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Author Topic: Formula 40 and 21 Sport questions  (Read 924 times)

Offline Ken Deboy

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Formula 40 and 21 Sport questions
« on: April 19, 2007, 11:38:38 AM »
What are typical wing spans/areas for competitive models in these classes? Has anyone built a competitive 21 Sport with fixed LG? WHere can I find pics of some of the planes flying in these 2 classes? Nova Rossi is engine of choice for 21? Run stock or needs rework?

thanks,
Ken
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Offline Bill Hughes

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Re: Formula 40 and 21 Sport questions
« Reply #1 on: April 19, 2007, 02:49:00 PM »
Hi,
Go to the NASS Web site... www.clspeed.com  There is a link to a WebShots page which has a lot of SpeedTimes photos on it...hundreds of pictures...
As far as a .21 Sport Speed with a Wheel...no one has built a competitive model with a wheel. But, any model would be competitive if you bring it to a contest and fly it!
The Nova Rossi RX .21 is the hot engine right now. There are several other alternative engines out there that do not cost as much $$$. The CS is $150.00. The OPS $275.00. Arny Nelson and his Son are getting good speeds out of the OS Max .21 also.
Bill Hughes
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Offline Ron Blombery

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Re: Formula 40 and 21 Sport questions
« Reply #2 on: April 19, 2007, 06:25:15 PM »
I also have a few 21 Sport questions, so maybe this is a good place to raise them:

Mini Pipes
The rules say constant internal diameter.  The exhaust from my engine is 0.5 in ID, and I expect that the optimum pipe ID will be greater than this.  What sort of transition from engine exhaust ID to mini pipe ID is desirable and allowable?

Engine Cooling
I notice from photos I have looked at that there are no cooling air inlets in the engine cowling, and cylinder head fins are exposed.  Is this adequate cooling? What is recommended?

Thanks
Ron
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Offline Les Akre

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Re: Formula 40 and 21 Sport questions
« Reply #3 on: April 20, 2007, 07:12:32 PM »
Hi Ron

 "What sort of transition from engine exhaust ID to mini pipe ID is desirable and allowable?"


None really. You can't taper the pipe towards the exhaust stub so just stick with a 90 degree angle. The 90 degree angle differentiates the header portion of the mini-pipe (where it attaches to the exhaust stub) from the actual pipe. Proper length and diameter of the pipe are far more important anyway.


"I notice from photos I have looked at that there are no cooling air inlets in the engine cowling, and cylinder head fins are exposed.  Is this adequate cooling? What is recommended?"



Most of the 21's are built on a large crankcase relative to their displacement. Consequently they take a bit longer to heat up and run properly than most other engines. The 21's seem to like to run hot, at least mine did.

I did not find engine cooling neccessary, however, you might want to cool the header area of the pipe. That might help "O" ring life a bit.

Regards, Les
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Offline Ron Blombery

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Re: Formula 40 and 21 Sport questions
« Reply #4 on: April 22, 2007, 06:53:04 PM »
Thanks for your response Les - that's very helpful info.

If you could please help with a few more questions about mini-pipes for 21 Sport.... 

What is the preferred method of fixing the pipe to the engine?  I've noticed various springs, clips and screw fittings, but I can't work out how to fit/remove the pipe with the cowl in place.

Is it normal to have a support at the rear (open) end of the pipe?

Thanks again

Ron

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Offline Bobby Poisson

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Re: Formula 40 and 21 Sport questions
« Reply #5 on: April 23, 2007, 03:34:40 PM »
I'm not sure what the preferred method is...but Barry Tippet's idea which my dad and I have both used on our .21 sports has worked fine for us.

The mini pipe ends up between around 1/2" to 1-1/2" behind the hold-down bolts that go thru the wing/crutch assembly. So if you put some brass tubing around the threaded part of the bolt just long enough to make the head of one of the bolts stick up above the hole slightly, you can bend a couple of "u"s at each end of a spring. One of the u's goes around the end of the pipe and the other goes around the threaded part of your bolt, under the head. This keeps some pressure on the pipe and keeps it tight against the exhaust stub.

Finding the right spring tension/length is a must. Too short and the pipe will be pulled sideways...too long and it will vibrate around inside the cowl
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Offline Les Akre

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Re: Formula 40 and 21 Sport questions
« Reply #6 on: April 23, 2007, 04:47:07 PM »
Hi Ron

Securing the pipe at the back is certainly the easiest.

The best retaining unit I've seen, (and one that doesn't damage the pipe), was a piece of Delrin that was slightly curved to match the pipe diameter, but with a lip that supported the back of the pipe and prevented it from moving backwards. This retainer was held by a counertsunk screw into a threaded insert in the top of the wing. Might have even been the same hole that the bellcrank bolted to.

Les
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Offline Ron Blombery

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Re: Formula 40 and 21 Sport questions
« Reply #7 on: April 23, 2007, 08:09:15 PM »
Now why didn't I think of that???

Thanks Bobby and Les for your excellent suggestions.

Ron
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