I have a 40-oz Primary Force ARF that I'm having trouble getting to turn briskly enough (Brodak 40) even with the CG at 25% and 45 degrees elevator travel.  I have a Shark 402 that is twitchy with the CG at 12% and 20 degrees travel.  The Skyrays and Flite Streaks I have flown have been similar to the Shark 402.  All these planes turned just fine, within the realistic limits of flapless planes.  What CG and how much elevator are others using with their PF ARFs?  Thanks.

Just add some weight to the tail and fly it.  Don't go over board with the weight.  If it helps add a little more until you get it where you like it.  Mine was built from the short kit Mike used to have.  Power is LA .25 with needle valve assembly moved to the venturi.  Am now flying with APC 10-4 prop and 10% fuel.  Seems this LA .25 likes the prop.   

Serge,

Is turbulating the tail different from using a sharper LE?  Such as adding turbulator threads?  And if so, where is itbest to add them?  I will try taping on a sharper cardboard LE this weekend. 

After putting several engines on the scale, I discovered to my surprise that the LA25, FP20, and Brodak 40 all weigh within spittin' distance of the same (w/o mufflers).  I tried the Brodak and have been pleased with the results.  One goal with this plane is to see if I can replace the higher pitch sound of the smaller engines running fast with a more pleasant-sounding 4-stroke (either with or without a break).  This is of course an "aesthetic" preference rather than a technical thing.  After trying several props, I'm now running an APC 10x6, 9.2K at takeoff, 4.8 sec lap, and the Brodak (late model) produces plenty of ponies to do the trick.  It has quite a hard break, which I am working on softening.  Originally I tried a 6 oz Hayes tank (because the Brodak needs more than 4 oz, even with 4 layers of nylon stocking over the venturi) and had considerable RPM increase as the tank empties, a problem which does not affect my friends running 3 oz Hayes on FP20s and LA25s, whether mounted outboard or inboard.  Go figure.  I tried switching to my trusty GRW chicken hopper and it works great.

Another factor is that with the stock ARF bellcrank setup, I have 45 degrees of deflection with the pushrod in the outermost horn hole and a fairly narrow handle spacing (actually it's an in-flight adjustable combat handle with no spacing adjustment that I like for initial flights to get a good neutral adjustment quickly in the air).  Next step is to install a longer horn and switch to a hard point handle with the wider spacing I prefer.  I started a long discussion some time back about the feel of a handle as it relates to line spacing and the resulting confidence (or lack of it) that the plane will make it thru the coffin corners (another aesthetic thingie).  I suspect the elevators are acting as airbrakes in hard corners with the current setup due to excessive "insurance" input on the handle.  Going to wider handle spacing may seem counterintuitive to some, but it's kinda hard to explain the feel thing.  Will have to try it to see.  The hard point connection will no doubt help too.     

I've learned a lot and had much enjoyment in the process of trimming this plane.

I will be interested to hear how your rebuilt PForce flies when you get it in the air.

Kim

Hi Kim,
The C.G. for the P-Force should be close to the 2” back from the leading edge of the wing, measured at the fuselage.  Going any further back can make the P-Force a little jumpy at times.  At forty ounces, yours shouldn’t have much on an effect on its turn rate at all.  I added 6 ounces of weight to the C.G. of the prototype P-Force without greatly affecting the turn rate of the model.
In your post, you mentioned that you have a full 45° of elevator deflection, if indeed you are using all of that deflection, then your handle spacing is way too much for the model.  The handle spacing should closer to 3-3/4” for the best all around performance.
The Brodak .40 is a good choice for the P-Force.  It’s lightweight and power should make good combo for the P-Force in most situations.  My thinking is that you have too much pitch on the prop and causing speed to go up when the engine breaks into a 2-cycle.  Switch to a 4 pitch to control the speed and the lap times of the model.  One of the best flying P-Force I ever flew was built by Jim Hoffman.  It was powered by a Randy Smith AeroTiger 36 and he was almost unbeatable with it (just ask the guy’s in the southwest).
Later,
Mikey