Fox 35 and 11X5 Prop?

And chances are good that the 11-5 I used then was a TF wood, so rated with more pitch than it actually had, as was the case until the "Power Point" series came along, maybe even then.

The last 15-20 years, with props other than Rev-Up and TF, I have liked using 10-5's with Foxes better than anything else. We do have a regular correspondent in the online C-L community who has never enjoyed running engines in the 4-2-4 "stunt run" mode. Jim Thomerson flies stunt with his engines set in a two cycle, just a bit richer than max speed, so it still speeds up with the nose headed up.

Jim says he uses modern 11-5 props with his Foxes.

Kiwi

You didn't mention your lap times. Going up in diameter and down in pitch will lower lap times and increase thrust. That may be exactly what you need, but maybe not. If you are below 5.1 seconds / lap (lots of variables in that), decreasing pitch is probably correct.

The smart guys do those kind of changes without even thinking. The rest of us struggle.

LAST EDITED ON Jul-29-01 AT 02:55 PM (EST)

Larry wrote "Going up in diameter and down in pitch will lower lap times and increase thrust". I think he meant that these changes would increase lap time and thrust. The correct prop depends on how strong your engine is, what altitude, humidity and temperature you fly at, nitro content, muffler (or not), etc. The BY&O 10-5, Revup and old TF 10-6, WOOD Master Airscrew 10-5 and the APC 10-5 or 10.5 x 4.5 are good props to try on a Fox. Zinger has 10-6W and EW's that are "kit props". Robbe makes a 10-5 with good blade area and it's worth a try if you're after a nylon prop. Graupner makes a 10-5 also. Taipan's 10-6 is too much for a Fox, but the 10-4 might be worth a try. Taipan's pitches seem to be a bit higher than rated and then increase toward the tip (which we want). Steve

EDIT: I looked through the last MA and found the part referred to in Joe Wagner's column on p.82. Editor Bob Hunt found it doubtful enough to warrant an editorial comment "Editor's note: Many would question those figures". Joe W. was known to fly light CHIEF's on anemic McCoy 19's, so be assured that he was comfortable with light line tension and didn't fly in windy conditions. Not very practical. Having some reserve power is always a good policy.

Thanks for translating correctly. I said it backwards, but some would figure it out, and some would be forever confused.

I have run 11-5's in the past, and it was right on the edge with a stock motor and 5%, at least in the Midwest. There are several issues. Most 11-5's are really not a full 5 - for instance the Rev-Up 11-5 Special Pro 2 is about 4.75 at middle, and tapers off to around 4.25 at the tips. The old Top Flight (regular blade) was pretty close to 5 at points. The Y&O was less, as was the BY&O.

It has a much better chance with either more compression, more nitro (up to 15-20% in the heat), and a light airplane in perfect trim, capable of acceptable performance at 5.5 second laps. This is *very slow* for a Fox-powered model, most need to go around 4.8-5.0 on the requisite 56-58 foot lines. The advantage to this is the same as the advantage of other low-pitch systems, much better speed control.

It's worth a try, if you have some time, and a good airplane with which to try it. Avoid wide-blade props to give yourself the best possible chance. It will be evident quickly enough if you have the poop required. I can almost guarantee it will need to be right at the break point in level flight. Try several differnt makes and models of prop. And don't do it on a hot, humid day.

Joe Wagner's comments are about right in most respects. A stock motor with a standard 11-5 and 5% fuel will go around 6 second laps with typical classic models. What I would tend to be skeptical of is the statement that this will provide adequate vertical performance because of the break. Of course, the definition of "adequate" is in the eye of the beholder.

For almost everyone, I would avoid the temptation to try to fly any slower than about 5.1 or on longer than 60 foot lines with their Fox models. Too fast is way better than too slow. The only Fox model I ever saw (which is A LOT of models, my own included) that would go slower than this and still perform was Don McClave's Green Box Nobler, which was very light, and had several non-stock items in the engine.

I more-or-less agree with that. The problem is usually that even though it's the same load at the same revs, it still has to spin faster. A stock engine running 5% doesn't have much breathing room to spin faster.

Nonetheless, it doesn't hurt to try - it won't blow the engine up, so why not try it and see.

Do not over prop a stunt engine. I run 5% nitro, 29% caster
and 1% syn for a fox 35. K&B 40 and Silver Foxx 40 5% nitro,
25% caster and 2% syn..........

True, do not overprop an engine.
False, "ALL 35 and 40 size Stunt engines run best on 10x6 props."

This last statement is characterized by the RC crowd that compares one engine of this size with another simply by how fast it can scream a 10 x 6 prop.

There is going to be a certain rpm at which any engine is going to be comfortable in running, and our stunt engines are usually set up to run at less than maximum rpm. Some will run the higher rpm, low pitch mode, and some will run in a lower rpm, more 4-cycle mode. Even pipe engines are set up differently by different people (not always correctly) these days. We set ours up to run 4-cycle in level flight, and, ideally, kicking into a 2-cycle near the top of the loop or in the corners of the square.

But, setting the pipe type runs aside, there is still a vast difference between types of engines in this size range and in the type of prop that each will turn.

I have not found a Fox 35 to run for me with much over a 10x6 prop. Even cutting an 11x6 Rev-UP (a notoriously underpitched prop) down to 10 1/4 seemed a bit much without some hop up to the engine. And I have, also, not found the correlation stated above, of dropping one inch in pitch and then being able to go up one inch in diameter. A Fox happy with a 10x6 Rev-UP or BY&O was not at all happy trying to swing an 11 x 5 prop of the same brand. Would that mean that a 61 pipe engine turning a 12 x 4 would only be capable of turning a 10 x 6 if we upped the pitch? I don't think so.

We were running a stock 35 FP with a 10 x 6 and it was happy, but we were not. With modification, we were able to turn a 12 x 6 and even a 12.5 x 5.5 Bolly on the same engine. The Silver Foxx 40 that you mention (at least one of Lew's later ones) will quite happily turn a 12 x 5 BY&O. It won't turn a 12 x 6, however, since it still wants to run a bit faster and has to in order to develop its best power. So it is happy with a 12 x 5 and our 35 FP is happy with a 12 x 6, and neither is happy with a 10x6, although you could run it blubbery rich, I suppose, and get away with it.

The oil content on your Silver Foxx 40 also seems a bit extreme. 20-25% total oil content, either all castor or 50-50 mix (or somewhere in between) will work adequately for this and other engines of its type. Anything more is definitely overkill and will lead to loss of power. For ourselves, we pretty much settled at 22% oil, although SIG at 20% seems to work fine.

For the Fox 35 and OS 35S stay with 25-29% oil and all castor is still best for these.

Anything less on the 35S and you will lose the rod. Beware of buying a used RC engine of this breed, as many of them have been run on lesser oil.