Getting Started in GSR, A Rookie's Perspective

[GR-7 Driver]

See...

[joeraf]

The cost of my polecat came in at $3800 not including ground equipment. Alcohol conversion is about $500 more including the prop. Most alcohol engines run 10 to 30% nitro with 6% oil content. The GT engines run low octane gas with 25 to 1 oil mix.

[RobertV]

Potential Flow Joe,
Your present and ongoing thread has great value in that you are telling everyone exactly what you personally went through to get going in this awesome sport. Most people may recognize your name as a top level turbine jet and scale modeler. So you entered into this with a bunch of building and flying experience. What you describe here exposes both the good and bad aspects of the current infastructure of this rather obscure facet of the hobby and thus will provide a great path to help others avoid some of the potential pitfalls and get in the air faster. It may also help the industry understand what is needed to help it all grow. Keep up the great work!

	Quote:
	The Polecat is a very fast airplane

I called for Joe in the F-1 alky class at Ashtbula and, when his motor was running right (the biggest learning curve so far) he could run with the big boys in his heat. Heck yeah it's a fast airframe....real good aero, Fred!

	Quote:
	In F-1 proper...(my bias is showing)...the RnR Polecat is a very good choice, and it's the only airframe other than the GR that has won in a few years. I think you will see the AR-6 join that club soon.

Kenny, the AR-6 storm is brewing.........

[KennyMac]

Most of the kits for the F-1 class are fully composite fuse,wing, stab, and are skin hinged. That reduces the work for putting one together but raises the price. The composite kits run between $2200 and $2900. You can still get kits from Ed Rankin and possibly Tom Easterday that are glass fuse with foam core wings and stab that are cheaper...I think around $1200 or slightly less. The tradeoff is that they require more time and work to build.

The engines for the F-1GT class are stock Zenoah GT-80s, and you can usually find them on sale for about $700. A used one is not a bad option either because they run better as you get time on them. It might need a ring or two, but it could be a good runner. If you go to F-1 then you'll need some engine work done and a different induction and fuel system. That's about $350 or so.

Servos are nothing too exotic...There are USRA specs for the power required for each flying surface and we usually use something around 100 in/oz per surface. The JR 4721, or even the 4131 work well. Some use digitals like the 8411.

I don't want to steal any of Joe Raf's thunder so I'll stop there, but If you go to the F-1 GT class, you can build a reasonably priced racer.

Take it away Joe!

KMac

[KennyMac]

Robert.....Stop teasing us with your AR-6!

I don't think Fred did the aero on the RnR Polecat wing, but he certainly helped fix some stab size related issues that early versions of the airplane had.


KMac

[joeraf]

Formula one allows a maximum engine displacement of 80 cc. The GT class allows only one type of engine, the Zenoah GT-80, hence the class name. These rules make it easy to switch classes by modifying the GT-80 to F1 alcohol using the same airframe.


At first the GT-80 engine seems somewhat pedestrian compared to other more exotic engines. But the point of the GT-80 class is to make the airframe and pilot skill the determining factor. Think of it as restrictor plate racing and you get the idea. The engine is rugged, reliable and the strongest running engines are those that have been broken in properly and had lots of gallons of gas through them.
It's a fact that Robert Vess who undoubtedly had one of the best running engines in the first two races this year, did nothing but slap the lump on his Edge 540, breaking in the engine by running Lawn Boy ashless oil in his fuel, and flying endless laps around his field.
No modifications are allowed to GT engines except removing the starter spring.
The engine is widely available at many hobby shops and internet outlets including Horizon Hobbies.

The standard GT-80 unmodified


SleepyC pic


There are a couple of engine experts who modify the GT to alcohol. Fred French and Ralph Cunningham have contact information on the USRA site.

A Fred French modified engine. Note the twin intake sliders and stacks. The cylinders are somewhat easier to balance with this setup. The engine cylinders are nestled in the close fitting cooling ducts. Air exits aft of the firewall below the cheek fairings.




GT props are available through APC or the USRA. If you buy these props through APC be sure to specify GT racing props. APC has a F1 prop that is not the same as the GT.


F1 props are available through Fred Sattler and again can be contacted through info on the USRA site.
Please note that GT props should never be used on F1 alcohol engines. The GT props are not stressed to take the higher rpms of the alcohol engines and may come apart.


Cooling ducts did not come with this kit. These are extremely important in properly regulating engine temperatures. Most of the painfull engineering work has been done by people like Ed Rankin and the ducts are available from him.

The gas engine needs a positive shutoff both external and radio controlled. The GT engine is hard to kill using the carburetor low idle. A good solution is to use a kill module such as the rc 100 switch from RCATS http://rcatsystems.com/electronics/rc100.php. The external switch can be a simple slider wired to the rc kill switch.

[joeraf]

I use Tru Turn spinners. http://www.tru-turn.com/ Tru Turn can custom cut the necessary prop slots. The spinner has to be able to stand up to electric starters.


Most of the other necessary hardware is common to the GS crowd. Rocket city/ Nelson control surface links, heavy duty servo arms, dubro 4.5” lite wheels, 1” tailwheel.


There are minimum specs for radio equipment outlined in the common rules section on the USRA site.
I used JR 8411 servos on all control surfaces, JR r2000 receiver, and Fromeco Peerless LIon battery with their reliaswitch regulator http://www.fromeco.org/.


Weight control is a constant concern in the F1 and GT classes. The closer you can get to the minimum dry weights, 25lbs F1 and 27lbs GT, the more competative the plane will be. I started out with a passing interest in building a light airframe and by the end of the project became as obsessed as any F3a pattern competitor. I eventually bought a digital gram scale and weighed everything that went on that plane.


Since I had to fabricate the firewall and formers and with weight savings in mind I decided to use composite materials. I used end grain balsa / carbon laminate for the firewall and mid fuse formers and nomex / fiberglass for the aft former and equipment tray. These materials are available from Aerospace Composites www.acp-composites.com or Composite Structures Technology www.cstsales.com

Next The Build

[KennyMac]

Hey Joe....

It looks like you are using the Eagle tree telemetry/datalogger system. Would you mind commenting on what you think of it. In particular, I'm interested in the cylinder temp sensors that they provide. I use an old AeroSport datalogger that has thermocouple wires that we attach to the top of the cylinder. I'm curious how the wraparound sensors on the eagle tree work.

Thanks and keep up the great thread.

KMac

[joeraf]

Kenny, I am becoming very attached to the Eagle Tree. Almost feel naked if I can't download the data after a race.
The cylinder head temp sensors work fine for what I think they should be used for. I would not trust them to provide absolute temperature values to judge an overheated head. However I do think they are great in judging relative values between the two cylinders. Very handy when balancing the cooling ducts.

I also use EGT sensors mounted in the header flanges to more accurately measure cylinder temps.

The Eagle Tree system comes with some very nice graphing software



I cut off the Rpm data to zoom on the cylinder temps and EGT's.

I had some problems with the telemetry system early on. The telemetry transmitter interferred with the digital servos mounted near it. When I moved the transmitter away from the servos the jitter stopped. This did not effect the analog throttle servos. Eagle Tree replaced the tansmitter and the problem went away.

[joeraf]

This isn't meant to be a full construction article. I'll only hit the points that I found somewhat different from normal builds.


I aligned and mounted the cowl first. This results in a solid nose ring and gives a reference point for the engine mount and firewall.


The Zenoah engine has an integral mount. It is a compact package that needed standoffs to properly position the firewall. I made the standoffs from 7/8” round aluminum bar stock. I simply cut the stock into 3/8” sections and drilled the centers to 1/4”.


The mount and stand offs shown. The engine has the longest prop shaft available from Zenoah. The total extra length places the firewall properly.





At the first GSR race that I attended as a helper, the two planes that I helped prepare showed signs of spinner ring rubbing in the lower quadrant. While the spinner ring gap was not excessively close, and the engines were firmly mounted the pilots concluded that the firewalls were moving enough to allow this to happen. This is understandable as the planes pull around 20 g in the turns. Whats worse is that the planes' turn rates are on the order of 2 to 3 seconds per 180 degrees. To take this kind of hammering the firewall has to be very rigid. I chose to use 2 layers of 1/4” end grain balsa carbon fiber laminate. I used 1/4” depron foam to make the template. I used 3/4” hardwood dowels glued into the firewall to prevent the end-grain from crushing when tightening the engine mount bolts. I also used fender washers glued front and back to cap the hardwood dowels.


I made a mistake in assuming that the nose ring was molded to set the thrust line correctly. After completely assembling the plane I did some final checks with a digital incidence meter and found 1 ½ degrees of up thrust. Correction required re-cutting the standoffs to differing lengths to account for the firewall misalignment and re-fairing the nose ring. Next time I will establish the fuselage waterline first and work from this reference point.

[joeraf]

After adding the mid fuselage formers I mounted the wing. The polecat being a mid-wing design requires a heavy duty hold down system. It has to be capable of supporting at least 540 lbs (20gx27lb) over many cycles. I had to figure out where the hold down blocks were placed in the wing first. Using a straight pin I started poking around through the wing skin where I thought the blocks should be placed. After many pokes, I was able to locate the center of the blocks. The fuselage hold downs were made from 1/4” ply stretched across the wing saddle and supported by false formers. Fiberglass cloth was then attached to these assemblies for added strength. I used 1/4-20 socket bolts and blind nuts to complete the hold down.

[joeraf]

The horizontal tail on this kit was unusual in that it was not a one piece assembly. The halves joined using 5/8 and 3/8 aluminum tubing. While mounting the tail was easy I think that it is excessively heavy.





The rudder also came out heavy mainly through my overbuilding the support structure and tail wheel mount. Steerable tail wheels are required and I hollowed the bottom of the rudder and fit a 1” wheel.





My paint job could have been lighter. The kit came with a white gel coat and only needed seam treatment. I used a gray primer on the seams and this in turn required a heavy white coat to cover the primer. The paint was PPG base coat-clear coat and 4 colors. I think a lighter alternative would have been to use a white primer and single stage paint system, no clear coat. Visibility and orientation is not an issue during a race.

[bodywerks]

	Quote: Originally Posted by KennyMac
	Most of the kits for the F-1 class are fully composite fuse,wing, stab, and are skin hinged. That reduces the work for putting one together but raises the price. The composite kits run between $2200 and $2900. You can still get kits from Ed Rankin and possibly Tom Easterday that are glass fuse with foam core wings and stab that are cheaper...I think around $1200 or slightly less. The tradeoff is that they require more time and work to build. The engines for the F-1GT class are stock Zenoah GT-80s, and you can usually find them on sale for about $700. A used one is not a bad option either because they run better as you get time on them. It might need a ring or two, but it could be a good runner. If you go to F-1 then you'll need some engine work done and a different induction and fuel system. That's about $350 or so. Servos are nothing too exotic...There are USRA specs for the power required for each flying surface and we usually use something around 100 in/oz per surface. The JR 4721, or even the 4131 work well. Some use digitals like the 8411. I don't want to steal any of Joe Raf's thunder so I'll stop there, but If you go to the F-1 GT class, you can build a reasonably priced racer. Take it away Joe! KMac

Well, it sounds like F-1GT is the place to start. I mean racin' is racin', right? What's the average speed in this class? 140-150?
Then, since the whole goal of racing is to win. I assume that if you meet your goal often you might be attractive to some race teams that can help you move up in class and such if you joint their team, right?

[bodywerks]

WOW, Joeraf! Thanks for taking the time to share this with us - very informative!
I do have a few questions, though.
1) What is the purpose of that black vent duct behind the two velocity stacks? I see that it goes into the fuse, but don't see where it ends.
2) I get the idea of the ignition kill, but I assume that is only for the GT class, asn the alcohol class uses glow plugs, right? Sow how do you kill them? I noticed a couple of rotary valves linked to a servo. I assume you can completely cut off fuel flow for a suitable engine kill? That brings me to a two-part question:
3) I see that, in the alcohol class, the carbs are replaced by a venturi, which I completely understand why. But I assume a positive fuel delivery is still desireable (but it is no linger coming from a pump in the carb since there are no carbs!) and you guys probably rely on more than just the low pressure in the venturi to draw the fuel, no? Which means you'd need a pump or a pressurized tank, right? If that is the case, how is the air/fuel ratio maintained at an optimum? I could see how the rotary valve setup might be mixed with the throttle so you can program it's fuel flow for a given throttle position? Or maybe I should just let you school me!

[joeraf]

bodywerks
The gt class is the most popular in GSR. There are a lot of very experienced pilots flying in the class and the reason, I think, is that they enjoy the close racing. A few of these guys also fly in the unlimited class. My opinion is that while GT is considered by some to be an entry level class, the only things "entry" is the gas engine and the lack of a cash prize, which I think is an oversight.
As for teams, the big benefit here is the availability of help and not necessarily sponsorship. You always know that you will have a caller, help starting engines, transportation to races, etc.
This is not to discourage solo efforts though. Most GSR racers are very willing to help new people. It is unlike any other racing community that I know of.