What is a good engine for the primary force to be flown in the rough east coast small fields?
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How do you tell if you have the too soft fusalage
thanks
Fred Q

My first one had a K&B28 Sportster/10/4.5TT prop and PM 10/22 and it flew better then a lot of the heavily overpowered examples that showed up.
I built a 2nd one and went down the the Sportster 20 and had a blast with it till I let it go to someone else.
This thing really doesn't need huge amounts of power and weight, an OS LA 25 will probably do as well as anything else.
dennis

In reality, I think Mikey designed it around an FP .25.

No, really, it was the .25LA-S. I wonder if his website still exists? I looked for it in "Favorites" and didn't find it. Googley might work. There was some info on the engine setup.

Bob Kruger has a Magnum XL .28 in one, pretty much reworked to SSW specs, I think. The stock XL .28 will do well, with a 10-4 APC., 10-22 sorta fuel, and a .265" venturi (for .156" spraybar). We only have one PF ARF around here. It started out with an XL .28, but ended up with an Evo .36 in it. The .28 went into an ARF Smoothie (which it flew quite well). The Evo .36 doesn't seem very popular 'round these parts. Dunno why. A Brodak .40 from the first batch would be a good match. The ARF is likely to turn out tailheavy with a .25LA, and by the time you add noseweight, you shoulda used a bigger engine.  Steve

Fred
Why don’t you dig one of your old rat engines out and make it interesting?
Willis

I had one with a Saito .30 four-stroke and couldn't imagine a better airplane/engine combo. It's a very light model and any two-stroke bigger than a .35 is really unneccesary.

   Agreed. A Magnum 32 or 36, for example, would work for a *biplane* version of this airplane!  We have flown a lot of airplanes this size and larger with 20 and 25FPs, and they work just fine. The "new" 25LA also seems to be pretty stout, much stronger than the early versions and that would be fine, too.

    The problem with 35/36/40's would be getting them to run properly at the low power required.
     Brett

Fred,
If you are flying on rough fields, I assume you don't want a long roll before takeoff.  I have a Primary Force with 3" light electric wheels that failed to take off in longer-than-average grass at the end of our rainy season with an OS25FX ball bearing R/C engine converted to C/L running wide open, while a Skyray and a Flite Streak got off the ground just fine.

I compared the proportions of the Primary Force to Ted Fancher's Medic and found them to be comparable except for nose length.  The PF nose is about 2 inches shorter.  I have another ARF PF partially built that test balanced more than 1" behind the CG shown in the instructions with a Brodak 40!  Eliott Scott weighed the components of two ARF PFs he built and found the stab/elevators to be about 3 times heavier than the replacement units he ended up building for them.  I question the idea that all ARF PFs are necessarily "very light".  It's possible that the weight of the wood used varies from batch to batch.  Others may have had success balancing theirs with lightweight engines, but my recommendation would be not to build yourself into a corner and end up with a modestly powered plane that takes a lot of noseweight to balance.  I agree with Steve's comment about that.  Better to test balance the whole thing including prop, spinner, tank, landing gear all taped or rubber banded together before getting out the glue.  If noseweight does turn out to be necessary after gluing it all together, it is still better to go ahead and add the weight than to fly an unbalanced plane.   

As for the too soft fuselage, first make sure it's not warped.  Several people have bought ARF PFs that had banana-shaped fuselages.  I would suggest both using aluminum engine mount pads and installing brass or aluminum tubes in slightly enlarged engine mount holes through the fuselage to prevent compression of the engine mounts and loosening of the bolts over time. 

Also, the iron-on covered fuselage is very susceptible to fuel soaking in the engine mount area.  Fuel WILL find a way to get into the wood if all covering edges are not thoroughly sealed.  I have a beautiful All Australian Old Time plane that is fully Monokoted but not well enough sealed in the engine compartment, and the wood became mushy all the way back to the wing after only two years of flying.

Hope this helps.  Good flying.  The Primary Force can be a really nice plane.     

I built mine several years ago from Mike Pratt's short kit. The plans showed an FP .35, which I had. So even though Mike had a .25 on his, I drilled mine for the FP-.35. I built a bit heavy, and with this engine (tongue muffler), overall weight exceeded 40 oz. Still, although I built heavier, I have 1/2 -3/4 oz of lead weight in the tail. The plane flies fine, but nose vibration, my metal uniflow tanks, and the characteristics of my FP-.35 and LA-40 make consistent runs hard to achieve. If I were as dedicated as I ought to be, I'd have switched to a clunk tank and/or reinforced the nose. On 60'-62' lines, it needs 4.8 - 5.1 second laps for preferred line tension.

SK

Better tail weight with that long tail than nose weight, with the short nose--so much less is needed.   

Serge, Would you describe your 40+ oz PF as overpowered with the FP-.35?

My experience agrees with yours as to lap times for line tension.

Thanks

Kim and Bill-

'sorry I'm late in replying. I'd forgotten this thread and was just taking time off from Newsletter editing to see what's going on. Fortunately, I just flew the P-Force yesterday for one of my few tries this summer and think I made some progress...

>Serge, Would you describe your 40+ oz PF as overpowered with the FP-.35?

If the engine runs away, then yes, but it's fine when it stays at the transition to wet 2-cycle. That goes for both the FP .35 and the LA-.40, although the LA-.40 seems slightly "friendlier". I ran the FP-.35 in club specialty events requiring extremes (slowest flight, fastest, longest lines, etc.) a couple seasons ago and it had the best overall scores. But it didn't balance on the "edge" well. I switched back to the LA-.40 and tried a lot of "stuff", some of which worked at certain temperatures, but problems persisted, even on the usual uniflow set-ups; it would lean out suddenly and not return. Yesterday morning I put on a Zinger 11" x 3" and wound it out a bit more, but still at that edge of 2-cycle. Ambient temperature was probably 80+ degrees F. I didn't however fill the tank, so results on uniflow are not conclusive. I got the 5-5.2 -second laps and just slightly leaner/faster inverted. The plane was consistent for the entire flight and handled well before running out of fuel suddenly inverted for a soft, slow landing inverted in grass - no damage. So, I liked what I saw with this less efficient, lighter, lower-pitch prop. Tension was nice. Tomorrow, if I'm far enough along on the club newsletter, I'll try it in the evening again adding 1/4 oz weight  back to the other 1/2-oz on the tail.

I would say that the power available, if harnised correctly is just fine, but when it leans out, the lap times on an APC 10" x 4" prop go down to 4.0 - 4.4 seconds per lap. I'm a bit old now to do many tricks at that vertical speed. If it were a 36-oz plane, I think the LA-.25 would be a fine match.

>How's it going?  As to the laps times around 5.0, traditionally that seems to be the area that most profiles like to fly.  I think some of the newer designs with newer powerplants,
>can be slowed down a bit (*maybe 5-2-5.3*), but it isn't by much it seems, and it is a model to model situation.  What cha think?

Things are pretty interesting and hectic as usual in this retirement thing, Bill. I'm enjoying designing and trying to mathematically analyze my very unconventional model design and have neglected my conventional, by-the-numbers stunter that's about 2/3 complete (has been for months!). This has been sandwiched among Gusti's LaCave reunion committee musical events leading up a big concert weekend that took place over three days a couple weeks ago. Super event! Now I'm back on airplanes for a couple weeks.

Anyway, I agree, it seems that the planes themselves seem to make a big difference. I first had the LA-.40 (which I bought from Richrd Oliver at VSC-14) in a heavier, flapped, hodge-podge profile made by Mike Alimov from his crashed Tucker Spl. wings and tail. That engine would not perform at all on the same clunk tank that had been used for his ST .51. Even Mike couldn't get it to work. So I put a GRW (?) metal uniflow tank on it and coaxed it into working really well. Some leadout adjustment made that plane fly just great with that engine on 60' lines. Then of course it went in inverted, when I got too comfortable near the ground and forgot which way I was flying. After flying Foxes on SkyRays for a while, I built the P-Force and never, got any consistency other than a handful of flights or separate days on a variety of tanks. So the LA-.40 liked the Tucker derivative, but has never liked the P-Force. The nose of the P-Force is a bit flexible, and I really should put a cheek on it; so maybe I just have myself to blame. Still, the plane does seem to make a difference. Possibly also the engine worked harder on the draggier Tucker than on this relatively clean 500 in^2 flapless profile. Of course, the model weight makes a significant line-tension difference, perhaps affecting line length as well. 'lots of "stuff" to coordinate in any plane and change.

SK

Kim-

Richard never mentioned modifying it. This was at the time he was transitioning back to CL with a Cavalier at VSC. He told me he really didn't likre the LA-.40's run characteristic, which is why he offered it for sale at a good price. I suspect that he had not modified it. I agree that the cowl cheek is a good thing to try. I passed up the opportunity to verify Sunday's results on a really nice flying evening, while finishing up the club newsletter; so I want to do that first, before tying up the plane.

I might write something, if my little project shows success. Now though, I'm just using it as an experiment in design techniques to decide construction feasibility and to choose theoretical c.g. positions for comparison to what really works. This is more important in this design, since predictive rules of thumb don't apply, and normal stunt design traditions would probably leave things way off. It's been slow and fun. I re-learned a bit of calculus and Excel again (boy, did that work great for integrals that I couldn't evealuate!). I think I can now find the c.g.'s of NACA 00xx-based bodies and their skin/finishes. It's that kind of weird stuff, to go with some comparisons of gliding tests to a.c. predictions. In many years of looking, I haven't seen anything particularly close to this in appearance - kinda surprised me when it showed up on my note pad. No advantages are expected; it's just one of those projects to see what can be done with something interesting.

Walter Mitty..."pocketa, pocketa, pocketa,..."

SK

For the original poster;
I used Fox 35's in my son's Primary Force. He wrecked it hard, so it is using another Fox 35 now. The model is light and flies very well, the smaller, newer engines suggested would be very satisfactory by my experience with my son's model.
Chris...

Hi Chris,

I *think* the LA 25 would be a good choice, especially since they are available, and take less fuel than the trusty Fox.  I saw Dave Hemstraught flying an ARF Nobler in Classic a couple years ago with the LA 25, no mods, and it flew great.

Bill

Ward,
Have you weighed your PForce?  My comes in at 42 oz., and the wing is the only ARF part.  The rest is scratch-built using 5-lb stuff.

Thanks,

I know that this is off topic but the remarks on the LA40 provided a few chuckles for me as I went through a 2 year period trying everything possible to tame the one that I had. I had up to 3 head gaskets, 25 sized venturis, pantyhose over the venture, modified heads, different fuels, different nitro contents. All of these mods gave non consistant results. The only thing that seemse to work well on this engine was to grossly over prop it. With a 12.3X4.75 prop is would finally loaf along in a wet 2 for the whole flight.You all know that cool weather works in your favor and hot humid days  make life miserable with an LA40.
Mine has sat in the drawer for a few years since it's easier to use the 46 and just go fly.
Dennis