Hello, I recently picked up a .60 size pattern plane ( not sure the model) with an OS 61V. I'm wondering if someone could help me with suggested prop size for pattern flying and maybe the practice rpm range. The headed appears to be full length which I assume based on my experience with tuning .45's is not correct. This will present a problem if I have to shorten it because the pipe is covered on top of the fuse with piping routed and exiting the bottom of the plane but I can probably resolve that.

Thanks in advance for any advice.

Cody

Cody,

if the exhaust is pre-routed and this is a second or third hand pattern model, there's a good chance that the builder and/or previous pilot set the engine up properly and the exhaust is tuned. Length of exhaust is dictated by RPM band of the engine which in turn is dictated by engine design and prop size.

The OS 61VF (front intake), which is what I presume you have (the VR, rear intake, is a different engine), is a short stroke classic engine which has a design power band in the upper RPM range - roughly 13,500-15,000 RPM. To turn those RPM's you will want an 11x7-11x9 prop. Before changing the exhaust length, you should run the engine with these props and record the RPM's. If the exhaust is long, it's possible that the previous owner(s) was running it in "long stroke" fashion with props like 12x8-12x10's. It was doable but this engine wasn't optimal on those props.

If you can get the engine to turn in the upper 14K RPM on a wood 11x7, the exhaust should be roughly right. If it doesn't make it to 13K, it likely was setup for the larger props.

In the end, what prop you use will depend on how you want the model to fly.

David

Thanks a bunch David. It is a front intake engine. Those are good numbers to look for and I have it on a test stand now to see what kind of rpm I get with 11-7,11-8,11-9. Thanks again for your help.

Cody

Cody,

Glad to be of help. Of course, if the engine and pipe on the stand are changed, then my commented approach doesn't quite apply. The engine, if it still has good compression and you prop, plug and fuel it right, will approach 15K rpm (probably more like 14.5K on 10% nitro and an 11x7) but that is dependent on proper pipe length as well. You might get the engine there but in so doing, you'll likely also have to change the header length. If the exhaust system is routed for the design of the model (it's nice to have the exhaust diverted out the bottom), getting the engine in it's optimal RPM range might result in the exhaust system not working for the layout of the model.

I have to say I'm sort of commenting in the dark a bit as we don't know what pattern model you're talking about nor how the exhaust is setup in detail. Can you take some snaps of the model with good side and top views. The design of the model also affects what type of flight envelope it should have. Given the vintage of the engine, you likely have a late 70's, 80's design (i.e., ballistic pattern) but it could also be that an older design engine (short stroke) was used in a later pre turnaround (long stoke) pattern design.

Looking forward to seeing some pictures!

David

Looks like 12,500 with an 11x9, and about 13,500 with an 11x7 on 10% nitro.
Here is a pic of the plane.

cody

That's a Wolfgang Matt GRANAT / ARROW 800

It sure is! Thank you, I had not been able to identify it.

cody

Yup, what Paul said - an Arrow 800. Looks like one made by Hobby Barn (Quality Line Kits) too.

I suspected it was an Arrow...

If I recall the dimensions of that fuse and it's pipe deck/tunnel, the exhaust setup you have shouldn't be "short" for the VF. Rather, just right.

Are your RPM figures measured (tached) or are you estimating at this point? If you measured, what props, fuel and plug did you use? What does the header and pipe look like?

Should be a fine flying classic!

David

These were tacked rpm with 10% powermaster fuel and APC props. Looks like a Mac header and pipe and I started shortening it and performance was increasing but I ran out of daylight. I'm taching 13,200 with an APC 11x9 now.

Any further progress on the tuning and model?

David