Flight report-Spring practice at the Zambelli Aerodrome-Taming a runaway fp.40

[RknRusty]

Windows of opportunity for flying have been precious few lately, with the Spring winds blowing every day. But with the season opener in our neck of the woods coming up in three weeks, it's time to practice some patterns and make sure the planes are in tune. Besides if I was afraid of the wind, I'd have no business entering contests. I've been flying a Cardinal pretty well this Winter, and can fly any of the stunts very well at one time or another. Stringing them together is another story. I suspect that's true of most stunt pilots in my Intermediate rank. I've been Int. for longer than I'd hoped, but I enjoy plugging away at it. I hope to someday Belong in Advanced. I might have seen a pilot or two in Adv, that look to me like they belong in Int. Lol... but I'm not a judge.

So Bob mowed a circle and we flew some planes. We flew in survival mode. The windsock at his neighborhood runway was rigid, standing straight out. Bob's buddy, and my recent acquaintance Jim Stoia, both aviators estimated 10-15 with 20 mph gusts. It was clean coming across the field, and slamming a row of trees on the other side. That made for a lot of excitement, a really close call, and one incident. I've never had a solid wind at my back and had the plane come in on me at the top of a loop like the Cardinal did once. Another time my Oriental was climbing into a wingover and the plane was swatted off course so bad I ran and finally caught it a few feet off the ground. We both had flights like that for most of the day. I aborted maneuvers I wouldn't have in a contest. Bob flies so fast and freestyle, I wouldn't know if he aborted anything or not. Jim watched. Being a recent CL retread, he was reluctant to risk his plane. Can't blame him after the show I'd put on, flying, running and dancing.

My Cardinal and its LA .46 are on song and ready to go. But my Oriental's fp.40 has been giving me fits since last Fall with runaway screaming lean runs that usually start while pulling up into the OH8s. I've added head shims, flown without the cowl to keep it cool, but can't tame it. It always ran great till one day it didn't. So a couple of weeks ago I gave Bob my spare .40 head to hemi out. This engine has always had really high compression, so much I can't usually hand start it. It's been finicky about priming and cranking from day one. The reworked head he handed me looked like a soup bowl compared to the original small combustion chamber. I realized what little a shim or three really does compared to opening it up like this. For the sake of comparison I put up a flight with the stock head and three shims. Sure enough it went lean right on schedule and worse than usual, so bad that it sagged any time I tried to climb overhead. And I flew and flew and flew flat laps.

After it cooled, we bolted the new head on with two shims(I think). I short-tanked it by an ounce because I'd turned way too many circles on that last flight. Off she went with a nice throaty 4 stroke, but good speed and felt like it had enough authority to climb into the wingover. As soon as I turned up, it broke into a clean two and powered overhead, breaking back into a 4-stroke on the downside. It turned on and off like Swiss clockwork on every stunt I flew for the whole flight. The only other engine I've had that sounded that pretty in a traditional old fashioned stunt run is my Max-S.35. And to add serendipity to the whole thing, it's more fuel efficient. I used to need all but a half ounce of the biggest tank I can cram in it to have any laps after the clover. Like I mentioned, I short-tanked it with only 4 ounces, and had plenty of laps left over. So for anyone with an unruly fp.40, I highly recommend opening up that combustion chamber. It did the trick for me. I wish I'd taken a picture of it.

So I'll call this day a success. Had a lot of fun, and mission accomplished, I have two strong planes ready for Huntersville, and hopefully a couple more days of practice between now and then... and try not destroy any of them between now and then.

I have some video crunching that I'll post later.





This is Bob's Icarus Jr. It has, of course, a very old badass engine. A rear induction dual BB engine, and the fuselage has air scoops on the side. I forget the vintage and name. Hopefully he'll drop in and tell us. It may be from the late 1940s, but that's close to the beginning of glow ignition, so I'm not sure.

[Geoff Goodworth]

Rather than mess about with sending Bob heads from Australia, he sent me the drawing and I had some Tower heads machined out. I have one on an otherwise unmodified ABN FP 40—6.75mm venturi and OS NVA—with only one head gasket, powering a Brodak ARF Cardinal.

It draws positive comments about the way it runs every time I take it out.

[RknRusty]

Rather than mess about with sending Bob heads from Australia, he sent me the drawing and I had some Tower heads machined out. I have one on an otherwise unmodified ABN FP 40—6.75mm venturi and OS NVA—with only one head gasket, powering a Brodak ARF Cardinal.

It draws positive comments about the way it runs every time I take it out.

I bet it does. Mine is ABN too, thank goodness. If it had been steel I would have cooked it by now with those screaming lean 15 minute runs. I measured my venturi at 6.44mm with the stock NV. That's with my new $10 Chinese caliper.

[Robert Zambelli]

I've been experimenting with FP combustion chamber configurations for quite a while and I feel that I've optimized the design.
I found it interesting how small changes in shape made noticeable differences in performance. What really surprised me was how angling the squish band upward improved the four-two-four break.
I've done this mod on quite a few Schnuerle ported engines, all with favorable results.

Regarding the Icarus Jr, it is, as I recall, a late 40s/early 50s design by John Coasby from Great Britain.
Mine is powered as was the original by an ETA 29 rear rotor speed engine.
It turns a Top Flite 10-6 wooden prop at around 11,000 RPM on 10-22 50/50 fuel.
I feel that in the right hands, this combination could be a top competitor in Old Time Stunt.
It does fly fast - around 80 MPH.
Bob Z.

[Scott Richlen]

Bob:

On the FP-40 head modifications, what do you attribute the better fuel efficiency to?  In your experimenting, what did you find to be the worst things to do, and what were the best things to do relative to head modification?

Would you please post the drawings for the benefit of the rest of us who have FP-40s and want to play around with them?  And how did you actually make the mods - Dremel, mini-lathe,?  Any cautions?

Thanks,
Scott

[RknRusty]

Scott, I noted better fuel efficiency based on just one flight with the modified head, so that's not really empirical evidence. I'll be flying it some more if the wind ever drops below 15. I would have actually guessed the opposite if I hadn't had that one flight. And it used to drink a whole 4 ounces by the middle of the V8 when running in a similar 4-2 fashion, which was launching at 10.8K. We did not tach it Sunday, but by ear it was pretty close to that. Bob never mentioned efficiency when we talked about the mod.
Rusty

[Robert Zambelli]

Scott - and anyone else interested.
I'm away from home now but I'll be back soon.
Glad to share any info on the head modification.

I use an old South Bend bench lathe with adjustable angle on the cross feed (IMPORTANT).

When I return, I'll post some sketches of the mod.

Bob Z.

[Robert Zambelli]

Quite a few people have called me and asked about the modification.
Here is a capsule summary.
The sketch below shows the mod after a few years of experimenting.
The photo shows the cutting tool that worked the best. I started with a .500 radius and just brought it down a bit.

Starting at the glow plug threads, gently enlarge the combustion chamber until the squish band is .035 to .040 wide.
Then cut the 5 to 10 degree taper as shown.
While the head is spinning in the lathe, use a small piece if fine sandpaper to round off the edge where the taper meets the combustion chamber. I'm not sure why but this made a very noticeable difference in the break. I remember from studies in fluid dynamics that a sharp-edge orifice can and usually does create turbulence. Maybe the round-off creates a smoother flow. In any case, it works.

I have performed this mod on both Schnurle ported and loop-scavenged engines, mostly with good results.
I recently did an HB 40 (loop scavenged) and was amazed at how well it performed.

Regarding fuel economy, I'm not sure exactly why it improves but it does.
When I applied this mod to an LA 46, not only did it run in a more classical stunt run but it ran nearly a minute longer on a 4 1/2 ounce tank.

When comparing the two heads while bench running the FP 40, the bypass was around 20 degrees cooler with the modified head, as measured with a thermocouple.
Possibly the improved fuel economy was due to less energy wasted on heat loss???

Something else to think about: As I experimented with smaller engines, the improvement was not as noticeable as on the larger ones.
I did a Brodak 15 and an LA 15 - minimal improvement.

The LA and FP 40s showed the most improvement.

I hope this information helps - it certainly worked for me.
My next project is a Super Tiger 35 Stunt (baffle piston)

Bob Z.

ps - a number of people asked if I had modified Fox 35s, OS Max S 35s or Brodak 40s.
Why bother? They run just fine STOCK!

[RknRusty]

......ps - a number of people asked if I had modified Fox 35s, OS Max S 35s or Brodak 40s.
Why bother? They run just fine STOCK!

No s#!t, my stock Max-S .35 is the best surprise I ever blew the dust off off and tried, after you outfitted it with a venturi. Makes me want to throw my only Fox away(Yak-9) and replace it with a Max-S.
Rusty

[Scott Richlen]

Thanks for posting this info Bob!  I have a couple of FP-40s that I want to experiment with and happily have a few club members who like to play with their metal lathes.  But, I wish I had a neighbor with a machine shop - wouldn't that be fun!

Scott