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Cruise Speed C4B5

rpigeek

rpigeek

Member
I’m researching a low serial F1 with a Sport Wing and C4B5 engine, and the cruise speeds seemed low. At 10,000 feet 2,300 RPM wide open it would hit 170 mph indicated (148 KIAS). Owner quoted that it cruises at 210 +mph true (185 KTAS) at 8-10,000 at 23-2400 RPM full throttle at about 13.5 to 14 GPH LOP. That seemed low and I was expecting at that fuel rate to be about 200 KTAS from reading other anecdotes. I was expecting 185 KTAS to be about 10 GPH. Is that typical for that engine? Were earlier, low serial number kits slower?
 
RPI,

That 185 KTAS claim at 8-10K and 2300-2400 RPM is very believable for a Sport Wing F1 Rocket, especially at 8K, with the fuel flow up there at 13-14 (which sounds ROP).

For a good comparison, can you provide a bit more info please. Is the IO-540 C4B5 a stock 260HP 540, or does it have higher compression pistons? What is the advertised HP if it is not a stock motor? What propeller is installed? What was the temp and DA at 10K on that flight? If you have a TAS readout from that flight, that would help too.

I rough calculated a standard day TAS at 10K for your flight, and it pencils out to 172 KTAS. I'm betting it was hotter than standard though, this time of year. Even so, that would seem on the low side...what was your fuel flow that day? Whether you were ROP or LOP will tell part of the story too. May have been very hot the day you tested, so how you were leaned could be a factor too.

F1 Sport Wing pilots will have the best data, but I've flown side by side a number of them, with both me and them testing various props (and the propeller can make a big difference). I have a Super Six that has an IO-540 C4B5, and had a Sport Wing until a couple years ago. However, I have 10:1 compression, and the original dyno in 1998 showed 322 HP at the engine builder's shop. Here are some round numbers from the first several years of ownership, before a lot of aero mods were done, but with a couple notes on propellers. All are 10K average speeds from various test flights and cross countries, and all are WOT and 2300 RPM round numbers. Back then, I'd say my plane was a little faster than most HRIIs, and some F1 Sport Wings, but a little slower than most F1 EVOs...just for a comparison point.

C4B5 with 80" 2-blade Hartzell D-twist that had been trimmed to 79" 189 KTAS, 11gph LOP. Add a knot or two going ROP @13gph
C4B5 with 80" 2-blade Hartzell BA 193 KTAS, 11gph LOP, same + a knot or two going ROP @13gph

In both cases, if I went further LOP, it would slow about 5 knots...so the above two data points were just slightly LOP, but up high, so safe for the motor (and it stayed cool).

That prop change basically added 3-5 knots. At the same time, F1Boss changed from an MT 3-blade to the same 80" Hartzell, and he gained 12-14 knots...no kiddin'. So what prop it has is an important factor.

I recently did a pre-buy and then some training with the new owner of an HRII with a stock IO-540 and D-twist propeller, and it cruised in the mid to high 170's KTAS up at 10K, similar power settings to what you were using. However, it needed a bit of aerodynamic TLC...as the fit and finish, and the paint...needed a little work. That is the other factor to consider...how clean is the airframe build?

Hopefully some F1 Sport flyers will add data, but hopefully that gives you some data to start with. Good luck with the search and the purchase!

Cheers,
Bob
 
It sounds low for a F1. What kind of speeds are you seeing on Flightaware?

It makes me wonder if the pitot static is leaking or the ASI is way off. I would do a 4 way test and compare my numbers against GPS ground speed.

A few minutes of flight time may save a lot of head scratching time if it’s instrumentation.
 
Webb said:
It sounds low for a F1. What kind of speeds are you seeing on Flightaware?

It makes me wonder if the pitot static is leaking or the ASI is way off. I would do a 4 way test and compare my numbers against GPS ground speed.

A few minutes of flight time may save a lot of head scratching time if it’s instrumentation.
Click to expand...

That's a good call Webb!

Cheers,
Bob
 
rvmills said:
RPI,

That 185 KTAS claim at 8-10K and 2300-2400 RPM is very believable for a Sport Wing F1 Rocket, especially at 8K, with the fuel flow up there at 13-14 (which sounds ROP).

For a good comparison, can you provide a bit more info please. Is the IO-540 C4B5 a stock 260HP 540, or does it have higher compression pistons? What is the advertised HP if it is not a stock motor? What propeller is installed? What was the temp and DA at 10K on that flight? If you have a TAS readout from that flight, that would help too.

I rough calculated a standard day TAS at 10K for your flight, and it pencils out to 172 KTAS. I'm betting it was hotter than standard though, this time of year. Even so, that would seem on the low side...what was your fuel flow that day? Whether you were ROP or LOP will tell part of the story too. May have been very hot the day you tested, so how you were leaned could be a factor too.

F1 Sport Wing pilots will have the best data, but I've flown side by side a number of them, with both me and them testing various props (and the propeller can make a big difference). I have a Super Six that has an IO-540 C4B5, and had a Sport Wing until a couple years ago. However, I have 10:1 compression, and the original dyno in 1998 showed 322 HP at the engine builder's shop. Here are some round numbers from the first several years of ownership, before a lot of aero mods were done, but with a couple notes on propellers. All are 10K average speeds from various test flights and cross countries, and all are WOT and 2300 RPM round numbers. Back then, I'd say my plane was a little faster than most HRIIs, and some F1 Sport Wings, but a little slower than most F1 EVOs...just for a comparison point.

C4B5 with 80" 2-blade Hartzell D-twist that had been trimmed to 79" 189 KTAS, 11gph LOP. Add a knot or two going ROP @13gph
C4B5 with 80" 2-blade Hartzell BA 193 KTAS, 11gph LOP, same + a knot or two going ROP @13gph

In both cases, if I went further LOP, it would slow about 5 knots...so the above two data points were just slightly LOP, but up high, so safe for the motor (and it stayed cool).

That prop change basically added 3-5 knots. At the same time, F1Boss changed from an MT 3-blade to the same 80" Hartzell, and he gained 12-14 knots...no kiddin'. So what prop it has is an important factor.

I recently did a pre-buy and then some training with the new owner of an HRII with a stock IO-540 and D-twist propeller, and it cruised in the mid to high 170's KTAS up at 10K, similar power settings to what you were using. However, it needed a bit of aerodynamic TLC...as the fit and finish, and the paint...needed a little work. That is the other factor to consider...how clean is the airframe build?

Hopefully some F1 Sport flyers will add data, but hopefully that gives you some data to start with. Good luck with the search and the purchase!

Cheers,
Bob
Click to expand...
I have an early F1 Sport Wing, stock Matituck D4A5, which should be 260HP. At 9000, 2250 RPM, 10GPH (which should be LOP as Bob points out above), I typically see 175-180KTAS depending on temperature. I have two sources of drag that I've been meaning fix that I just haven't had time to address. At best power I peak at about 205. It's a challenge to nail numbers to less than 5kts unless you are willing to take a lot of data. Turning the mixture knob up makes a big difference.

A bit different way of looking at the numbers: I recently spent some time with Vince in Indiana, and the round trip average (including two intermediate stops on each leg) my average was 170KTAS, 11 gph.
 
It’s a C4B5 stock compression, so assuming max 250 hp. It has a Hartzell 3 blade (HC-13YR-1 RF/F7693FB) and I questioning it in this airframe. I believe static is about right as there is consistent ADSB data that is in the same ballpark and matches the numbers.

Ok, sounds like for this airframe / prop / hp combination cruise is a notch better than our RV7 With arguably better handling and better climb.
 
I think the “250” HP of the C4B5 is due to the max RPM limit of 2550. Spin it higher like a D4A5 and it should regain that lost 10 HP. That said, @2300, both should be the same. Seems slow.

For comparison, my “dirty” HR-2 will routinely do 198 - 202 KTAS at 9500 feet LOP 10-10.5 GPH. Just flew it today and full fuel, 425 pounds of pink flesh and bags I got 203 KTAS @11.5 GPH at 8500 feet. OAT was 70f too. My typical cruise RPM is 2300, as it was today.
 
I only looked at the aircraft in question briefly last time, and happened to spend more time to look at it more closely. I can see where it might have a lot of drag cleanup that needs to be done. All the intersection fairings, wing root fairings and wheel pants have a lot of gaps and discontinuities. There's also a large lip under engine/exhaust exit that is generating a lot of drag. At those speeds I can see that eating 10-15 knots.
 
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