RV-8 with IO-320 and Catto Prop
I just finished my RV-8 with an IO-320 and a Catto 2 blade prop. I also came into a good deal for this engine, magnetos, AFP FM-100 F/I and prop, so I built my 8 as light as possible to match the expected performance of both the engine and prop characteristics. My previous RV experience had been a 9A with 160 HP C/S and a 6 with a 180 HP C/S. I had never flown a FP prop RV before. The Catto prop is very light, but you do need to add a prop extension, which adds some weight, but no where near what a CS prop weighs. The weight difference between a 320 and 360 is maybe 10-20 lbs depending on installation and engine model. My IO-320D2A is a light weight version, with no provisions for a CS prop. I ended up with an empty weight of 1028 lbs. I asked Craig Catto for a prop that allow me to cruise at 150 KTAS and take off from a 2000 ft grass strip at gross wt, with the 160 hp / RV-8 combination. Craig suggested the 2 blade 70" dia, 70 pitch prop. I am now in Phase 1 testing and here are my performance specs; Static RPM is 2100, T/O RPM at un-stick is 2400 RPM, The Dynon Skyview EMS says the engine is making 89% power (2400 RPM, W/O throttle, 12.5 GPH fuel flow, 1000 ft DA). Take off roll on pavement is about 400-500 feet. Grass is 700 feet. Adding flaps, helps cut about 100 feet off. I have found that Vy is about 115 KIAS, mostly for engine cooling and letting the prop RPM wind up to at least 2500 RPM. Climb at Vy is 1200 to 1500 FPM. At lower climb speeds (80-110 KIAS), the engine and prop are not efficient and it wallows along at 500-800 FPM climb. Vx is with 50% flaps at 90 KIAS, but engine cooling is problematic. Once airspeed is above 120 KIAS and the prop RPM is 2500 or greater, performance improves significantly. Cruise climb at 140 KIAS is about 800 FPM. As far as max speed, at 2000 feet, max power, I have achieved 173 KTAS average, using a 3 segment, 10 mile leg course. Prop RPM stabilizes at 2750. At 8000 feet, the same speed test provides 171 KTAS, but the prop over speeds slightly at 2800 RPM. Cruise power (70%), leaned to 8.5 GPH fuel flow and 2475 -2525 RPM yields 155 KTAS at 6000-8000 feet. Note, I have not installed my wheel pants and these specs are not at gross weight, so I expect some minor increase in speed with the wheel pants, but lower climb at higher weights. Some thoughts on making the airframe light and using a light engine prop combination; With full fuel and just me, my CG is 79.83 and 1436 lbs. This is a somewhat fwd CG and the handling characteristics in pitch was as poor. The pitch axis was heavy and 3 pt landings were problematic. By adding about 50 lbs in the aft baggage area, CG now becomes 81.79 and the pitch forces lightened up and 3 pts were now easy. Max airspeed went up slightly, possibly by reducing some trim drag. Stall speed went down from 48 KIAS to 47 KIAS. I suspect adding more weight in the back will possibly improve the pitch axis handing to a point. One possible negative aspect of the light weight airframe / FP prop combination is that when you want to slow down, it requires you to reduce power much sooner and you have to manage the pitch and power much more closely to manage approach speed and glide path, because the FP does not provide much drag. My initial idle RPM was 700. After a few white knuckle floater landings, I set it at 600 RPM and that helped significantly. So a FP prop on a light weight airframe may lead to significant floating if you are too fast on approach. I have found that making 3 pts landings at 65 KIAS, at idle power, seems to work out well. Wheel landings seem to result in more float, due to the slightly higher approach speeds. Aerobatics will probably magnify many of these issues depending what you are doing, prop over speed being one of them. Another issue that I have run into with light weight prop is the lack of "flywheel effect". At low idle (600 RPM), my engine runs a little un-even. At 700 RPM it idles smoothly. I have asked a few folks about this and it has been said that the slower the engine runs, the more the flywheel effect of a heavy prop helps smooth out the firing impulses. So on landing roll out, I have to let the engine due its thing until I get slow enough to idle it back up to about 700 RPM. Also, during engine "almost starts", the starter drive may stay engaged to the ring gear, I sometimes get kick back, that causes the prop to spin backwards a blade or two. It is possible that a heavy prop would resist this kick back more than a light weight prop due to its greater inertia. Possibly, make sure your starter is capable of handling kick backs. I have an S-Tec lightweight on my 8.
In summary, if you build a lightweight RV-8 airframe and use a 160 HP engine, with light weight prop, first decide what you want it do, cruise fast or climb well. It may not do both well with a single FP prop design. I wanted to cruise efficiently and be able to operate out of 2000 foot grass strip. I have not operated mine at gross weight, on a hot day, off of grass yet. I suspect that I may have to be careful. In the tandem RVs, it appears that there is going to be trade off, put a large engine and prop up front, add weight in the back to compensate for CG. Even a light weight engine and prop requires a bit of weight in the back. Overall, it will comes down to handling qualities, once airborne, based upon the difference in gross weight. The heavier RVs I have flown seem to settle down on landing better (less float), seem to ride a little better, (higher wing loading), but they all have CS props and more HP. A light weight airframe with 160 HP and FP may have a higher useful load based upon the delta between empty weight and max gross, but it may not have the performance to actually operate in certain conditions at max gross weight. In the end, I am very happy with what I have built and how it flies. I knew ahead of time what to expect. My total cost for the 400 hr engine, prop, 4 new cylinders, F/I unit and mags was $7800. So far I am happy with the fiscal performance of my RV-8, IO-320 160 HP engine and Catto FP prop.
