air is flowing over it. The method of propulsion is irrelevant. 

[Hank]

My C has a 3-blade Hartzell, and I take off at the speeds shown in my Owners Manual (printed for the original 2-blade McCauley prop).

Push everything forward and the book says to pull back at 65-75, so unless I'm loaded heavy, I use 70.

People have reported that planes with 3-blade props take off in shorter distances and to climb to altitude faster. But the airspeed should be the same, we just get there quicker 

[PT20J]

The airplane takes off when lift = weight. Lift is a function of TAS^2; it is not affected by thrust and thus choice of propeller which only affects how quickly the airplane accelerates to takeoff speed.

I cannot account for the differences in the POH speeds; there are some anomalies in the Mooney performance data between various models. Perhaps it is a difference in technique by different test pilots. 

Rotation is really a concept from jets and not directly applicable to piston powered airplanes. The best technique is to apply a small amount of back pressure during the takeoff roll and let the airplane fly off when it's ready rather than trying to yank it off the ground (i.e. rotate) at a particular speed.

[buddy]

I rotate at 62 when light to mid weight 67 when heavy. 310 hp

[GeeBee]

The entire thing is runway vs obstacle dependent. There is value in leaving the airplane on the ground and allowing additional acceleration to come off the ground with "extra mash" for acceleration to climb speeds if you have the runway. If you notice in your data, just to achieve Vx minus 2 at 50' the two blade has to stay on the ground and extra 9 knots to come off with the extra mash needed to accelerate while the three blade smartly accelerates the airplane to Vx at 50'. The problem with the long body Mooney is after about 67 knots on the ground, it gets squirrely so the three bladed prop creates greater ground control in its performance.

We see the "improved takeoff" technique often and in particular for under powered transport aircraft or at high altitudes. We used to use "Flaps 1 Improved" takeoff out of Guatemala City  for instance. Yes the runway is a little short, but the climb is the limiting factor so we would set our flaps to minimum, accelerate to a very high rotation speed then take it airborne. It takes a lot of intestinal fortitude in those situations because you will see the numbers at the other end before you rotate but the airplane will gather itself very quickly to the required climbing speeds. If you fail to keep it on the ground your climb performance will deeply suffer and you may depart via the Cholorphyll One departure hopefully without the branch transition

On a humorous note at old Denver Stapleton we were doing a flaps 1 improved takeoff to the west in a 737-200 with dash 8 engines (read slug) We waited for the needed 6 knots of headwind and rolled. Scheduled rotation was 161 knots! As we rolled down the runway midfield, the midfield windsock turned to a tailwind. My Captain who was flying looked at me and said, "There is no stopping this S-O-B now!" and off we went looking at the numbers underneath us as the wheels left the ground! 

[Fly Boomer]

19 minutes ago, GeeBee said:

Cholorphyll One departure hopefully without the branch transition

Sounds exciting!