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The best prop will be one with a 4 inch pitch. A 10x4 Master Airscrew would be ideal. The 4 inch pitch lets the engine rev up higher to produce the same amount of thrust, which makes it easier to accurately control the engine RPM needed to maintain controlled flight.

A 40FP running at 1/4 throttle will be easier to precisely control with a 10x4 than a 40FP running at 1/8 throttle with a 9x6. The 10x5 will work in a pinch, but will be harder to learn to control the throttle than with a 10x4.

Again, keep in mind that the PBF with a 40FP is WAY over powered, so accurately controlling the throttle around the 1/4 to 1/3 power mark is critical to your success. If you try to fly the PBF like a "normal" plane and let it go too fast, the wing and/or tail will begin flexing and the plane will quickly get out of control, and probably crash.

You can add a full span piece of 5/32" music wire inside the front flute of the wing to let the PBF fly at a faster speed without flexing, but the stiffer leading edge screws up the locked in stability when flying in harriers or during hard core aerobatics.

The goal of learning to fly the PBF is to learn how to coordinate the throttle and elevator sticks to maintain good control during harrier flight, and the PBF does that very well. Plus it will teach you how to use the rudder during harrier flight. All of these skills learned on a PBF will transfer over to flying a 3D profile later on, and will also improve your overall flying skills even on non-3D planes.
 

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I completely concur with the 40fp/10-4.
Even then i find it has some spool up hesitation with a stock muffler, any exhaust that lets it breathe more really helps with throttle response. However with the stock muffler it is still good.
 

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You're going to break a LOT of props if you're having fun! Buy bulk....
 

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If you can find some of the old genuine Grish or Tornado nylon 10x4 props, they work great on a PBF and will survive far more crashes than the Master Airscrew props. But good luck finding any of those.

Hobby Lobby used to sell an imitation Tornado prop that was made by Kavan. Hobby Lobby sold it as an "indestructible prop". My experience with them was that they were more fragile than wood props.
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Are those old 9x4 and 10x4 nylon props?
 

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Discussion Starter · #96 ·
Dog gone,got to be something better being built than plugs pbf. Show us some balsa being glued. Was thinking of low speed round wheel sander for trimming joints. Something you set on table,small, 4in wheel flat pad,maybe batt powered. Think 1/2a el motor on 4.8 volts. Like a miter box size to sand joint ends. Could make from light ply. Small motor spinning ply backer instead of prop on 4.8v,easy to move around. My dremel batt died bit back,rewired for a old 4.8 batt,slower,but works great,might work ok if glued in a box.
 

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Discussion Starter · #97 ·
To much wind to fly today so test ran os 40 fp. Started right up,closed main needle a bit,ran great,ldle was great,a first for me. Out the box runs that good,wow. Only thing,just how to best to hold pbf while starting,needs a handle or something,a short fin like rudder in front to hold while working on it.
 

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I just lay the PBF on the starting table and use my left hand on top of the wing to pin it down, then use my right hand to start the engine.

You can add a handle to the top of the PBF if you want to, although I haven't personally done it. A cheap kitchen drawer handle from a hardware store will work. Here's an example from the old PBF thread on RCU.
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Discussion Starter · #99 ·
Well got in some flying today. First flight of p bf. To say i was surprised was understatement,it flys very nice. At 1/4 speed i went in big circles,even some rolls and loops. No,no hovers yet,just careful getting use of it,will say have to stay on the sticks,and neighbors 3 year old chatting beside me did not help. What a great engine,the easiest i have ever had,and ten minutes flight with 1/2 a tank left,outstanding. Got to close to ground and bumped at the last,yep,broke prop,nylon at that. lol Thank you McDDD i see how much can be learned from the PBF. Only thing i shall add is handle on top. There be a os 20fp with main needle on back cover,would that also be a good engine for no 2 if i go there. Over all,flys way better than should,no meannest in it,just play with all your fingers ready. Thanks again,bros helping bros,if your new here you need build one,fly this well,shucks,you be ready for most anything. Sparkplug still learning new stuff,64 and still trying to grow up to be a bro. lol
 

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Congrats on the successful test flight of your PBF! It sounds like you did just fine!

It sounds like you are describing the OS 20LA instead of the OS 20FP. The LA has a plastic backplate with the needle valve assembly molded into it. The older FP engines had the needle in the carb with a metal backplate. The newer FP engines had the needle valve assembly mounted on a bracket, which was held in place by two of the bolts that secured the metal backplate. I'm not a fan of the OS LA engines at all, but the OS 20FP with a 9x4 prop would work just fine in the same size PBF.

If you can get a 20FP powered plane down to 2 pounds even, you will find it flies even better than the heavier 40FP version. It won't have the raw power to pull out of and through some hard core maneuvers, but the lighter PBF will simply be more fun to fly.
 
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