Skyray

All of which shows what I and others have been preaching for a long time: Using larger engines does not mean you have to go faster. You control lap speed with rpm and pitch. In fact, your lap speeds with your 40 on those 62 foot lines are lower than most are going with an FP 20 aboard.

And, as to weight, with a tongue muffler, your engine weighs 8.5 ounces. The 20 FP with the stock muffler that some are using weighs...you guessed it!

OK, you have more fuel aboard with the 40. But the critical points in the pattern are at the very end of the flight when you have burned the fuel off. It's a wash.

And with the 40, you can even use a little bigger prop, if you want to. Pat Johnston has been experimenting with the 11.75 x 4 Bolly and the 11.5 x 4 APC on my 35 FP conversion, 2 props I never tried with it. He reports the same 5 second laps on 60 foot lines and a nice 4-2 break.

Now, if you would just lengthen the lines a little bit...

The problem with the "control with rpm and pitch" theory is that while it's sound in principal, the 40FP is not known for it's flexibility or controllability. In fact, the entire rework industry sprung up to try to tame it's generally difficult characteristics. To fly a stunt plane at a reasonable speed with available (4" pitch) props, the engine will have to turn anywhere from 10500 to 12000 in the air (depending on the diameter and the airplane). The 40FP wants to run 14000+ with 10-11" props, meaning you're going to be running 3.5 second laps. You can't usually needle it down, that just makes it run away more abruptly.

The likely solutions to these issues are also problematic. The first guess, lower pitch, is possible if you get a $37 prop and depitch it to 2-2.5". This is way beyond the the call of duty for the likely target market for 40FP's. Another possible solution would be to reduce the diameter, which would hypothetically reduce the speed by decreasing the advance ratio. Unfortunately, the reduced diameter just lets it spin faster still, obviating the speed reduction. Adding a filter or adding head gaskets would also help, but not enough most of the time, and then you start getting into modifications.

I've seen this scenario played out dozens of times. MrWhizkers has had a unusual experience. Most of the time, it just doesn't work.

The only other solution is to give up the idea of using a stock motor, and get it reworked. This will work, if you get a reputable reworker (such as Leonard, Randy, or Big Art, etc.). While I have no problem with reworking in general, I do find it hard to believe that I'm going to tell every beginnner I see to go out and buy modified motors as their first act. Additionally, a whole lot of people try doing it themselves, which very occasionally works, but usually just results in even more problems.

Compare this to the 20FP, which bolts in and runs with available props, fuel, etc, and is a completely turn-key operation. Additionally, having done it and flown them both ways, I think the airplane flies a lot better with a 9-4 at 5.0 second laps than it does with a 12-6 at 5.0, in almost all respects. The 9-4 is just a better prop for the airplane.

Asking a beginner to try to figure out how to dump the excess, unusable power* from the 40FP is just not likely to work in most cases. The 20 already runs right. That's why I discourage anyone, and particularly beginners, from trying a stock 40FP on "35-sized" airplanes.

Brett

* note power and speed are inextricably linked - the power the airplane absorbs is proportional to the cube of the velocity. If the engine is putting out more power than the airplane needs at the current speed, it will speed up until they balance out. The only variable is the efficiency of the propellor, which is poor for model airplane props. Larger props tend to be slightly more efficient, meaning the larger the prop, the less power the engine is developing at a particular RPM and airspeed.

To fly a Skyray at 55 mph only takes so much power. Any more that the engine develops has to be dumped somehow via modification or reduced prop efficiency. Needling it is a fine-tuning adjustment, but won't get rid of 50-75% of the power without incurring other problems.

I would also note that most stunt fliers equate power with either line tension or prop diameter. This is not technically valid, and while the effect tht people feel is real, the use of the word power to describe it is not accurate.

We have bult a few Skyrays and have used fox 25 baffle piston motors on them, and they fly great. An APC 10x4 is about the best, once the motor is broke in, but a 10x6 is too much. 52 or 60 x 015 lines, depending on the wind are no problem. I can't do the hourglass yet, but it has enough power a vertical 8 just fine, ( well I don't do that one well yet)

Now I am building 2 more (scratch) and will try a fox 19 baffle motor for my kids. I would expect no problem if i keep it light.

Even with this setup, it was nose heavy, so with the 40 and 4 oz of fuel, I would expect tail weigh is needed.

If all I had was the FP 40, I would have used it. But I had a 25 and got another for almost nothing. They start easy, and run a nice even rich 2 stroke and 2.5 oz is more than enough.

I'd like to build something 5% larger like a skyray just for the FP. Any ideas?

Disclaimer: Just a sport flier, but will fly a contest or 2 next year.

This whole engine vs prop vs airplane size is still a big black hole for me. I used to just put the largest motor in the airplane and would always get rock on a string, but this combo flies very well.

How big a flat prop will an FP40 turn in an application like this?

Good Luck and let us know how it turns out.

Go ahead and use the 25 with the rc carb (aka adjustable venturi). It also will give you a way to tune down the power, if necessary. The amount of tailweight that you MIGHT need to balance off the rc carb (instead of a lighter venturi setup) is pretty small.

To hold the carb setting, I simply used a cut-off rc servo arm, using one of the engine mounting bolts (okay use a washer to help distribute the load). Attach a standard rc pushrod setup between the carb and the arm, and away you go.