Converting OS20 FP and OS15 FP from RC to Free Flight

[binbag]

I have just picked up an OS15 FP and OS20 FP which I want to run in SLOP models. I have a full workshop (lathe/mill etc) but would like to know if there is any information on how to convert these engines to use in FF. The OS15 I may use in a Dixielander or Dreamweaver for classic power rather than SLOP, but thats a side issue.

I am relatively new to power, and bought the engines on advice as good start points for SLOP engines if converted.

Can anyone help?

Many thanks
Binbag

[I hate trees]

Hi Binbag,
Welcome along!

I too am relatively new to power and have also got some OS15 and 20fps to use in free flight power models. Originally OS did parts to convert the engines for use for free flight and control line. These are the venturi and needle valve assembly. Please see photo attached of a 15 and 20 with the venturi and needle valve. These parts are still available from Tower Hobbies in the USA, I think the venturi is about 7 dollars and the needle valve 12 dollars. Until recently they were listed as still available from Ripmax in the UK, as soon as I inquired in my local model shop they were withdrawn from the Ripmax website!

The venturi is a simple thing to machine in a lathe. The bottom part of it will be the same as the rc carb fixing, only the holes for the retaining screws will be used by the needle valve assembly. The needle valve holds it all in place. For the 15 sized engine the hole in the venture to let the air in should be about 6mm to run on suction (so I'm told) and about 7mm when on pressure. I don't think the actual shape of the venturi is that critical (I've seen lots of variations on different people's models.

I presume it would be possible to use a needle valve from a different source, for instance Nelson ones from Doug Galbreath, but I haven't got around to finding out about that.

I hope this is of some use, I'm sure there are people out there with a LOT more info to impart on this sort of thing, I too will be happy to hear it.

See you on the flying field sometime soon
Adam

[Scottl0413]

Adam and Binbag, If you are going to run SLOP than you'll need a venturi in order to run suction. But if your running Classic gas and can use pressure you don't need a venturi at all. Both engines will run fine on bladder pressure without a venturi.

Scott

[binbag]

Adam ;

thanks for the help - i have contacted tower hobbies to try to get a needle valve assembly so I can get this engine converted for free flight.

Pity nothing is available for the UK.

I have sent you a PM by the way
Thanks
Ralph

[glidermaster]

Hi Ralph,
You've picked 2 great engines to start flying power with.

My original 15 and 20 are fairly old, and have the original OS CL venturi/spray bar assemblies, but the last 15FP I picked up a few years ago has the rear mounted needle, so my fingers are a long way from the prop. I really like that, even though these things are not real speed demons! I like it so much that I am experimenting with wick feed carbs for F1C engines (the CL Speed guys seem to like them).

I personally don't like the OS needle assy. with its clicky detent spring. I much prefer the Rossi/Super Tigre friction lock system, and Doug Galbreaths versions are excellent, and not a bad price, either. Adam is about right on choke dia. for suction, but it is better to start out a little on the small side, I think, to get consistent runs and above all, confidence.

You can shave a few grams of the engine weight by removing/reducing the cooling fins on the head. You don't need them for f/f.

Moderate nitromethane (15% is plenty) and Master or APC props. My 15FP uses a 8x4 Master cut down to 7.5 inch dia. On the 20FP I use a full 9x4, but that might be a bit much, I'm thinking to trim a 1/4" off the diam. and see what happens.

Good luck.
John

[jbfly]

On APC props the tip is EVERYTHING. Why not buy a smaller prop, and or different pitch? I fly RC also & if I loose more than .25 in, the performance goes way down.

jbfly

[glidermaster]

I agree w.r.t. APC props - I only ever crop the square tipped Master props - I should have made that clearer.

[Scottl0413]

Ralph, I'd suggest using the APC 7 x 4 or 7 x 5. I use them on my OS MaxIII's and Fox 15's and they are real performers.

Scott

[I hate trees]

John Thompson recommends via email a Master 8 x 3 for classic events, where you get 12 sec runs. This will accelerate the model faster but the model will not keep accelerating, thus trimming for the end of the run and transition are less critical.

There, John, I've told them. Why don't you join up and tell them yourself instead of lurking in the background?

[binbag]

Adam;

thanks for the John Thompson (or is it Lord Thompson) information and I must admit I have been digesting his information in the 1066 Clarion. I say digesting, but I think gorging is more of an appropriate word. It all helps with experts like this supplying information hard earned over a umber of years.

Ralph

[PeeTee]

There, John, I've told them. Why don't you join up and tell them yourself instead of lurking in the background

Adam

That's because he likes baiting us poor minions in the background - much more fun I have received a missive from him this morning, and replied in kind. I intend to call him later for a quick wind up on electric models!

On a more serious note, and without personal experience, I remember reading an article by a noted power flyer saying that retaining the RC carb isn't a bad thing. The slow running position can be adjusted so that the motor stops, and as this cuts off the air supply, it means that neat fuel is drawn into the motor acting as a 'sorta' flood-off. Instead of the pinch-off or valve mechanism, just attach a spring to the slow running arm on the carb so that it closes when the timer releases an a mousetrap arrangement. I realise that the venturi is likely to be of smaller diameter, but that might not be a bad thing if you are stuck with suction feed as in SLOP. Also I recall that you used to be able to buy 'tune-up' carbs (for RC) giving more power. They were made by Perry if I recall correctly and must surely be as cheap as chips on the pre-owned market! Anything to get rid of the burble on the over-run now that the FFTC has kicked the rule change into touch again.

Peter