Unloading

I've never found the 25's and any brand of 9-6 to be matched up, not even 40 years ago when the TF wood props were over-rated by about an extra inch of pitch. I used 10-4's and 9-5's with the 25's of that era, and now use 8-4's, 9-4's with the modern 25's.

>Is it possible that this motor/prop combination might "unload"
>in the air sufficiently to make the combination viable for
>flying?

My opinion: No.

>Or, if not, can it generally be concluded that a prop that
>overloads an engine on the bench will overload it in the air
>as well?

That would be my own expectation, yes.

>Or, is this just another one of those situations where there
>is no valid generalization and only in flight experimentation
>can be relied on?

Let's say this: modelling has not yet become an "exact" science and some generalizations may not cover every eventuality. With experience in making ad hoc tests, the number and breadth of testing may be greatly reduced.

Kiwi

It depends on what you are expecting your model to do. If you are after top speed in level flight, maybe. But as this is a stunt forum, I doubt that's you goal, and think you need to revise your prop choice. If GMA gave you any suggestions for prop or static rpm, I'd give that a try. A 9-4 should work ok as a starting point. Steve

My opinion on maximum top speed is to let it wind up and the 9-6 is too big for that. An 8-6 of some sort is more like it for that purpose.

>But as this is a stunt forum, I doubt that's you(r) goal,
>and think you need to revise your prop choice. If GMA gave
>you any suggestions for prop or static rpm, I'd give that
>a try. A 9-4 should work ok as a starting point.

Fox still makes an iron piston 25 (as well as an ABC one?) that requires a certain amount of break-in. Unless GMA prepared the engine for you with a pre-broken-in honing job?

I've always used a couple of "sizes" smaller in diameter for the break-in props, made from large props, such as cutting down 11-4 props to 8-4's to break in 29's and 35's. That provides some flywheel effect, even with wooden props, but a LOW torque load.

One of the signs of incomplete break-in is always a sag, usually related to overheating. Iron piston engines aren't ready to run a good, solid max rpm setting without several runs in which they heat up slowly and are allowed to cool down to room temperature before running again. This need for break-in is one of the big reasons that modern ABC type engines are so dominant today (they require comparatively little breaking in and it's almost opposite, needing a quick heat-up instead of a slow one).

Don't get your nose out of joint if you do know and understand all of what I wrote, and none of it applies in your case. Remember that like most forums of the nature this one has, lurkers who don't write any messages at all are practically the majority of readers. Many of those folks may be inexperienced enough not to know about the break-in procedure for iron piston, steel cylinder engines.

K.

Some people prefer less load on the engines and will, thus, use smaller props, but with any engine and any prop there has to be some type of unloading in the air which is totally independent of the prop changing its pitch. (Most of our props, be they wood, or plastic or carbon fiber will have a certain amount of depitching due to flex when they are under load with the engine running.)

APC wood 9X4 - Fast. I tried running a 4-2-4 but when it went to 2 it never came back.(imagine that)

Nylon APC 9X4 - Basically the same as above.

BY&0 10X3 - Rev the motor up to get your lap times down & line tenion up. This prop is showing the most potential so far. I just need to figure out why the motor falls off on the outside manuvers. I am using a GRW uniflow with no muffler pressure. The outside manuever problem improved when I restricted the uniflow tank vent.

BY&O 10x4 - Fast with plenty of line tension. Unable to get a 4-2-4. I run it in a fat 2.

Wood Zinger 10X4 - 4.6 second laps in a fat 2. Too fast for me.

I have a BY&O 11X3 which I'm trying tonight. I'm convinced my solution is somewhere between the BY&O 10X3 & 11X3. My engine is stock and I have concluded that it neeeds to run in a 2 cycle fashion. I am also waiting on 10X3, 10X4 & 11X3 nylon APC props to try also. I have been told that the nylon APC is a popular prop for the 2cycle run. I will send you my results. I would also like to know what GMA did to your Fox? Tim

>> Having tried some other props and developed some baseline
>> data, I put a Top Flite 9X6 Power Point on the motor and fired
>> it up. Attempts to lean out the engine simply made it sag,
>> noticeably.
>> It seemed pretty clear to me that this particular prop was
>> simply too much for the motor, at least on the bench.

> I've never found ANY of the 25's and any brand of 9-6 to be matched up, not even 40 years ago when the TF wood props were over-rated by about an extra inch of pitch. I used 10-4's and 9-5's with the 25's of that era (Foxes), and now use 8-4's, 9-4's with more modern Schneurle (OS) 25's.

>> Is it possible that this motor/prop combination might
>> "unload" in the air sufficiently to make the combination
>> viable for flying?

> My opinion: No.

>> Or, if not, can it generally be concluded that a prop that
>> overloads an engine on the bench will overload it in the air
>> as well?

> That would be my own expectation, yes.

>> Or, is this just another one of those situations where there
>> is no valid generalization and only in flight experimentation
>> can be relied on?

> Let's say this: modelling has not yet become an "exact" science and some generalizations may not cover every eventuality. With experience in making ad hoc tests, the number and breadth of testing may be greatly reduced.

JAK> I have been messing with these peculiar little critters four decades now. There is nothing that is a hard and fast rule ("this is ALWAYS so") about them. The only place you will find the answers to your question, is indeed in flying YOUR particular motor/ plug/ prop/ airplane/ fuel/ fuel tank/etc. combo in YOUR local environment. What works for you won't necessarily work for the guy in Alaska or California, and the reverse is true as well. Generalized ideas are of course always valid, but a specific ultimate answer is really yours alone to find.

JT> What you want is to be up on the power curve of the engine where a change in RPM doesn't change the amount of power the engine is putting out by much. When you load the engine in a maneuver,you move it back down the power curve. I understand that the combat guys underprop their engines so that in level flight the engine is running past the peak of the power curve. So when it loads in a maneuver, and slows down, it actually increases in power output. Anyway, I would tend to underprop an engine a little, rather than overprop it, for stunt use. As Kiwi says, none of this works for everyone.

TB> To date I have tried the following props on my stock Fox .25BB:

(I didn't see what in my opinion would've been "enough" 9 inch props listed, and not even a single 8" has been mentioned at all.)

TB> APC wood 9X4 - Fast. I tried running a 4-2-4 but when it went to 2 it never came back. (imagine that)

{Remember that if Leonard is correct about thick-bladed props having an "effective" pitch greater than the actual pitch, you'd need an APC 9-3 to compare to most 9-4's (I have yet to run across any wood APC's anywhere, or any wood Master Airscrews).}

TB> Nylon APC 9X4 - Basically the same as above.

There's a huge difference between a composition prop like a Master Airscrew and a Grish white nylon or a Grish glass-filled prop, so I don't know WHAT 9-4 that one of yours might be! The Grish props have been used successfully by various combat pilots whose planes/ engines have been at Houston when I was there, including that self-same Fox 25BB Peter H. is frustrated with.

TB> BY&0 10X3 - Rev the motor up to get your lap times down & line tension up. This prop is showing the most potential so far. I just need to figure out why the motor falls off on the outside manuvers. I am using a GRW uniflow with no muffler pressure. The outside manuever problem improved when I restricted the uniflow tank vent.

I think that one's too big to allow the engine to run at its peak power level. I can't imagine it being happy with a majority of the 10" props available.

TB> BY&O 10x4 - Fast with plenty of line tension. Unable to get a 4-2-4. I run it in a fat 2.

Still too much prop blade, compared to a 9".

TB> Wood Zinger 10X4 - 4.6 second laps in a fat 2. Too fast for me.

Several of our NW region stunt pilots like to refer to Zingers as "prop kits," by which they mean the prop isn't close to being usable as delivered. I haven't seen that brand on the dealer shelves around here in years!

TB> I have a BY&O 11X3 which I'm trying tonight. I'm convinced my solution is somewhere between the BY&O 10X3 & 11X3. My engine is stock and I have concluded that it neeeds to run in a 2 cycle fashion. I am also waiting on 10X3, 10X4 & 11X3 nylon APC props to try also. I have been told that the nylon APC is a popular prop for the 2cycle run.

Only if you can find an APC 9-3, but an 8-4 would probably be interesting to try. If a ten inch prop is overkill, lugging the poor engine down slower than its peak, then an 11" is naturally even worse.

TB> Driving home from the site last night I would have thrown that .25BB in the river if my friend had not grabbed it away from me. As much as I love a Fox engine (and I do Kiwi), and doing things differently.

There have been a lot of Foxes that I never liked much at all, like the odd "Sport 36" that followed the 36X's and used the same basic guts, and I've never liked the very odd carb mounting that Fox started using, with flanges instead of a drop-in adapter.

TB> I'm saying UNCLE. The fastest lap time I could get with a BY&O 10X3 was 5.6 seconds which made outside & overhead manuevers real scary. When the engine would quit it would just die not lean out and run faster. That told me there was no more power left in the engine and I would never make it down to 5.2 or 5.0 with that prop. The 9X4 ran at worst a 4.6 which is too fast for me. The engine will make some serious power but not in a stunt friendly manner.

Sure seems to me that there ought to be a way to put it to use.

My absolute favorite Fox of any kind and /or size is the 1957 model 25. They are little marvels, running a sweet 4-2-4 with good torque at low rpm's.

I still do have about four kinds of OS Max 25's, and a Magnum, and a couple of Thunder Tiger 25's. All but one are Schneurle ported. The Max-F 25 ABC engines are Schneurles related to the FSR series. I have been running those fairly close to flat out on 8-4 APC's and Grish 8-6's, and getting a very solid 75 mph with large Arrowplane-type foamies. With the APC's, the engines actually accellerate in maneuvers instead of losing speed (the APC 9-4's were a lot faster in use, more like 85 mph, which seems to agree with what Leonard suggested, at least for that size).

Right now, you could say that I am "engine poor," with a lot more of them on hand than I can put to immediate use. So, I don't think I'm going to get another of the model Fox 25 involved in this discussion again, at least not real soon anyway, just to see what it will do in regard to the high performance area I've been using 25's for the last year or so. (I'm just too well pleased with the Max-F 25BB's.)

Both Phil Cartier (I think) and Larry Driscoll (for certain), however, and others as well, have had good things to say about the applicability of these Foxes to the SL Combat events I'm using the OS Maxes for.

Kiwi

P.S. November 30, 2001:

When I ran across the whining kiddie's complaint below, it is now a couple of months later, and I'm sure I never saw it before. That (missing the complaint) must've happened at another time when my connection to SSW was in the slowest of slow motions (as seems to be the case today). The kid REALLYgot his shorts tied in a knot, didn't he?

Who knows what sets someone off to act like that? Fact is, I still compete and will drive a hundred miles (Georgetown) for an ordinary flying session, but with a nice crowd of enthusiasts, instead of just Jeff and I, the way we did last Sunday.