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OS FS 91 is it capable ?

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OS FS 91 is it capable ?

Old 08-22-2008, 02:01 PM
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rthalls
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Default OS FS 91 is it capable ?

I just finished a sig senior that I enlarged to 93 inch WS and longer and bigger all around , finished it with dress lineiing and painted.
my question is that it came in at 14.5 lbs and had plan to use a OS FS 91, will this be enough engine for this weigh and size ?????
thanks for any input
Old 08-22-2008, 10:21 PM
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chashint
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Default RE: OS FS 91 is it capable ?

10 pounds is my absolute limit on the 91 four strokes, and I am a lot happier with them when the weight is 8 pounds or less.
It all depends on what you want out of the plane.
At 14 1/2 pounds the 91 will probably be able to fly the plane but you will have to fly on the wing 100% of the time and to do loops you will have to dive first to pick up enough speed to get over the top.
What kind of ballast is required to balance the plane ?
If it is taking nose weight I would bolt on what ever engine would eleminate the ballast.
Old 08-22-2008, 10:45 PM
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Gray Beard
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Default RE: OS FS 91 is it capable ?

Me too, under 8 pounds or it's going to be A dog. I use mine in 60 size planes under that 8 pounds and it is just OK.
Old 08-22-2008, 11:55 PM
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Gringo Flyer
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Default RE: OS FS 91 is it capable ?

For trainer type flight 10+ lbs for a 91 four stroke is very reasonable.

I built my son a trainer for his birthday and its powered by a magnum 91 4stroke. 60" long, 94" wingspan, 10.5+ lbs and it flies like a dream. Putts around at 1/4 throttle. Capable of nice loops and slow rolls. Its not an acrobat... but its not supposed to be. I think the airframe could easily handle several more pounds and fly well. Right now with a little headwind and lowered flaps it will almost stand still in the air at just above idle. I will eventually take the magnum 91 off for another project and will put a 72 in its place. I am sure it will still be plenty of power.

For trainer type flight I think it will be ok. 14.5 lbs is a little heavy but you have a high lift wing on that plane. If you are looking for a slow Sunday flyer I think you will be fine. If you want it a little sportier better look at a 2 stroke 91 or 4 stroke 120.

If you look in my profile gallery there are some pics of the trainer.
Old 08-23-2008, 05:00 AM
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rthalls
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Default RE: OS FS 91 is it capable ?

thanks guys , the only other engine I have not being used is a US 41 gas whitch I am sure would be a over kill ..... I added 1.34 lbs to the nose to get it to balance
Old 08-23-2008, 11:52 AM
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Default RE: OS FS 91 is it capable ?

I'm the one that always says that too much power is just about right. The 41 would change my mind!! What the Gringo said is quite true, the 91 will fly that plane and do A great job as A trainer type of floating around. It just doesn't have any OS factor for those times you would like to have some extra power. Then again, those SIG high wings always surprise me with the low wing loading how well they fly without power over kill.
A US 41 you say?????? Just thinking.
Old 08-23-2008, 05:51 PM
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Default RE: OS FS 91 is it capable ?

It depends on how you want the plane to fly.

I have an OS .91 Surpass II on the nose of my Westerner and it flys quite well, although the vertical performance is somewhat restricted.

145" Span, I estimate the weight around 13-15 lbs, but I've never put it on a scale. Comes up in less than ten feet and floats around 1/4 throttle.
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Old 08-23-2008, 07:15 PM
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Gringo Flyer
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Default RE: OS FS 91 is it capable ?


ORIGINAL: Lomcevak Duck
145" Span
WOW!

And it still flies on the 91. I would have guessed that would have been a little big for the engine.
Old 08-24-2008, 06:21 AM
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rthalls
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Default RE: OS FS 91 is it capable ?

I would put a picture of my plane on here except I don't understand how to do it
Old 08-24-2008, 11:17 AM
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Default RE: OS FS 91 is it capable ?


ORIGINAL: Gringo Flyer


ORIGINAL: Lomcevak Duck
145" Span
WOW!

And it still flies on the 91. I would have guessed that would have been a little big for the engine.
Well, a little big for an engine is relative to how you want it to fly. It's an old-timer free flight design from 1938, so it was never intended to fly like other sport planes. It does not understand the term "accelerate" or "vertical" and its aerobatic capabilities include the touch and go, gentle stall, and slow spiral. It is built so lightly that I'm afraid to loop it, but I'm sure it has enough power; if it was preceeded by a slight dive.

Most pilots would consider it under powered and, compared to a modern trainer, it is. But the design was not meant to behave like modern planes, and it does what it was designed to do quite well: that being climb to altitude, and glide forever. For it's purpose I would not have imagined putting anything larger on the nose, but you really have to understand the spirit of the design to see how relative 'under powered' really is.

And, 145" of undercambered airfoil creates a LOT of lift even at very low airspeeds.
Old 08-24-2008, 11:19 AM
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Default RE: OS FS 91 is it capable ?


ORIGINAL: rthalls

I would put a picture of my plane on here except I don't understand how to do it
You have to use the 'post reply' button instead of the quick reply box. From there you can click 'Click here to upload images and files' and then browse your hard drive for the file you want to upload, and post like normal.
Old 08-24-2008, 01:03 PM
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chashint
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Default RE: OS FS 91 is it capable ?


ORIGINAL: Lomcevak Duck


ORIGINAL: rthalls

I would put a picture of my plane on here except I don't understand how to do it
You have to use the 'post reply' button instead of the quick reply box. From there you can click 'Click here to upload images and files' and then browse your hard drive for the file you want to upload, and post like normal.
[sm=thumbup.gif]
Old 08-24-2008, 01:09 PM
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Gringo Flyer
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Default RE: OS FS 91 is it capable ?

That plane has definetly peaked my interest.

Did you build it from plans?

It looks like a good candidate for a scratchbuild.
Old 08-24-2008, 01:46 PM
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Default RE: OS FS 91 is it capable ?

I wish I could take credit for that kind of craftsmanship, but I did not build it. I am at least the third owner, and have been unable to find out who built it. I have spoken with several others interested in building one of different sizes, and I know there are plans available, but at the moment I cannot recall who sells them. Do a search in the vintage/antique forum and you might come up with better information. If you do decide to build one send me a PM and I can email you lots of pictures and more specifics about the model.
Old 08-24-2008, 06:28 PM
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Default RE: OS FS 91 is it capable ?

I am thinking about something a little smaller but the same style. If a 91 can fly a 145" wingspan a 70" or so with a 40-52 4 stroke would be a lot of fun.
Old 08-24-2008, 07:05 PM
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Default RE: OS FS 91 is it capable ?

With a 70" span I think you're right on with the .40 four cycle. My father has a 72" Buzzard bombshell with an OS .40 Surpass, and that plane is WAY over built. Almost like they carved it out of a solid block of wood.

Depending on the model and how lightly you build it, you might even go with a smaller engine. I've got a 67" Quaker with an OS .26 Surpass, and my father flys a 70" span Playboy on a .15 two cycle.

The best flying old timer I've had my hands on is the Dalaire Sportster. Penn Valley Hobbies sells a decent kit for it with 108" span and as I recall, they recomend a .52 four cycle or so. The build took my dad maybe a month. He hung a .70 Surpass II on the nose of his, and it is quite a bit over powered for an old timer design.
Old 08-25-2008, 08:00 AM
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Default RE: OS FS 91 is it capable ?

I'll start looking into those.

Nowadsays everyone so overpowers their planes I think it would be fun to show how well an "underpowered" plane can fly.

It would like to see what is the biggest possible plane that can be flown with the smallest possible engine
Old 08-25-2008, 09:54 PM
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chashint
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Default RE: OS FS 91 is it capable ?


ORIGINAL: Gringo Flyer

I'll start looking into those.

Nowadsays everyone so overpowers their planes I think it would be fun to show how well an "underpowered" plane can fly.

It would like to see what is the biggest possible plane that can be flown with the smallest possible engine
I have flown a couple of different planes that were "underpowered" and it just is not my cup of tea.
One of my club buddies has a couple of planes that I think are very underpowered, but he loves flying them and I like watching him do it.
So if that is what you or anyone else likes to do then I cheer you on, but give me some extra cubic inches on the nose
Old 08-26-2008, 07:39 AM
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rthalls
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Default RE: OS FS 91 is it capable ?

well guys I got my answer yesterday , it stalled out on clime out - so need more motor , plane survived OK guess I need a small gas engine
Old 10-23-2008, 03:12 PM
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Default RE: OS FS 91 is it capable ?

I have a Saito 82 w/ a Apc 14x6 pulling a H9 Sopwith Camel that weighs in at 9.5 Lbs and it flies it very scale at 1/2 throttle. I just bought a 2nd hand GP Tiger Moth which has the recommended OS FS-91 up front and it spins a 14x6 MAS at 10250 rpm. I'm shooting for a 15x6 APC in the high 9's and I have no doubt that it'll fly scale at 1/2 throttle when I finally get the chance to maiden it.
Old 10-23-2008, 06:54 PM
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Default RE: OS FS 91 is it capable ?

I flew A 66 inch Toger Moth with an OS 70 FS and it was very scale, I did have to dive for A loop though. A lot depends on the plane itself and the moth flew almost scale.
Old 10-25-2008, 07:44 AM
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Default RE: OS FS 91 is it capable ?

You guys are forgetting that we used to think that a .60 was a biig engine. The Kadet Senior is designed to fly on the wing and not on the motor. I have a couple of models in the 70-80" range that fly extremely well (within the design envelope) on a .26 4 stroke. Not all models are supposed to 3-D. The Pica 1/5th scale Waco YMF was designed to fly on a .60 2 stroke. You are talking about 2 wings at 72" each, and a high drag fuselage. Normally the weight of one of these is around 14 pounds. You don't need a small block Chevy engine to fly a model airplane.

Bill, AMA 4720
WACO Brotherhood #1
Old 10-25-2008, 12:41 PM
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Default RE: OS FS 91 is it capable ?

In answer to the original question in this post, an OS FS-91 on a 14.5 Lbs plane is definitely pushing it and your flying skills would be "tested" to say the least.
Old 10-25-2008, 08:57 PM
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Default RE: OS FS 91 is it capable ?

I couldn't resist posting about my newest plane that I hope to maiden tomorrow. It is a 40 size Sig Rascal with an OS 70 surpass. This is the biggest engine I could fit in the cowl. Hope to eventually put it on floats. Was toying round with it in the yard the other day. Took off in less than 10 feet with high grass. Should be a ball on the runway. Now that is over-powered if you ask me (actually if the Saito 91 would fit ...................... =;-)~.
Old 10-25-2008, 09:44 PM
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Default RE: OS FS 91 is it capable ?

s_s,

I flew a club members 40 size Rascal with a Saito 56 and it flew with power to spare.

Beware of too much throttle-too fast on take off which will result in annoying ground loops and tricky bumps of rudder correction.

Yours should be a handful at 1/2 throttle.

Good Luck, I hope you have the chops to handle it.

Bob G

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