After some reading here on the site, i broke down and bought an FP 20 rc off EBAY.  Folks have really spoken well about this engine, so i figure i would try it. I have built and am finishing a Brodak Vampire. I have an OS Max 35-s Installed, but many folks have said that this it too much engine. Im not sure I totally agree, but I am ready to try good advice and switch to a smaller engine. As the 25 FP weighs similar to the 35s, I didnt want to switch to that engine (I have two of them) Also, the 25fp would seem to be the same power as the 35s, so no real help there.  If the 20 is slightly less power, and thereby appropriate for the vampire, I will think it is a good swap if it weighs less. 

What say yall to that? Does the 20 fp weigh less than the 25 FP? Is this a better engine choice for the vampire than my 35s

Any information would be greatly appreciated.

V/R

Randy

... Also, the 25fp would seem to be the same power as the 35s, so no real help there. ...

Think of modern Schneurle-ported engines as "apples" and 1950's technology baffle-ported engines as "oranges".  Now, ask yourself how valid a comparison of the 35s to the 25FP (a Schneurle-ported engine) is.

The 25FP is way more powerful.

Actually, for sport flying, if you're willing to use all-castor fuel, the 35s may be a good choice, if possibly a tad big.  Based on my experience with Ringmasters, a 20FP is going to be a bit big (because - Schneurle).  If I had one, my next Ringmaster would get a 15LA or a 15FP.

Looking at my FP manual, without muffler the 20 weighs 6.81 ounces and the 25 weighs 6.52 ounces. I have no experience with the 35-s. I have used the FP 25 with stock muffler, 9x4 or 10x4 APC prop on a Ringmaster and Barnstomer.  Neither seemed over powered.

   25FP on a Ringmaster? If the engine is running right, I would expect about 3.5-3.8 secs/lap on 60ish foot lines. Do you have an aftermarket spraybar (ST, probably)?

    Brett

I am using stock OS venturi and needle valve with one layer of hose to keep crap out.

After rereading my reply,  I realize that I should have stopped after giving the different engine weights or should have expanded with my experience/results. So with apologies to Randy for halfa** info and to Brett for reading and agreeing with his advice how to run these engines then completely ignoring that advice (not out of "I know better than Brett", but out of willing ignorance and desperation), I'll relate my experience.
My first 2 flights (I was smart enough to short tank) with the 25 on the Ringmaster was the BBTU way. Using  9x4 APC prop at the recommended 11,000 or so rpm the engine ran great and consistent  thru my meager attempts at the beginner pattern. Brett is right, I was spinning around like a top at 3.5 second laps and couldn't keep up.

I then did what shouldn't be done with a stunt engine and backed the needle off to 9,600 rpm. What happened was that level flight laps where a comfortable 5 seconds. During maneuvers the engine would speed up to the point making it impossible for me to fly same size loops, squares and anything else. The end result of my trying run the 25 the wrong way was inconsistent runs, excessive speed up during maneuvers and a pile of parts and pieces of a Ringmaster and Barnstomer.

   You may well know better than me! I learn a lot from these threads, too.   

   3.5 is right in the ballpark, when we tried the 20FP on Jim Aron's 20-point (at VSC) Ringmaster with a 20FP with the standard setup, it went 3.8 seconds a lap. Note that this was trying to solve a problem with the original Veco 19bb - which went too fast, too, 4.2 seconds a lap! Some solution that was. A 25FP is substantially more powerful than 20FP, so faster.

    In fact - 25/20 = factor of 1.25 (25% bigger). The power ratio between 3.8 second/lap and 3.5 second/lap is (3.8/3.5)^3 (cubed) - which is factor of 1.27!

     It's too bad about the crashed airplanes, but that was sort of the point - either it goes like a bat out of hell, or you needle it down, which works for a while until it takes off again - with whatever results. Point is, you are fighting the engine to try to reduce the power to something usable. In this case, 5.0 seconds VS 3.5 seconds is *a factor of 3* in the power - meaning, to first approximation, you are trying to needle it down to one-third of its capability.

  The original RC carburetor would do that OK, by restricting the venturi at the same time as reducing the fuel flow, more-or-less without changing the mixture. Our version of that is to reduce the fixed venturi size to something smaller.   An alternative is to use a similar much smaller engine - like a 15FP.

    Brett

     

" The 25FP is way more powerful. "

Think thisisa bitofa fallacy , ACTUALLY .

The FPs most renowned feature is its ' RUN ' being said to be the best of the FP's .

Incidently is a capital 'S' on the 35 .

A OS 30 ( same as the 35-S ) will haul a Demon combat wing around authoritively . On a 9x6 .

A Good Traditional 35 CAN work good on a 11 x 4 , maybe a 11 x 5 .A little Fp / FSR OS WONT .

Dunno if theyll swing a 9 x 6 as hard as the 35 - S .

Down on a 9 x 4 theyll pull away more revs tho .

My FSR 25 was blown around till 1/2 inch disapeared off the 10 x 6 prop . Maybe thats about the max . 
Used a 9 x 4 black ridgid tornado prop on it , over 18 Kt. breeze . Tho takes a bit of footwork & co-ordination .

Found the 10 x 4 3 blade tornado ( the stiff silver ones )  good on long lines on the 35-S .
 
Putting a 5/32 ( 4 mm ) drill through the bearers carefully , as far as the nuts , can get both motors in the same plane ,
if angled bolts wont spoil your day .

Moral of the story being if you get the revs of the 35 S up where they are on the FP / FSR , youll find its got a few balls .
and the Std. intkes even o.k. then .

Thanks Brett for getting into the discussion here. After extensive reading, I realized that I bought the wrong FP 20. The one I have is the steel sleeve engine, not the ABN that you have written about extensively. I am hopeful it will still work well though.  Do you find the steel sleeve engine to produce a good run, or should I continue looking for the (now elusive) FP 20 ABN.

Thanks a bunch.

Randy

Thanks Brett for getting into the discussion here. After extensive reading, I realized that I bought the wrong FP 20. The one I have is the steel sleeve engine, not the ABN that you have written about extensively. I am hopeful it will still work well though.  Do you find the steel sleeve engine to produce a good run, or should I continue looking for the (now elusive) FP 20 ABN.

   I would just try it and see!  I haven't run the iron-liner version enough to really explore it. It will care more about the oil content, and you want more oil than the ABC version.  Based on testing a large number of these small engines, the important finding was that *they all work pretty good*, as long as you don't try to "improve" them too much, and run them like modern engines rather than trying to replicate 1955. That means running 4" of pitch and in a two-stroke.

    Brett

I've been flying a Ringmaster for the last couple of months with an FP 25 and a Brodak wood 9x3 prop.  On 60' lines  I get 4.6 sec laps with the engine running in a solid 2-stroke.  While the engine seems to be screaming, the lap times are just right for a Ringmaster (in my opinion)

Rick