I'm building a 64" wingspan Stearman PT-17. An old Sterling kit as a matter of fact. Ha, I should say that I am building the plane from scratch with the kit as a guide since most of the kit is unusable, it being 40 or 50 years and all. Anyway, I am wondering if anyone has any 4-stroke engine size formula or recommendations? I typically use 2-strokes and the kit calls for a .56 to .65 2-stroke but I would like to use a 4-stroke in this plane. I have a saito .80 and a Saito 1.20, but it seems to me that the 1.20 is too big and the .80 is too small. Any thoughts?

Hi,

The Saito 120 may be too heavy not too big! If you can balance it with the Saito 120 without adding extra weight that is what I would use. If the budget could stretch to it I think a 90 four stroke would be ideal IMHM.

Cheers,

Colin

buy a magnum .91 fs http://www.hobbypeople.net/gallery/210868.asp they are decent engines
although the 80 may be enough, at least for the maiden, if you dont like the power get a bigger engine. just dont cut the cowl till after you fly it

The Saito should be about perfect for that Stearman. You still may have to add some nose weight. Any Bipe that had a round motor is going to be short coupled, they were designed that way to compensate for the very heavy weight of those round engines they used. Most of the 17s or Supers I have seen have the 1.20 OS up front, pretty much required due to the added drag of the Bipe. Anything smaller and your going to be walking around saying things starting with, I wish in front of them.

Thanks for the replies. I think I will stick with the 1.20 like I originally planned even though it seems big. I had a smaller bipe several years ago the flew with the .80 and thinking back on that it did not seem overpowered at all so I believe the .80 is out of the question. That Super Stearman did require quite a bit of extra weight up front also so possibly the bigger engine will be a plus. Thatnks again for the info and opinions.

I was going to say the same thing, that Stearman is going to need nose weight. If you need nose weight you might as well make it an engine.

The OP is talking about a kit too and not the GP ARF. I have set up a couple of the GPs and they are a very light plane, with the OS 1.20 they both still needed nose weight. The Super Stearman I set up used the pumper so there was a tad of extra weight there already. A trick I used to do when I was still building Bipes was to make the fuse nose section longer then mount the engine on the fire wall. The fire wall was the last thing I mounted while the plane was still in the bones but the controls were installed, I would just pin the fire wall in and move it around until I found the spot where the plane CGed. Then mount the fire wall there and cut away the unused section of the front of the fuse. It isn't true scale but it kept the lead out!!

Thanks again. Grey Beard I like the way you think. I was considering moving the firewall backwards because the length of the OS 1.20 is way more than the original design allows but your last post made me rethink this. I think instead I will readjust it to help compensate for the weight needed. Scale is great but not absolutely necessary for me.

That's my thinking. One of my friends and I went through this not too long ago. I built a big Boeing F4B-2 from plans some years ago. Even with the big OS 1.20 pumper I couldn't get it to CG, everything of any weight was right behind the fire wall and it still needed several POUNDS of lead in the cowl to CG. Last year I sent the plans to one of my friends and mentioned making the fues nose moment longer but he was shooting for that scale look. He ended up going with a 40cc gas engine with everything up front but did get it to CG. An inch or two of length on the nose moment would have made life easier for the set up. Also if you have any way of making the back of the plane from the CG lighter do it. On that plane I didn't and it's now just hanging on a wall, it never was a very good flying machine and was never fun to fly at all. The people at the IMAA events liked it on a fly by but that was about it.
Gene

I am enjoying my GP PT-17 that I restored from someones wreak and it flys wonderfully well with its new OS umpumped 120FS and yes the airplane has a lot of nose lead. I just did the maiden on a friends super stearman version (basically two more ailerons, a cowl and wheel pants) with a G-23 gasser with no nose weight. It flew well also but really no better and if I had a choice I much prefer mine with the 120FS.

Now those are much, much larger airplanes than your Sterling and I am convinced that the a 120 is a gross mistake for that ship and Pardon me while I go take cover from the incoming flak that is certain to come my way

John

I have two PT-17s, one with a OS 120 and the other with a Zenoah 38 magneto. Both have advantages and disadvantages, the 120 version is a bit lighter and sounds great, but uses expensive glow fuel. The Zenoah is heavier, but uses gas which is a lot cheaper. Forget the 80 as it actually uses about as much fuel at full throttle as the 120 uses at 3/4. You really swing a big prop on the PT-17 to make it fly right. If you need anything bigger I would suggest you go to a gas motor.

The bottom line is the fifty year old Sterling design is a far smaller airplane and any 120 four stroke or gasser is wildly inappropriate.

John, can I ask how big the planes you are referencing are? I Believe the GP Stearman is around 70" if I remember correctly. Does it have extra power or is the flight more "scale". I am a fan of more power than more scale flight but do not want to be overpowered. I typically do build with a larger engine than recommended , within reason, as long as I feel the airframe can take the extra load. I have manufactured my own I-beam spars and have added carbon fiber strips. If the larger engine would help balance the plane then my only limitation is being able to keep my finger off the throttle. Any thoughts on this? Of course I would rather use an engine I already have in my workshop then purchase a new one, but it's not out of the question.

Both of the ARFs from Tower are listed as 71.5. When the plane is in the bones try putting it together and mount the engine and find out how it is going to CG. I do that anyway, I don't hard mount my gear until the plane is almost finished, I just tape servos and stuff around the plane and see where it all wants to go then I hard mount. John may or may not be right, I learned never to argue with him a long time ago but I'm thinking he could be wrong this time. Maybe???? Can't wait to find out though.

GB is right I may be all wrong but lets see? Seems to me its a long ways from 64 to 71inchs especially a biplane. And I'am betting that long way is just about the differance between a ninety and a one twenty FS's.

If one makes the asumption that a 120FS is ideal for GP's airplane and I do based upon flying two of them with a 120FS and the other with a G23 that would seem to indicate the ideal engine size for the Stirling airplane would be a 90FS considering modern taste's and power to weight ratios. Another factor here while yes the GP airplane is light by modern standards but its a tank by RC standards of half a century ago. I don,t know your airplane but its quite probably a lighter built structure.

OK guys sorry for playing the devils advocate but hey whats the point of asking if everyone agrees, So yup I think a 120 is overkill. Gotta run now Incoming, I.am heading for cover

John

A 1.20 4 stroke would be great as it'll give you the low end grunt without having to rev. EXACTLY what you're looking for.<div>Use a MAS 16x6-8 classic prop. You'll be amazed at the sound. The prop blade, even though looking great, is such an inefficient design compared to todays APC's, but for once this is to your advantage. Those big ol' bades move so much air and create such a wonderful sound compared with a modern design, APC or similar. Try the combo: Stearman/1.20 4stroke/MAS classic 16x6. You'll be SUPER happy.</div>

No need to duck John, it has been a couple of years sense I have built the Sterling, maybe a few more then a few?? I don't recall much about the plane but I'm going with how much lead and shifting stuff around in the GP was needed. I have yet to get enough weight forward in any of my Bipes that had a round emgine without lead. I can't wait to see how this one turns out.
I'm covering my new {old} little Gasser today, my first stab at electric power. My first stab at LiPo too!![8D] I'm moving slowly out of the stone age. This is fun, I don't have a clue as to what the heck I'm doing but all the cool stuff is working! The prop turns and the servos go flippy flop. As long as nothing burns out I guess I'm doing something right?
Gene

Oh good grief the sky is falling and you are going over to the dark side! I know the Trogladite ladies just don.t appeal to you any more?[8D]

I have a few electrics and enjoy them a bunch but sure would not want to make it a habit! Hm maybe its a sound thing? or perhaps its the aroma of a pinch of nitro in the medication cup on my nebulizer thing, Ya thats it[X(]

John

It would be useful to know what the projected finished weight of the 64" completed kit is and also the wing area.

Just a little follow up on this question. I have decided to purchase a Saito FA-91 engine to power this plane. I found several examplesof other builders who used 90 size engines with good results. So wish me luck and I will try to post a follow up following the up.

CADgeek,
You should have no problem with the Saito 91.I restored a Sterling PT-17 several years ago from a basket case.Covered it with 21 Century fabric and used plastic coated wire fishing leader for all the scale rigging.I used the dummy scale redial from an unbuilt kit I had;mounted my 91 and tucked the dummy in around it.The 91 was an older one with the small diameter muffler pointed downward.When finished;the engine and muffler blended in beautifully with the dummy radial.To get the CG,Ijust bent Great Planes stick on's and secured them around the face of the firewall. As for flying; it is a little twitchy on rollout until some speed is built up so manage your rudder and keep it fairly flat on liftoff until you have good airspeed. On landings,carry some power over the threshold and flare about 2 off the deck.A little up elevator after touchdown will keep the tail on the ground.
The PT-17 is no speed demon with this engine but it does do some sweet scale aerobatics.Power management is the key.
Good luck,keep in touch,
JB Stearman
[email protected]

This was posted almost a year ago but I would like to find out what the finished weight of the plane was.

Gary, still trying to figure these forum dates etc. out. My Stearman is about 12#and really does flies well.My Saito91with a 15-8 prop at idle remindes me of my Ford Model A engine with the spark lever retarded.Check my gallery if you want.I'm kind of partial to stearmans.
regards,Jim

I went through a bipe period that lasted for years, I was called the bipe guy at my field. I got out of that period of my flying but still like them. Last week I gave away my 60 size Aeromaster to one of the guys I fly with. I ended up using the OS .91FS in that plane but it started out with an MDS .68 two stroke then a YS 1.10FZ, the OS .91 was the best choice for how I wanted to fly the plane.
I fly 60 to 90 size IMAC or pattern planes now but I'm now looking to build something different?? I have plans for golden age racers that look like fun but I keep looking at the plans for a nice giant scale RYAN??? I just can't decide, I want something different but there are so many choices!!!!!!!!!!!!

Grey Beard, This Bipe is only now getting to the mechanicals of the plane. The coming of summer, family events and way too much work led to this project getting shelved for a while. I am just now dusting it off and getting my head back into the build. I will keep this post updated on the progress. As it stands right now I am still going with the Saito .91 for power but have not done a mock up yet to try to determine CG and weight.