O.S. 20FPs on 35-sized profiles

[dirtydan]

To All,

As this has become a banned subject on St*ka Stunt site, and yet there is quite a lot of interest in the subject, I thought it might be best to gather up my marbles and come on over here.

While I have not yet seen latest Stunt News, it is my understanding that the second of two articles dedicated to proper use of this outstanding engine has been included, along with some relatively substandard pictures.

News since writing second article: The Super Combat Streak was used in winning (narrowly) Classic Stunt at the VGMC Western Canada Stunt Championhips in Vancouver, B.C. Most everybody smiled when realizing I was serious about using such a model in Classic. Yep, looks odd. But the 20FP with what has become known as Brett Buck Tune-Up tranforms the model.

Not enough, it turns out, as at NW Skyraiders Roundup a mid-pack finish was the best I could do. However, with a score of 498--no fidelity points were used at this contest, so it's straight-up scoring--and in reviewing my score sheets I am confident the model can be put into the low-500s with relative ease. On a really good day we might be looking at 515 to 520. Not too bad for an ancient Combat design with a "too-small" engine.

I have been having pretty good results with the oft-discarded remote NVAs. The units I am using have metal bracketry and old-style remote needles, assuming there was some sort of change made when the LA series of O.S. engines came out.

There has been a bit of hit-and-miss when running bought-used engines. Just being able to start and fly them doesn't seem quite enough indication they are at the peak of their game.

I have come to the conclusion that if you are fixin' to buy a used 20FP, converting it to CL Stunt, look for the cheapest, saddest piece of junk you can find, planning in advance fit it with a new piston/cylinder set.

More likely is to find a really trashy 25FP, these having been sold in higher volume. Assume piston/cylinder set is artificial reef material, convert it to a 20 with new piston/cylinder set, wrist pin, head, gasket set.

Having purchased a few 20FP parts from the Tower seb site, it appears as if they are replenishing supplies of parts on an ongoing basis. Inventory on heads was noted as low for a bit, is now back up. I think wrist pins are shown as currently out of stock, to be coming in this month.

Anyway, as the 20FP is very nearly bullet-proof in a CL Stunt application, especially if one uses real good fuel, a rebuilt engine ought last for a very long time.

If you've got questions, I'll drop in and try to get them answered.

Dan

[oldcajun]

Dan,
I'm a little puzzled about your experience with the FP25. I have an ARF Streak and set it up with an FP20 per your and Brett's instructions. Here at Tucson, the airplane was somewhat underpowered. I switched in an FP25 without changing anything else and found that it runs just like the FP20 only a little (800 rpm) faster on the 9X4 APC. Same steady fast 2 cycle and very consistent runs.

Is your preference for the 20 due to the 25 having too much power at sea level?

Steve

[dirtydan]

Steve,

Honestly, it is a little hard to imagine a 20FP-powered ARF Flite Streak being under-powered on top of Pike's Peak, let alone in Tuscon. I'm not doubting your statements, just giving you an honest reponse.

True, I fly mostly at nearly sea level. But I am using Bru-Line air filters to knock off a little power. And tending toward longer lines, 62 feet CL to CL, will try 63s and 64s down the road.

How healthy is your 20? I had slightly substandard results with one bought-used engine that started (electric-assist!) and seemed to run fine on the ground. It was inconsistent--not weak, by the way--in the air and didn't settle into Basic Goodness mode until fitted with a new piston/cylinder set.

Were I you, new parts are an option, as is sticking with the 20 but going to 15% fuel, maybe even 20%. I have seen Derek Moran run his 20FP on 25%, and while definitely to the fast side (2004 NW Regionals) the speed regulation, which is the marvelous side of the 20FPs, was excellent.

Again for consideration, my Skyray 20 really is 34.5 ounces all-up, no fuel. And it's noticeably bigger with the (slightly) extended chord, block-style wing tips, all of this coming in at 450 square inches.

It flies superbly, although I will agree that it might not be as good in your area as mine. *However,* removing air filter, going to 15% fuel would, I believe, bring it right back on line.

I think there's something wrong with your 20FP setup. But if the 25 works, ya better use it...

Dan

[oldcajun]

Dan,
Both the 20 and 25 were brand new engines, R/C converted to C/L using OS venturis and needle valve assemblies. I normally use 10% nitro, 22% oil (half castor) on these engines, but I did try some with 15% nitro. Lines were .015 flex, 60' eyelet to eyelet. I did not check rpm on each flight, but I did check the 20 on a 9X4 APC at 11,400 rpm on the ground. The run was great as you described, but line tension was light overhead, to the point of some free flighting. The 25 turned the same prop at 11,800 rpm without being fully leaned out. What surprised me, was that the 25 run was every bit as good as the 20, just more power. The airplane was faster, and line tension was great.

My question was really, if the 25 runs as well as the 20, why modify a good 25 to convert it? This is, I know, a sample of 1 pair of engines under specific conditions, but I'm really happy with the ARF Streak/FP25 combination. I know I can slow the 25 down a bit by richening the needle without losing the steady run characteristics, but I don't have engough tests to know how much.

I did try a 10X4 APC on the 25, and it flew the same as with the 9X4, only very slightly faster.

Thanks to your comments on the "Brett Buck" tune-up, I did try these engines with the stock muffler instead of the tongue muffler everyone else suggested. I'm now a believer that the muffler is key in the engine runs. It seems, at least with my engines, that it works much the same for the 25 as it did for the 20.

Steve

[dirtydan]

Steve,

Well, you're certainly approaching this in the proper manner, thus I don't find much to disagree with. Your comment that the stock mufflers are key to proper engine runs is exactly correct.

There are points to be made, although this sort of thing is difficult. I'm just barely able to make my own stuff work, and I am doing it hands-on...

Assuming a stock ARF Flite Streak, that's what we refer to as my "demo" ARF as being, there are points of interest.

With stock leadout position, I found .012 x 60' (cl to cl) to be a lot better than .015s. In going back to the larger lines--some of my friends have been known to use model in competition--I eventually added what amounts to TE stock whittled from 3/16" x 1" x 3" stock and added it to fin/rudder. At one time I thought rudder offset to not be needed; but I was wrong.

After a couple friends said they liked the model, but it was pretty light on the lines, I measured for offset. And found some *left* offset had crept in. Stupid soft engine mounts sticks. Didn't take a lot of shimming--way less than one would get with washers, for example--to get it set straight ahead again, and line tension is now all one can expect with a model like this.

You may indeed be down on power. While you didn't specifically say so, is 11,400 a peak setting? I am setting the needles for 11,200 to 11,400 and there are quite a few more revs on tap if I choose to fully lean the mix. Don't have my notes with me, but I'm thinking 13K plus is not a stretch.

Gotta be careful richening the mix to slow things down. It sounds as if you will readily recognize the problem if it comes up, but ideally it appears as if either the 20 or the 25 need to run at about 12K in the air, possibly higher as that won't hurt run stability, but lower can put you on the wrong side of the curve.

For reference, I have an informal chart on the front of my toolbox which shows ground setting for my model/engine combinations. I will set as noted for the first flight, and chances are it will be quite good. But if it's off, my experience has been that it's only rarely off by more than two clicks, much more likely to be one click in either direction, if it needs adjusting at all. Opening the needle three or four clicks to the rich side would be regarded as a flight not worth taking.

Bottom line is that as you have new engines, are following the BB T-U to the letter, might be a good idea to fiddle with trim of the model.

And at the end of the day, if the 25 works better for you, use it. There doesn't seem to be much problem in selling good used 20FPs...

Oh, and your specific question: Around here the 25s simply go too fast. And my lone example tends to run away on me. If I can get a 25 to run in the same manner as my four 20s, I'll put the 25 in my Zilch X, and it will be absolutely deadly at the OTS circles.

Dan

[eklee]

Hi Dan, I really enjoyed folllowing your posts on the FP20. I learned A LOT. Thanks.

I have been flying a FP20 ARF FS for the whole summer and really improved my flying because I didn't have to tinker with the power train. Unfortunately, I totalled the FS yesterday when my vertical 8 became a vertical 9. I now learned never to chew gum while learning new manuvers. My simple brain couldn't focus properly.

Anyway, I just want to express my appreciation in your dedicated promotion of this setup.

My next ship will be a Skyray 20. Any recommended mod to the plane?

BTW, both my FP20s are of the iron piston version. Do you know if there is any difference in run characteristic?

Eric

[dirtydan]

Eric,

You are welcome, although I hastily interject this is being done for selfish reasons. Locally we've got guys who are just on the cusp of getting more seriously involved in CL Stunt. But their equipment sucks, and so they think they can't fly worth a rat.

A 20FP on a wide range of profiles is in almost all cases, up to and including Advanced PA fliers, demonstrably better than whatever they are flying currently.

So, again with selfish motives, learning something new myself, passing it along to others, has added to my database while seeing increased (successful) participation in an event which needs as many as possible.

As long as I was on some sort of campaign, very little additional work to take it national, especially via my column in Stunt News.

So here we are...

I would like to correct you slightly. Not only have you not had to tinker with the 20FP, the power delivery is such that one can actually concentrate on flying the maneuvers, whether chewing gum or not. There are no blasts of excess "power" to screw up your timing, overall the regulation of speed is simply outstanding.

As I don't know what I am doing in any real sense when it comes to design, my best advice is probably to just build your Skyray stock. At least we *know* that works. Brett's own Skyray 35 is indeed stock, although after first crash he replaced lite-ply ribs with balsa pieces. Yeah, it's now lighter, but that hasn't seemed to change the way it flies; it is, however, now much more crash resistant.

Second best piece of advice is not to re-name the model "Skyray 20" as Derek Moran and I have done. Instead, trick 'em, re-naming it "Skyray 3.5," as a 20 is close enough to a 21ci/3.5cc...

Sept./October Stunt News--I haven't seen my copy yet--said to contain Bad Boy Stunt column, and this was submitted with pictures, some of which show my current Skyray. So third piece of advice is to look there for ideas. And on SSW Derek (mpa) has recently posted pictures of his new Skyray. Definitely worth checking, especially semi-weird tank setup.

Finally, totalling contradicting myself, the actual *best* piece of advice is to note that *it's not the model which is imporant, certainly it is not any minor changes to an existing design which is known to work.* Instead, it truly is all about the engine. Keep your priorities in their proper order.

I have been told the iron piston engines, other than hugely different fuel requirements, operate in the same manner. I do not know this from personal experience, all four of my engines being ABN.

Dan

[oldcajun]

Dan,
On a slightly different subject, how did you modify the Combat Streak to fit the narrow engine? I have an old kit I was going to build for a Johnson BB Combat engine I have left over from the good old days, but based on your BBS column in Stunt News, it looks like it can be a seriously good flying airplane.

The nose is so short, I never could get a very long run on any kind of 35, but with the little FP's, a 3 oz tank should be plenty.

I have looked at narrowing the whole front of the fuselage about 1/8 th inch, but then the 2 inch wide tank won't fit. Unless you have some better advice, I'm planning to shim the mount beams in by using 1/16 X 1/2 inch plywood strips glued between the fuselage doubler and the beams. I also plan to replace the lite ply doublers with 1/16 plywood.

Since I fly mostly off grass, I plan to use an aluminum 2 wheel gear bolted to the bottom of the engine mount crutch.

I sure wish I had put narrow motor mount spacing in the El Diablo I'm finishing, but guess I will go with an L&J Fox for it.

Steve

[dirtydan]

Steve,

My Super Combat Streak was actually built for a 25FP, so there was no problem in slipping in a 20FP. I used a 3-ounce hard tank, and as I recall relieved a portion of wing LE in order to get come clearance for rear of tank.

I am currently building another SCS, quite likely for Classic Stunt next year, so well has what began life as nothing more than a sport model worked in a couple contests. I am building the fuselage to match external dimensions, gaining more room inside by using 1/16" balsa laminated to 1/64" ply fuselage sides, the ply suitably cut up and/or holed for lightness. More 1/64" ply will be used as triplers from nose to about high point of wing.

Part of the reasoning here, other than a suitably stiff fuselage, is that hard tank can be cocked slightly, rear outboard corner to outside of circle. Better cut-offs, more reliability in cut-off loops at end of pattern.

As to narrow engines, the case for 20 and 25 FPs is not actually all that narrow. Still, I never make engine crutches from specified stock. Instead, over-size material, sometimes 3/4" x 3/4", is used to make assembly, when dry is shot through a table saw until it has desired proportions. And it's always flat...

Also, Steve, I noticed that I overlooked the fact that you are flying on 60' lines measured eyelet to eyelet. With my "demo" (read: stock) ARF FS, I am using 60' center-to-center lines. That extra 2 1/2 to 3 feet of line might be part of the problem. I once inadvertently flew this same model on .018s and it was truly horrible.

And last night I looked over my notes on 20FPs. Of the bought-used engines, the strongest pulled 13,400, on 10% fuel and APC 9-4. With 15% fuel it went up to 13,740. No air fliter. I have since rebuilt two engines with new piston/cylinder; allthough both have been flown a fair bit, have not taken any readings. Would be a little surprised if 13,200 to 13,500 is not representative of what these engines will pull, assuming all is mechanically correct.

Note that if your 20 won't even come close to above performance, but the 25 does, well, you're probably running in-air revs pretty close to what I use, meaning you are making the engine happy by running in the range where muffler works as designed.

Dan

[Garf]

How close does the Magnum GP25 come to the OS 25. I put a TT 25 P&C in one. I need a good venturi for the magnum, but I don't think the OS will fit.

[dirtydan]

Phil,

If you're asking me, I have no experience with the Magnum 25.

Dan

[Kiwi]

Garf said:

How close does the Magnum GP25 come to the OS 25. I put a TT 25 P&C in one. I need a good venturi for the magnum, but I don't think the OS will fit.

AFAIK, the last Magum GP's were the same engines as the TT-GP's, before the change from Taiwan to mainland China for Magnum. If I'm right, check with your fellow Floridian, of Plastic Fantastic fame. He sells the CL versions of TT-GP's and has the NVA's, venturi inserts, etc.

I'm having one of those Senior Moment kinds of things and cannot for the life of me pull Mike's last name out of any mental file! As soon as I sign off at the Hangar here, though, it'll probably come back to me! Last name starts with an "O".

I can tell you that the post-changeover XL-A Magnums (BB shaft) are probably 5-7% stronger (in 25's, anyway) than the FSR/SF/FX OS Maxes that they are based upon.

[catdaddy]

Kiwi said:

I'm having one of those Senior Moment kinds of things and cannot for the life of me pull Mike's last name out of any mental file! As soon as I sign off at the Hangar here, though, it'll probably come back to me! Last name starts with an "O".

Mike Ogden isn't it?

[Kiwi]

Thanks for the reminder. It's Ogren with an "R" instead of a "D", but you were close.

I might as well pull up the Phil Brown CL Clearing House and get the web site URL for Mike's company and make the reference complete, though.

(Clearing House is home.att.net/~philbrown36641s5/clhomepage.html)

Mike's company is Pro-Tech Aero at www.protech-aero.com/

Incidentally, the last time I heard from Mike, the 40 size Radical Speed Limit foamies had been dropped.

[dirtydan]

While we don't hear a lot on this topic, there is something happening out there. I have one 20FP in need of a new piston/cylinder set, another which has a lot of crash damage, needs new case and piston/cylinder set.

In ordering these parts and a few other 20FP-related pieces, it turns out that wrist pins are not in stock. And I got the very last piston/cylinder set in stock. Needing two, of course.

The good news is that latter assembly is only temporarily out of stock, former is expected in early October.

I guess the message is that the 20FP was fairly recently a current, in-production engine, Tower is still supporting it with parts, even though O.S. is firing up old tooling in order to supply.

I mention this as one of the best ways to get a 20FP seems to be in buying the sorriest 25FP you can find, possibly even getting it free. Rebuild with 20FP piston/cylinder, head, wrist pin and gasket set. Assuming an RC motor, fit with *stock* CL venturi and NVA.

It also needs to be noted that a rebuilt 20FP, assuming reasonable care and Stunt-specific fuel, can be expected to last a good long time. Certainly enough time for us to "discover" an engine which offer same levels of power, same superb speed regulation.

Dan

[tperry2054]

Dan,

I may be telling you something you already know, if so, your trash can is as good as mine. You can go to an industrial supply place and order some drill rod the same size as the wrist pin you need and make one (or more). Even if your wrist pin hole is slightly egged out you can drill it out round and go to a larger size. And the best news of all ta da! Give me your snail mail and I’ll send you a case. Chaching: freebee, nada, no cost said in my best Monster Garage fake voice. I don't know if it is a twenty or 25 case but if you're not fussy should work fine. Matter of fact rather than take it apart I'll just send the whole engine and you can use what ever you want off of it. It doesn’t have good compression but the wrist pin might be ok. I will keep the venturi and NVA though; I’ve got my limits.

Tight Lines,

Tom Perry

[dirtydan]

Tom,

A free 20/25FP?! I am so, like, there.

Dan Rutherford
4705 237th Place SE
Bothell, WA 98021

While I don't want to spoil any surprises, no one has ever given me anything but what there was not a response in kind. Do be sure to include your return adress. Cannot be a P.O. box...

You are correct, there is an easy recourse in many cases when it comes to gudgeon pins (there are Canadians and Brits reading this, after all).

The real problem is the tubular pins are fitted with teflon pads at each end. And these have been known to wear, get lost, etc. I actually only need the pads, but if buying a wrist pin or two is the only way...

Thanks again,

Dan

[tperry2054]

Dan,

I forgot to mention this link:

www.udisco.com/hobbies/index.htm

They are out of the 20FP piston Cyl sets the US price is 15.32. They do have the writ pin for the 20 FP. Sells for 2.76. The Gasket set sells for 1.19. From your previous posts I don't think you have a lot of use for the .25 but they are in stock and the US price is 14.24.

Click on the Radio Control box on the left then when the OS logo comes up on the main page, click it and feast your eyes. I think the prices are much cheaper than your source but check and make sure.

Tight lines,

Tom Perry

[dirtydan]

Tom,

We continue to be on the same page, although others will surely benefit from this link.

Yes, I have looked at the selection. And the prices.

However, I order so infrequently from Tower that they are currently slobbering all over me, almost begging for an order. Or at least their computer noted my absence, triggering a very attractive discy kewponn.

Funny enough, another reason for ordering from Tower was the Canadians being out of stock on piston/cylinder sets, U.S. dudes (supposedly) having them in stock. Well, 50% of what was needed anyway.

Another issue is returns. While I have never had to return any of my buying mistakes, learning to use single-edge razor blades when intending to by #11s in bulk, for example, some of the parts ordered were done so on a guess. A corporation can deal with trans-border shipments; it has never been quite so easy on my end and as a consumer.

Still, the good news is simply that one can get all the parts for 20FPs; and judging by listings for other motors long out of production, this will be the case for years.

Maybe I'll just order a bunch of writspins from Canadian distributor, strip them of needed teflon pads. And send you pins themselves, whereupon you can grind them into drills. Works for me...

Thanks again,

Dan

[TreeTopFlyer]

Dirt,

Cool spot...and you can talk about the FP20.

I know you hate smileys, especially short ones, but there it is.