[brad walton]

I have seen the U tube video of round shapes and streamlined shapes in a wind tunnel and the difference that streamlining makes. In the real world, how much difference does streamlining make on, for example, an RV tank drain valve? How about the pitot mast and the tank vent tubing? Maybe a couple pounds of drag reduction at 165 KTAS? And how much power does a couple pounds of drag reduction equate to? I know it depends on many factors, but wondering if there is enough drag reduction to be had from streamlining these protrusions to have a noticeable effect.

 

[David Paule]

Drag in pounds, times speed in feet per second, equals power in foot-pounds per second. Divide that by 550 to get horsepower.

For example, your 2 pounds of drag at 300 feet per second (about 178 kts) is 1.09 hp. That is actual horsepower. Due to prop efficiency losses, it takes more engine horsepower than that to pull that drag through the air.

An excellent reference is "Fluid-Dynamic Drag," by Hoerner. The classic in the field.

Also spend some time here at VAF searching the archives for speed improvement, drag reduction, cowling modifications, etc. There's much of interest.

Dave

 

[flyingriki]

From Barnaby's Seminar

 

[rgmwa]

Interesting figures. There are 12,500 round(ish) rivets in the RV-12, of which probably 75% are exposed. By my reckoning that suggests that at 115kts (I assume Barnaby's values of 0.4 and 0.9 are for 100 rivets), something like 17 Rotax hp is required to overcome their drag. Surely not?
Where's my trusty file?

 

[rvmills]

Brad,

All those things add up. I doubt you could ever measure the effect of one item in this realm of drag reduction, but the "sum of the parts" really makes a difference. Incremental, but not incidental?its worth the effort! I'd take a couple pounds of drag reduction anywhere I could find it?that's no chump change?but I'm kinda afflicted when it comes to that stuff!

Just do it?those little touches get noticed by discerning eyes too!

Have fun!

Cheers,
Bob

 

[brad walton]

Thank you

Thank you for your responses. Just the info I am looking for. And I have been following the various other threads, especially the cooling drag reduction where it seems there is a good bit of reduction possibilities. I am ordering drain fairings today and will work on making something for my pitot and tank vents. Still on the fence on the tail wheel because it is probably in rather turbulent air where gains may be hard to come by.

 

[Saville]

Wow 5hp for a rotary beacon light at 200 kts? That's a lot. I wonder what would be saved if the outer casing was teardrop shaped?

 

[pierre smith]

Very few builders even use rotating beacons when strobes at three corners are superior and flush mounted.

Best,