Denny Dart II Prop Bearings

Monday, October 4, 2010

I was thinking about what to do with the second DD II Ding sent me. The single hole L prop hanger puts the prop shaft parallel with the stick. To get the angles I want between the prop shaft and the wing and tailplane, it would be necessary to put a 3/32″ shim between the stick and the trailing spar of the tailplane and a 5/16″ shim at the leading edge of the wing! Having to put shims on is a nuisance, adds weight and is hard to keep square.

The DD II with the plastic prop hanger has 5.7 degrees of downthrust. The wing is flat on top of the stick and the tailplane taper at the back of the stick is 1/4″ in 3 5/8″, 7.6 degrees. This flys very well. If you have a bunch of props without hangers for making DD IIs, I suggest making the prop hangers with a 1/4″ length of aluminum tube glued to a balsa block with 5.7 degree taper. As noted before, I am using a 1/2″ x 1 1/2″ paper rudder angled slightly to the left to maintain the turn after the torque runs down.

Cold and windy today, so I am not doing any flying. Maybe I will make a couple balsa props for my Cloud Tramp.


Tuesday, October 5, 2010

Ding measured the downthrust on thebearing of a Sig 5 1/2″ prop assembly and got 1/2″ in 3 5/8″, which is 7.85 degrees. That is what is actually there on the AMA Cub, Delta Dart, Squirrel, Dandiflyer and Dennydart when they use those prop assemblies. I would assume that is what Ehling had in mind when he chose that prop assembly for the AMA Cub. It is essentially the North Pacific prop assembly that had been used for years. I assume North Pacific chose that downthrust angle for a reason, too. With the wings at 0 degrees, that is the effective attack angle, measured to the wing chord. Aerodynamic charts for thin wings show the maximum L/D at around 5-7 degrees and the maximum power factor at 6-8 degrees, so this is a good attack angle to fly.

I generally measure somewhat less than 7.85 degrees for the usual prop assemblies. Here is how I do the measurement. I put the prop assembly on a stick so it is snug. I put a pin in the stick and loop a rubber motor to pull the prop shaft the way the motor will pull it. There is a little play in the prop shaft bearing, so I want it pointing the way it will point when the motor is pulling it. The shaft is usually visibly less than the angle of the bearing hole. I measure the perpendicular offset h between the stick and the prop shaft at the back of the bearing. I mark the stick where the offset was measured. I put the steel straightedge along the shaft and align it carefully. I mark the point where the straightedge crosses the bottom of the stick. I measure the distance d between the marks. The downthrust angle is the inverse tangent of h/d. In Ding’s example, h/d = 0.5/3.625 = 0.1379 and arctan(0.1379) = 7.85 degrees.

The tailplane taper of 1/4″ in 3 5/8″ implies arctan(0.0689) = 3.95 degrees measured to the chord, about 4 degrees, as Ding said.

So I would say the AMA Cub design includes about 6 degrees downthrust, 0 degree wing incidence and -4 degrees tailplane incidence. The zero lift lines are maybe 1-2 degrees above the chords.

What is important to flight is not the incidence measured to the stick, but the angle of the zero lift line measured to the direction of flight. If the plane is following straight behind the propeller, that would be relative to the prop shaft. We call it downthrust because it is down relative to the construction reference line. In flight, it should be straight ahead, not down. The stick makes a slight angle to the direction of flight. We should refer to the angle between the prop shaft and the stick as propdown incidence.

The Dennydart is not well trimmed with 0 on the prop, 0 on the wing and -4 on the tail. It is necessary to make up for the aerodynamic imbalance by putting the CG in a bad place and fly the wing at a poor attack angle. It will zoom under power and dive when the power comes off, severely limiting flight times. With -6 on the prop, 0 + 1 on the wing and -4 +/-2 on the tail, it flies well; slow steady climb, steady cruise and gradual descent. If you want 0 on the prop, the other incidences must be changed accordingly to be in the same relative angles to the prop shaft.

Prop performance may be compared if you fly the props on the same plane at the same angle with the same settings and the same CG and measure the torque required for level flight. The balsa prop may be lighter, and that is a big advantage in itself, but that is not the whole story. What is its aerodynamic performance?

Duration is our criteria for performance. I design and trim for duration.

My estimate of the zero lift line on tissue over stick airfoils is based on tests of the Big Pussycat tailplane. The plan calls for the tissue on the bottom. Several people have said it should be on the top because it will lift better. It does! I tried it that way and the plane dives. I had to deflect the tailplane down in front 4 degrees to get the same glide back. This tells me the zero lift line is midway, or 2 degrees from the chord. It so happens that the 2 degree line passes through the top of the leading edge and the bottom of the trailing edge. We may adopt this as a way of estimating the zero lift line on similar wings. It may not be exact to sixteen decimal places, but it is likely as close as we can measure and build angles.

Sunny, but windy.


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