Looking for help Computing Drag Impact

[lr172]

I will be adding an ads-b antenna, similar to my transponder antenna. I am wondering if it is worth the cost/hassle of going to blade style antennas.

According to Rami, the rod and ball antennas has .41 lbs of drag at 250mph and the blade has .09 lbs at 250. I would be reducing .8 lbs of drag at 250 mph.

Not being an aeronautical engineer, I have no idea how that number translates to improvement in airspeed on an RV cruising at 160 knots.

Can anyone help me ballpark the speed improvement?

Thanks,

Larry

[scsmith]

250 mph = 367 ft/s

The equivalent power for the small drag you describe is 0.8 lb x 367 ft/s = 293 ft-lb/s.

1 Hp = 550 ft-lb/s. So your drag change is equivalent to a power change of 1/2 Hp.

Thats at 250 mph.

160 kts = 184 mph = 270 ft/s.

At that speed, the equivalent power is 0.8 lb x 270 ft/s /550 = 0.4 Hp.

For constant drag coefficient, power varies in proportion to velocity^3, so assuming you are using 180 hp to go 160 kt at sea level, the speed increase is approximately, (0.4/180)/3 x 160 = 0.12 kts.

[lr172]

Originally Posted by scsmith

250 mph = 367 ft/s

The equivalent power for the small drag you describe is 0.8 lb x 367 ft/s = 293 ft-lb/s.

1 Hp = 550 ft-lb/s. So your drag change is equivalent to a power change of 1/2 Hp.

Thats at 250 mph.

160 kts = 184 mph = 270 ft/s.

At that speed, the equivalent power is 0.4 Hp.

Thanks for the help and detail. Can I assume that the speed obtained with that added .4 hp is a small fraction of .4 in MPH?

Larry

[scsmith]

Originally Posted by lr172

Thanks for the help and detail. Can I assume that the speed obtained with that added .4 hp is a small fraction of .4 in MPH?

Larry

Original post updated to include estimated speed increase

[6 Gun]

Looks like a great project for our VAF bros. with the 3D printers to make a blade cover for transponder antenna.I want one. Hope Steve Melton sees this.
Bob

[ChiefPilot]

Originally Posted by 6 Gun

Looks like a great project for our VAF bros. with the 3D printers to make a blade cover for transponder antenna.I want one. Hope Steve Melton sees this.
Bob

Why not just buy a real blade antenna from Delta Pop? They're super cool, reasonably priced, and work as expected.

website

[rv7charlie]

Originally Posted by ChiefPilot

Why not just buy a real blade antenna from Delta Pop? They're super cool, reasonably priced, and work as expected.

website

Homebuilding mentality?

DIY philosophy?

300% price delta over the spike?

[Bill Boyd]

1. Steve said, "speed increase is approximately, (0.4/180)/3 x 160 = 0.12 kts." Not sure where the "/3" term comes in. Feel like there should be a ^-3 function in here somewhere. But I'm NOT a math guy.

2. Have noted for some while the auto industry's use of spiral-fluted automobile antennas, presumably for reduction in vibration seen in smooth rod broadcast receive antennas on car fenders. Is there a drag reduction to be gained from tripping the boundary layer with a spiral flute like this? Might be a compromise design between blade and rod-and-ball antennas that we could implement?

[airguy]

Originally Posted by Bill Boyd

2. Have noted for some while the auto industry's use of spiral-fluted automobile antennas, presumably for reduction in vibration seen in smooth rod broadcast receive antennas on car fenders. Is there a drag reduction to be gained from tripping the boundary layer with a spiral flute like this? Might be a compromise design between blade and rod-and-ball antennas that we could implement?

The spiral flute on the auto antenna is there to prevent vortex shedding which will cause the antenna to whip back and forth perpendicular to the airstream.

[rv7charlie]

Originally Posted by Bill Boyd

1. Scott said, "speed increase is approximately, (0.4/180)/3 x 160 = 0.12 kts." Not sure where the "/3" term comes in. Feel like there should be a ^-3 function in here somewhere. But I'm NOT a math guy.

2. Have noted for some while the auto industry's use of spiral-fluted automobile antennas, presumably for reduction in vibration seen in smooth rod broadcast receive antennas on car fenders. Is there a drag reduction to be gained from tripping the boundary layer with a spiral flute like this? Might be a compromise design between blade and rod-and-ball antennas that we could implement?

I bet he just got lazy with the keyboard when he typed the formula. I did a cube root of that ratio, & got around .13.