Broadway Bod Busters

Combat props

By Ken Burdick

Since the first day it became available, the Nelson .36 changed Fast Combat forever.

Gone were the days of reworking engines, buying after market parts that were stronger and faster. Gone were the "good old days" of tailoring the airplane to the engine's power output. We have never really explored the true output of this engine and for good reason; it's difficult to fly combat that fast. Henry solved all of the problems we used to face.

All this history is fine and good, except we have lost something in all of it. Power and speed are not much of a consideration anymore, at least in Fast Combat.

What happens if you fly another combat event and you want more speed?

D-bat, F2D or 1/2A -- how do you get more bang for the buck?

You can't add nitro to the D-bat, and the 1/2A's already use a little more than people like nowadays so where can I get mo-go from my engine? The answer is what the speed guys have always known: the prop. How many times have you heard, "experiment with different props"? I think it has been printed on every set of combat plans that ever came with a kit. I'm not going to pull out the stops and talk about how the little finger cutters should be shaped and which ones are best for what engine. That is the fun of modeling. What I am going to do is touch on the basics so you may better understand some of the important features that can gain you more thrust. Don't hop up the engine, hop up the prop instead.

By understanding what you are using, and when you make a change you will know what has happened to make your wing go faster, turn better in the corners and best of all, pull that streamer more consistently through the turns.

There is a link to a chart from Iskander Taib on the Yahoo combat group files page that shows a typical F2D prop. (Editor's note: This is an xls document that works only on PC computers. We hope to get a pdf to upload here shortly.) Notice that the stations on each blade are measured by a pitch gauge. Prather Products still makes them for about $90.00

"But I fly Combat..not speed or racing!!!! I don't want to spend the $$$$"

Right, you don't need to but it is money well spent if you want to get a formula for a particular prop.

Most props we use are molded and work well as is, if however you want a lighter and stronger prop to reduce the rotating mass, then you will likely end up with a hand molded glass/ polyurethane prop made in the Ukraine. In general, the work is good and they perform well, but they are made with as few fibers in them as possible perhaps to keep the cost down. The downside to this is that it can causes cracks splits and in some cases the blades to shed while running. The upside is the blades are thinner and can be more efficient. While the props are well made, we have found that there are gains to be made here as well.

So our choice is a glass/nylon molded prop from a hobby shop, a hand made prop from Russia or the old standard wooden prop. The wooden props are at the root of the hand made ones and have been carefully shaped into the plug for the first generation molds. Light weight is a wood props only advantage and as rpm's creep up in the performance engines, you see less and less of them used. A well balanced and "tweaked" Rev-Up 8x6 is still one of the highest performers in Fast Combat. They can still offer some advantage in speed limit combat.

Knowing what you have is most important, so you will need to measure it and compare the various stations along the length of each blade.

Have a look at the props and associated data on Isky's chart. You will see some minor differences between blade one to blade 2. Realize that the stations are marked with a felt tip marker but are measured accurately. Each station shows pitch that is recorder by a pitch gauge. There are a variety of props measured here, all are F2D props but gives you the idea of the criteria used to judge a prop by.

Some props blades are way off while others are nearly the same, it just depends on how well the plug and mold were made.

You can trim blade length, reshape your prop to make all stations equal, balance it, thin the blades so they are the same thickness and on and on. Reshaping can be difficult if the prop is thin like the one shown.

By now most of us have decided to just bolt on the prop and forget it, I agree.

I have however outlined a quickie list of things that will give you some improvement and you do not have to spend all day reworking your prop of choice.

1. Make the blades equal length.
2. Balance it.
3. Check blade tracking

"What?" "Tracking!!!?"

The problem is related to one blade having more pitch than the other. A simple triangle template can tell if one blade is more than the other.  If so, the hub can be sanded to make both the same.  If you do this you can also check to see that the prop sits flat on a flat surface. If not that needs to be tweaked as well by sanding the hub surface a bit.

I use a fixture to check for tracking.

1. Bench vise
2. Engine mount
3. Dial indicator
4. Holding fixture for dial indicator.

Mount the engine into a mount that will hold it and not move. Put this in the vise and clamp it. Next put on a prop to be checked and tighten it. You will need to make a fixture to hold the dial indicator so it can not move but can be adjusted closer to or further from the engine in the set-up. I use a hobby vise to hold it and move it into position until I get a reading then clamp it to the bench at that spot.

Next get a reading by rotating the prop until the front of the blade bumps the dial indicator at your determined station. (Record the reading) Now rotate the engine so blade 2 bumps the dial indicator. Compare the two readings and determine which way the prop is tipped, remember to account for any thickness variations between blades.

Use a fine tip marker, mark where you will need to take a swipe or two off of the flat side of the hub that mounts against the thrust washer. Remove the prop, make one or two light cuts with a fine file and reassemble. Be sure to mark the engine so you can align prop where it was. Check the readings again. When you have it close .005 you are about there. (The flat surface and triangle is much faster)

Props are not easy, but if you want more power from your engine these are the things you must do. Combat flyers are notorious for taking short cuts due to the nature of their event, and as a group are much lazier than speed, racing or stunt flyers. The time spent for improvements as shown in the quickie, will cost you about 3-5 minutes per prop but will add performance.

Here is a typical combat flier's response to checking props as pulled from an F2D site:

"I never even check my shipments of props. I have a really good prop and stick to it. I throw it on the engine and go fly, never check pitch or balance or nuthin'. Never had any reason to."

Here is a reply to this by a combat flyer who actually tries new things"

"Well, you might want to start checking the props. I've only been messing around with the Seryogin (Moscow #1??). I can't afford to buy dozens of a dozen different props, and all the other ones I've tried haven't shown any appreciable improvement. I had to trim them down a few millimeters to get 15 Top to run well. One particular prop was half a second faster in level flight and pulled half a second faster in tight maneuvers. A second batch, from a different source, none of the props came any closer, even though they looked identical. Only difference I could measure, the better prop was about .010 thicker 30 mm in from the tip.

With the motors all being pretty darn close in power, the only place left to get anything is picking the right prop/head clearance/head shape to match the plane and the weather. Or figure out what makes the good AKM's produce 15-20% more power with a much wider useable power band."

This is an F2D post, but the same applies to all events including those nasty diesels.

We'll go over how to measure head clearance in a future article and correlate it to weather conditions.

Good hunting.

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This page was upated March 11, 2008