Author Topic: Good Size for OS Max .25 (Read 1884 times)

Tim Wescott · « **on:** November 29, 2009, 07:14:06 PM »

I'm looking at a 3-view of a Beech Bonanza, thinking that it'd be a good starting point for a semi-scale stunt profile (or maybe semi-scale semi-stunt). I expect that I'll shorten the tail a bit, and increase the V-tail span without increasing it's height, to get the tail volume coefficient up to around 0.45 or 0.5.

I plan on using an OS Max .25 that I have lying around, either with the RC carb held open or with a venturi.

Any guidance on a good size for the airframe? I'm thinking that 400-450 square inches of wing is probably good, but I haven't done this before, so I'm open to suggestion. I'm looking more for stunt trainer sort of performance than out and out 'best you can get'.

Steve Helmick · « **Reply #1 on:** November 29, 2009, 07:53:04 PM »

The "Max" series (aka "S series") OS's weren't terribly powerful, but I think your .25 would do alright on a Flite Streak ARF. For that reason, and because I've never run one of this series OS except the .35S, I'd suggest you get a FS ARF and give it a go. There are some fixes that need attention, but a little searching...or an email to Dan Rutherford will turn up all the info needed. Don't expect blazing reply speed, because his computer is down at the Public Library, and they only let him in there for special occasions.

Steve

Air Ministry . · « **Reply #2 on:** November 29, 2009, 10:00:22 PM »

theres a PeterMiller 'THRIFT' design for a OS 25 Max .
I pirated the airfoil for my Folkerts , which likes wind,
calm and middling air .He pirated it from Bill Wewage , and thought it was his best stunter by far .
(by chance rather than intent ,he said in the builing artical )

Performance re LIGHT weight. Worth a look for a basis.
Published in Aeromodellor mid 70s.

Flew a twin with 20 and 25 in it yesterday,
Not a bad combination. Need a few hours use to loosen of,
and good oil in the old iron piston/linerengines.

Trostle · « **Reply #3 on:** November 30, 2009, 12:58:41 AM »

Quote from: Tim Wescott on November 29, 2009, 07:14:06 PM

(Clip)

I plan on using an OS Max .25 that I have lying around (Clip)

Any guidance on a good size for the airframe? I'm thinking that 400-450 square inches of wing is probably good,

(Clip)

I campaigned fairly successfully in OTS for several years (placing at the Nats and several VSC's) with the Pagan design from the 1951 Aeromodeller Annual. I had two of these models, each powered by the OS 25 Max which is like the little brother to the OS 35 Stunt. It is a smooth running engine, but as has been mentioned, it does not have the power of the OS 25 FP or even the OS 25 LA. These engines used a Super Tiger needle valve in a custom made venturi, the diameter I will have to check. Ran on 9.5/4.5 props at a fairly high RPM. I did not use a muffler. It was noisy and flew on 58 to 60 foot lines.

The Pagan has a 46 inch span and about 380 sq in. My models weighed around 25 oz and were a delight to fly in OTS. Even without flaps, these would do a presentable modern pattern.

I would recommend not going much over 400 sq in with this engine, and keep weight to as much below 30 oz as possible even if you are using flaps.

The extra power of the FP or LA would carry a bit larger model, but these engines are at least an ounce heavier than the older 25 Max. If your weight comes out to more than 30 oz, you will be limited in performing a good pattern.

Geoff Goodworth · « **Reply #4 on:** November 30, 2009, 01:34:44 AM »

Keith, I always liked the look of your Pagan, thanks for the pic and the info.

Matthew, I was talking CL with Peter Miller a couple of months ago and he mentioned the Thrift. He is still very fond of that design.

Tim, if it interests you, the Brodak Shoestring is about 400 sq in and mine, weighing 30 oz, flies fine with an FP 20. I would hope that a Max 25 would produce similar power but I have never used one so I'm guessing.

Cheers, Geoff

Tim Wescott · « **Reply #5 on:** November 30, 2009, 10:16:48 AM »

Trostle, Geoff, thanks for the comments!

It sounds like my guess at 400 - 450 square inches may have been a bit high -- I'll aim for 400 or a tad less.

Trostle, if you could check the diameter of that venturi I'd appreciate it, particularly if you could let me know how much that needle valve assembly is occluding the air passage. I'll be making my own venturi (or a few, to experiment with tuning); knowing what worked for you should be a good starting point.

I'm mostly interested in a good looking plane that's a bit more compact than the modern crop of stunters, but if I end up with something fun that I could shop around to the mags for publication I won't say no.

Brett Buck · « **Reply #6 on:** November 30, 2009, 10:32:34 AM »

Quote from: Tim Wescott on November 30, 2009, 10:16:48 AM

Trostle, Geoff, thanks for the comments!

It sounds like my guess at 400 - 450 square inches may have been a bit high -- I'll aim for 400 or a tad less.

Trostle, if you could check the diameter of that venturi I'd appreciate it, particularly if you could let me know how much that needle valve assembly is occluding the air passage. I'll be making my own venturi (or a few, to experiment with tuning); knowing what worked for you should be a good starting point.

I'm mostly interested in a good looking plane that's a bit more compact than the modern crop of stunters, but if I end up with something fun that I could shop around to the mags for publication I won't say no.

I would put even 400 at the very top end of the scale and only from a master modeler like Keith. The baffle-piston 25 is not a very strong engine at all by current standards, and I don't even think it's enough to provide good performance even on an ARF Flite Streak (which is darn light). The last airplane I had my 25-S on was a Shark 15, and it was a decent performer and not overpowered. I would suggest something a little larger than that with a similar (pretty thin) airfoil -maybe 325-350 square inches and as light as you can make it.

Brett

Trostle · « **Reply #7 on:** November 30, 2009, 12:08:04 PM »

Quote from: Tim Wescott on November 30, 2009, 10:16:48 AM

(Clip)

Trostle, if you could check the diameter of that venturi I'd appreciate it, particularly if you could let me know how much that needle valve assembly is occluding the air passage. I'll be making my own venturi (or a few, to experiment with tuning); knowing what worked for you should be a good starting point.

(Clip)

Tim,

The venturis that I used were 0.280 and 0.275 with the ST spraybar which measures about 0.165. Hope this helps you get in the ball park.

Thinking back on the performance of the Pagan, I think 400 sq in is the absolute max area for this engine. 360 to 380 might be about right. And as Brett emphasized, and I cannot overstate it either, weight is a significant factor. You will need to keep it in the 25 oz range or less without flaps, maybe 28 oz or so with flaps. Even with the light weight (compared to the more modern .25's) of the OS .25 Max, keeping the weight to those minimums will be a challenge. If you keep it light, you can have a nice performing airplane.

Keith

Brett Buck · « **Reply #8 on:** December 01, 2009, 10:34:04 AM »

Quote from: Trostle on November 30, 2009, 12:08:04 PM

Tim,

The venturis that I used were 0.280 and 0.275 with the ST spraybar which measures about 0.165. Hope this helps you get in the ball park.

Thinking back on the performance of the Pagan, I think 400 sq in is the absolute max area for this engine. 360 to 380 might be about right. And as Brett emphasized, and I cannot overstate it either, weight is a significant factor. You will need to keep it in the 25 oz range or less without flaps, maybe 28 oz or so with flaps. Even with the light weight (compared to the more modern .25's) of the OS .25 Max, keeping the weight to those minimums will be a challenge. If you keep it light, you can have a nice performing airplane.

   What's the wing thickness on the Pagan? That will matter a lot too - thinner and you can get away with more wing area, but at the cost of weight (since it has to be built more sturdily) and potential separation at the LE. To the surprise of no one, I would recommend a smaller thick wing over a larger thin wing - maybe something like 16-18% and 340 squares. I would make the aspect ratio relatively high, like 5.5, to keep it from dying in the corners. ~7 7/8 average chord, 43" span. Curiously enough, that's about the dimensions of the first airplane I ever designed and built back in 1975 - for an OS25-S! I had forgotten about that until now. 8x42, NACA 0018.

   BTW, those venturis have more choke area than is usually used on a 40VF! So much for the magic formulas relating displacement and venturi size.

   Brett

p.s. Or, you could just build a Pagan! Here's Keith's first version:

Tim Wescott · « **Reply #9 on:** December 01, 2009, 12:56:07 PM »

Quote from: Brett Buck on December 01, 2009, 10:34:04 AM

BTW, those venturis have more choke area than is usually used on a 40VF! So much for the magic formulas relating displacement and venturi size.

Brett

p.s. Or, you could just build a Pagan!

I've been wanting to build a V-tailed doctor killer for just about my entire modeling career -- and for some reason I just love the looks of a ship with dihedral coming around the circle. So, a Bonanza it will be.

Is there a web page out there that details this magic venturi vs. displacement formula? Surely there's more to it than that -- like RPM, fuel pressure, how flyable you want it to be, etc.

Brett Buck · « **Reply #10 on:** December 01, 2009, 11:21:33 PM »

Quote from: Tim Wescott on December 01, 2009, 12:56:07 PM

I've been wanting to build a V-tailed doctor killer for just about my entire modeling career -- and for some reason I just love the looks of a ship with dihedral coming around the circle. So, a Bonanza it will be.

Is there a web page out there that details this magic venturi vs. displacement formula? Surely there's more to it than that -- like RPM, fuel pressure, how flyable you want it to be, etc.

I was using the word "magic" ironically. My point was precisely that all the things you mention *are* important - you can't just go by displacement. There is no formula that tells you what size venturi to use - the example is of a 40 that uses a smaller venturl than a 25. Oh, and by the way, the current World Champion's PA75 venturi has less choke area than the 25, too!

Bretr

Trostle · « **Reply #11 on:** December 01, 2009, 11:50:38 PM »

Quote from: Brett Buck on December 01, 2009, 10:34:04 AM

What's the wing thickness on the Pagan? That will matter a lot too - thinner and you can get away with more wing area, but at the cost of weight (since it has to be built more sturdily) and potential separation at the LE.

(clip)

Brett

p.s. Or, you could just build a Pagan! Here's Keith's first version:

Brett,

Thanks for posting the picture of that first Pagan. The Aeromodeller plans for this thing show the wing root section to be just over 13% and the tip to be almost 14%. Needless to say, even with that 25 oz weight, it needed to fly fairly fast to present a good pattern, but it never once hinted of a mush or stall, no matter how hard you cranked on the handle.

For those who might be interested, it had wing dihedral. It is an interesting design from 1951 with a tapered wing, inverted cowled in engine and sort of a semi-scale look patterned after the small deHavilland TK-04 racer from the late 30's.

Keith

Neville Legg · « **Reply #12 on:** December 06, 2009, 01:54:17 AM »

Keith, I've just had a look at my Aero Modeller annual for 1951, as its such a foul day here in England! no flying today!! I take it you didn't use elastic bands to hold the wing on the Pagan? and the plan doesn't show any form of tail skid! Did you use 1/8" sheet for the tail, seems a bit thin?

Cheers Neville

Tim Wescott · « **Reply #13 on:** December 06, 2009, 04:14:35 PM »

Quote from: Brett Buck on December 01, 2009, 11:21:33 PM

I was using the word "magic" ironically. My point was precisely that all the things you mention *are* important - you can't just go by displacement. There is no formula that tells you what size venturi to use - the example is of a 40 that uses a smaller venturl than a 25. Oh, and by the way, the current World Champion's PA75 venturi has less choke area than the 25, too!

Bretr

-- presumably any such formula would just be a starting point. But it's better to have some starting point for such things. Most of what I would like to know is what might make me want to increase or decrease the area once I'd tried one, beyond the obvious issues of suction and engine speed.

Author Topic: Good Size for OS Max .25 (Read 1884 times)

Tim Wescott

Good Size for OS Max .25

Steve Helmick

Re: Good Size for OS Max .25

Air Ministry .

Re: Good Size for OS Max .25

Trostle

Re: Good Size for OS Max .25

Geoff Goodworth

Re: Good Size for OS Max .25

Tim Wescott

Re: Good Size for OS Max .25

Brett Buck

Re: Good Size for OS Max .25

Trostle

Re: Good Size for OS Max .25

Brett Buck

Re: Good Size for OS Max .25

Tim Wescott

Re: Good Size for OS Max .25

Brett Buck

Re: Good Size for OS Max .25

Trostle

Re: Good Size for OS Max .25

Neville Legg

Re: Good Size for OS Max .25

Tim Wescott

Re: Good Size for OS Max .25