OS engine ?

Thank you Jason for the information. Appreciate it.
Jim

No- there are major differences in footprint between the two: weight-25FP 244g 35FP 335g length-25FP 3-1/2 inches 35FP 4 inches
crankcase clearance 25FP 1-1/8" 35FP 1-1/4" muffler length 25FP 4-1/4" 35FP 5"

All actual measurements taken from my own examples-with regard to the weights quoted-these are the R/C variants-with a standard C/L venturi and NVA they would be slightly lighter.


the 20/25FP have comparable footprints-as do the 35/40FP but not the 25FP vs 35FP

ChrisM
'ffkiwi'

Thanks Chris for that information on the differences between the OS engines?

Is this a FP engine? I don't see the FP marking.

https://www.ebay.com/itm/164436440373?ul_noapp=true

Looks like an OS 25 FSR series engine. Ball bearings and the older style muffler.

Thanks Jason, is it as good as a Fp .25? Looking for one to put on a sig Skyray.

FSR was probably OS best engines.

Here are the mounting lug cards for the 20-25 FP and 35-40 FP.

oldguy wrote:Thanks Jason, is it as good as a Fp .25?  Looking for one to put on a sig Skyray.

Comparable-but a bit heavier, being a twin BB engine. 254g (FP) vs 279g (FSR)....the FP might well have a slight edge on top end power, being a later design.

ChrisM
'ffkiwi'

Also keep in mind that the early FP's were iron piston and not as high rpm's as the later ABN versions. The exhaust side of the case of a iron piston version has a large bump in the casting. A LA .25 has virtually the same power as a FP and also shares the same footprint so if you can source the LA, it's equally as good. One thing that I will say about a LA vs a FP is that if your off on your needle and your too lean it won't be as much as a issue. The FP on a lean setting can run for what seems to be a eternity on 2 ounces of fuel and it's going to be real fast.

Here is a OS max FP.40 example. Is this an early iron piston, or the newer ABN version FP. I would think the FP .25 would look the same is that right. So the ABN newer version FP's would be the one's to look for?



https://www.ebay.com/itm/OS-40-FP-RC-model-airplane-engine-40-glow-fuel-/143762234910?_trksid=p2047675.l2557&ssPageName=STRK%3AMEBIDX%3AIT&nma=true&si=%252FHR5gGl5cHU5mC2rWj4xZ9733Vo%253D&orig_cvip=true&nordt=true&rt=nc

That FP .40 in your picture  is the ABN version and the other sizes will be identical to that as far as the bump is concerned. It all depends on what your doing with the engine. I like the iron piston versions. I feel they can swing a bit more prop and pitch but they will vibrate slightly  at higher rpm's. Dan Rutherford aka "Dirty Dan" was quite popular many years back for exclusively using the ABN  FP.20. The FP.20 makes good power for it's displacement. I own both versions and I was using the iron piston version on a original kit built Flite Streak. The ABN version is a lot more forgiving in regards to a bad needle setting.

       There's been quite a bit of testing using the suggest FP and Skyray including launch rpm's. Using this information with the iron piston version may not yield such good results. I'm currently using the FP .20 ABN version on a vintage combat plane made by Consolidated which is the Jubilee. While not a period engine, it runs excellent and I enjoy using it.

So then I'm looking for the ABN version FP .25 for a sig skyray and a super Satan. I have two of the ABN FP .40, but there too big for these two planes i will be building this winter.

I have a club member who uses the Tower .40 exclusively on many profiles. He currently has one on a smaller profile which is the Brodak Galaxy and Twister and Brodak profile Oriental. The Tower .40 is a beautiful running engine. All parts from the Tower .40 interchange with the FP .40. The Tower is a true ABC engine . I have a few FP's with the Tower .40 piston sleeves in them. Either or, hold onto your FP .40's they're excellent engines to have and work well .

How does the OS MAX 25 SF stand up to the .25 FP?

I never owned a SF, I've had the FSR and appearance wise they're quite similar. The FSR did have a true chrome cylinder ABC and that engine was the Cadillac of OS. The SF was a ball bearing engine vs the FP with plain bushing. The FP is a lower performance engine as far as r/c goes so I would venturi to say the SF offered more power. At the time the SF was being offered, the FP was also probably still in the iron piston vs the ABN.

If you have an iron lunged FP 20 or 25 with the correct muffler and venturi it will run like the Brett Buck setup for the box stock LA25.  The OS website calls out the muffler and venturi in the LA25 parts list.  The older muffler will not provide the desired run.  

While I am sure the ABN vs iron has other differences, for flying pattern on a Flite Streak or Skyray the power and performance will be there.

http://www.aeromaniacs.com/brett.htm
APC 9-4
11,300 rpm

The "2030" muffler (silencer in the instructions) is the important part of this system as it is a tuned pipe.  (the 871 does NOT perform) the venturi diameter is not listed on the osengines website.

The footprint is close enough to a fox35 that it should bolt right up unless there is no wiggle room in the bolts.

Phil

how does the OS MAX .20 compare to the OS max FP .20?

Similar physical size and weight-but a cross flow engine compared to a schneurle-so somewhat less powerful. Timed for a lower rpm peak-so might in fact be a better choice for your proposed CL stunt use than an FP. You can find the engine test on the Max-20 here: http://sceptreflight.com/Model%20Engine%20Tests/OS%20Max%2020.html

Tens of thousands of them in use during their day....I still have three or four in my inventory. They were the direct successor to the OS Max-19-so the design dates to the mid 60s....though-as the engine review points out, it didn't appear on the market till quite a bit later-in the early 70s

Unfortunately there is a bit of a dearth of data-in terms of english language engine tests-on the smaller FP engines-the 10, 15, and 20-so you have to rely on the manufacturers figures....which are often...how shall I put it...optimistic....

If the intended purpose IS for CL stunt-then I would happily recommend the Max 20-and running on a 10x3-1/2 or 9x5 (depending on your preferred flying style-slow or faster) which it should turn around 10.5K.....and they are still plentiful on the second hand market. Spares of course are entirely another matter....

The FPs do have something of a reputation for not being entirely tractable when use for CL stunt-ie a reluctance to hold the 2-4-2 break reliably...and a tendency to 'run away' once they get a bit warm during the flight. You would probably have to drop the prop size slightly compared to the Max 20 to get one in its power band....which might or might not affect the way the model flies as a consequence...on the plus side-being more recent, spares are more available-but I should qualify that by saying that earlier OS designs were much better supported in terms of spare parts than more recent ones...I suppose thats a simple result of the OS range having proliferated vastly over the decades-and the more engines and variants you have, your spares requirements increase exponentially....as does the time required to both produce them and manage inventory..

ChrisM
'ffkiwi'

A good analogy would be like comparing a Vega to a Corvette. The older Max engines are well built but they don't offer the rpm's like the FP series. If kits state that it was made for engines .19-.35, you elect for the .35. The same goes for power, you can de tune, you can't extract more. If you want a twin, you choose the most anemic engines to do so, the early Max series works great for doing just that.

ffkiwi wrote:Similar physical size and weight-but a cross flow engine compared to a schneurle-so somewhat less powerful. Timed for a lower rpm peak-so might in fact be a better choice for your proposed CL stunt use than an FP. You can find the engine test on the Max-20 here:  http://sceptreflight.com/Model%20Engine%20Tests/OS%20Max%2020.html

Tens of thousands of them in use during their day....I still have three or four in my inventory. They were the direct successor to the OS Max-19-so the design dates to the mid 60s....though-as the engine review points out, it didn't appear on the market till quite a bit later-in the early 70s

Unfortunately there is a bit of a dearth of data-in terms of english language engine tests-on the smaller FP engines-the 10, 15, and 20-so you have to rely on the manufacturers figures....which are often...how shall I put it...optimistic....

If the intended purpose IS for CL stunt-then I would happily recommend the Max 20-and running on a 10x3-1/2 or 9x5 (depending on your preferred flying style-slow or faster) which it should turn around 10.5K.....and they are still plentiful on the second hand market.   Spares of course are entirely another matter....

The FPs do have something of a reputation for not being entirely tractable when use for CL stunt-ie a reluctance to hold the 2-4-2 break reliably...and a tendency to 'run away' once they get a bit warm during the flight. You would probably have to drop the prop size slightly compared to the Max 20 to get one in its power band....which might or might not affect the way the model flies as a consequence...on the plus side-being more recent, spares are more available-but I should qualify that by saying that earlier OS designs were much better supported in terms of spare parts than more recent ones...I suppose thats a simple result of the OS range having proliferated vastly over the decades-and the more engines and variants you have, your spares requirements increase exponentially....as does the time required to both produce them and manage inventory..

ChrisM
'ffkiwi'

Chris thanks for all the breakdown on the differences on the OS eng. I'm looking to put a .20 on a Super satan combat, so i would be wanting the Fp series.