Sorting this plane out has been a trip. Due to the softness (I believe) of the ply doublers on the nose. It's been quite a process to keep the FP 40 from running away. A lap or two and a berserker run. This from an engine that had proved extremely reliable on a Twister I built up a few years past, one with five ply in the nose and maple motor mounts going back to the leading edge. Conventional thought would have been head gaskets, blocking the boost port, cutting the liner etc. etc. and a posting about the awful no good for stunt FP. Luckily a top twenty flier in our club recognized the issue for what it was, a vibration harmonics causing the fuel to bubble badly. He had tamed a problem like this on his Foxberg racer by putting 1/8" steel plates under the engine mounts. These plates extending 3/8" past the engine mount front and back. Also a plate on the back side of the profile allowed the mounting bolts to be cinched up tight. Doing this to the Primary Force has largely solved the run away problem. I still have to check the bolts and tighten before every flight. Perhaps an engine of less power (or a different FP40 for that matter) would not have caused this vibe trouble. The plane should work with a less powerful engine. Even when loafing rich the FP pulls the PF up and over, no problem. I did add another head gasket to soften the break (up to three now) but that's it.

I made a cheekcowl out of some tough "Jerry Who" balsa to stiffen/damp out vibration on my Primary Force. I am runnning a Saito 40 and it never was running very well until I made this change--just before the Sig Contest. Now the engine performs quite well, so well I  can't blame my crummy flying on it! I'll try to take a photo of it.It sort of resembles the cheek cowl of the Pathfinderprofile, except I do overlap it on the wing to give some bracing.

For what it's worth, I had a  similar problem on my 3 year old P40 ARF. When I first built it, I mounted a Brodak 40 swinging an APC 10.5-4.5 prop and the plane was just a dream to fly. Then at the Sig contest that year, the Brodak engine mounting system came loose during my flight. Turned out it was the insert threads pulling out. I tried to fix it but finally gave up. I think I sort of gave up CL the rest of that year and last year (year 2) I spent most of my time fighting the afore-mentioned Primary Force engine run.

Well this year I pulled out the P40, installed blind nuts and remounted the B40. Ran like crap! Never the same twice. Changed tanks etc but no good. Finally I mounted a LA46 with hopes that it would be better, but no, same problem--inconsistent runs with a 11-4 prop. Finally realized that I was probably seeing a resonance in the front end at those rpms (10k-11k range) in both the B40 and LA46. Changed the prop to a Rev-UP 12-5, rpm's dropped, engine runs a lot better--needle range significantly larger.

So I asked what happend between year 1 with good runs with the  B40 and the start of year 3 with crappy runs. My only answer is that the original Brodak clamp mounts must have actually added a little stiffening to the front end of the P40. Recall they are reasonable long as compared to the bolt holes on a standard engine (even with pads). The  P40 already has a reasonable doubler/tripler on it, so I am not sure how to stiffen that up.

I think that what I see is that a profile front end is a little of a crap shoot.  Unfortunately I have been on the losing end of that now for some time!

To remove the resonance problem you need to add lateral stiffness to the nose ahead of the wing.  The easiest way is to mount a "cheek cowl", inboard, attached to the doubler and the wing.  A simple triangular web of plywood would do it but a "cheek cowl" looks a lot better.  I think there were pics of a cheek added to a PF recently, but I don't remember the thread.

I have been dealing with the "bad vibes" issue on my most recent Cardinal ARF with ST 51 (actually 3 different ST 51's and 3 different tanks; different props, spinners--and anything else I could think of).  I have replaced the "exotic asian hardwood" beams with maple beams (maybe too short) without much/any improvment.  Anybody want a Cardinal cheap??  This is a good place for this  !

My old, crashed/bashed/trashed three times Cardinal ARF with ST 51 just keeps flying right along, nice as you please, with nice, steady runs.

Jim

Seems lots of engine runaways blamed on engine set up or engine type are a function of bad vibes. First advice of many (not members of my club, since we've seen many FP40s doing effective stunt duty) was dump the engine. When issues with the PF nose were sorted out a few had a chagrined and mystified look watching my Force run dead rich lap after lap. A setting sufficient to power the plane in mild conditions. My Galaxy stated to act up big time in June (just in time for Brodak, ouch!) exhibiting what appeared to be classic FP40 runaway. Plug change helped some. But it was still weird. Finally we located a hard to see leak in the hopper tank. Leak down was initially slow enough to not be obvious when pressurized while mounted. After pulling the tank, filling, etc. the drips showed up. Dunking tank, looking for air bubbles, uncovered the hopper seam that had sprung a leak after two years of steady flying. Fix took a few minutes. Engine runs predictably now.

What's that plywood?

Update on my softnose PF: We tamed the run doing various strategies mentioned above. A wood Zinger 11x5 also helped. Good, good enough, but not great. The FPs tried until then were cast iron piston engines, veterans, who ran fat and happy on other planes. Functional here, but not sweet. On a whim we switched out the cast iron engine and mounted an aluminum piston FP that spent a season powering a P40 ARF. This engine proved a gem. In the PF level flight is a nasty burble switching back and forth between 4x2 or 2x2. Definitely a classy stunt sound. When doing the tricks the engine powers up quick. The sharp crackling music of a good running stunt FP40. Even the sardonic and funny Peabody locked me in his sights and sputtered a compliment. Current performance of the PF/FP40 combo is a thrill. This winter (perhaps sooner) I'll do the laser kit and toughen up the front. Sig lists the ARF PF wing as available separately. Also an intriguing option. Easy enough to fabricate a fuse (with an improved front end, made of KNOWN materials) and a tail.

This is an inresting thread; I've read many times onthe internet that only Fox .35's wouldn't run on profiles.

Evidently other engines have the same problem.

De Hill

This is an inresting thread; I've read many times onthe internet that only Fox .35's wouldn't run on profiles.

Evidently other engines have the same problem.

De Hill

Dee,
Not really sure where this comes from but a Fox .35 does run really well on any profile that I have ever built.  The burp that many talk about has not been an issue although vibration can be a problem at times.  Anyone with any make engine can have problems if not properly set up.

Later,
Mikey