Fastback & Performance

I built my -4 fastback because I liked how it looks. If it used a little less gas, bonus. I just finished reading Dave Anders presentation <click here to download his notes> on his mega-speed RV-4 and two things stuck out at me.

#1 - He went fastback and painted together and it netted +6kts. Anders himself stated he thought it would be more.

#2 - I imagine he probably did dozens of big and small mods to both the engine and airframe, with a very high attention to detail. In the end he produced a measurable 27% reduction in drag. He summarize all his gains as:

"Flying with other RV?s w/o any of the mods uses ~ 22% less fuel per stop especially when were high and lean." and "27% of increased performance from drag reduction"

He also calculated that for the 14,800 gals of fuel he's burned, the decrease in fuel burn from his mods should have saved him $11,100 @ $3.75/gal. Forget the $5+/gal we pay now.

Looking over his changes, no one mod made any huge difference in performance. The big winners, in no particular order:

Engine:
- Electronic Ignition (for fuel burn at altitude only)
- Plenum air box
- Inlet/outlet ratio from 150% to 76%. He decreased inlet slight and outlet a lot.
- Cooling tunnel fuselage mods (smaller, see previous)
- higher compression from stock 9:1 to 13:1. He's also running ceramic heads.
- Smoother flowing cold-air induction
- 4:1 exhaust

Airframe:
- Fastback
- Paint
- Improved Gear leg fairings
- Lamb Tires
- Pressure recovery pants
- Sheer wingtips
- Aileron cuff position (re-rigged)
- Aileron hinge bracket fairings

I feel like having my airworthiness is yet another license to learn. Or stated differently, I've climbed K2 only to look out and see Everest.

-Bruce

Ive flown with (behind) and talked To Dave Anders alot. My impression with his speed mods is that if starting with a clean airframe and strong engine that the engine plenum and cooling presented the greatest potential for increase in speed. Next would be turtledeck. YMMV
Tin Man

Anyone know where I can find pictures of his engine and lower firewall / belly?

-Bruce

Dave is a VAF member, doesnt post a lot. You could try a PM, or e-mail.

He is very easy to talk to, and quite willing to share his speed "secrets".

Check out this thread. http://www.vansairforce.com/community/showthread.php?t=26615

I found those pictures from the Anders thread. thanks. Here's the direct link

http://www.flickr.com/photos/arffguy/

nice plenum!

More specifics

Here are my results from a more detailed analysis. All %'s are increases in dynamic pressure (0.5 * air density * velocity ^2). This is a direct function of flat-plate area & power, so it's a normalized indicator of overall increase in performance for any given speed... See a 5mph improvement at 100mph isn't nearly as impressive as a 2mph increase @ 200mph. With dynamic pressure we can compare drag & power alterations as apples/apples for any given speed.

1992 - 6%: Plenum, Y-stack
1993 - 8%: An early firewall cleanup, more revs (2980), electronic ignition
1994 - 4%: Racing tail, gap seals, lamb tire & 2-piece pants
1995 - 10%: 10:1 comp, new FI, new plenum, new gear leg fairings
1997 - 2%: 11:1 comp, new inlet rings, aileron work, sheer wingtips
2000 - 7%: fastback, paint

This just confirms what Dave and others have already stated. Spend your time cleaning up the cowl (especially the exits) and go fastback. Those are the two really big wins.