Scaling of designs for different engine sizes.

[Glen_Alison]

Hi everyone, If you wanted to build a replica of a typical 60 sized F2b model for a smaller engine (say a 35), by what percentage would you reduce the size? Should you do it by wing area or just make everything 10% smaller for example.
Thank you for your thoughts, Glen Alison (UK)

[Gerald Arana]

I'd do it by area. But that's just me..........

Jerry

[Brett Buck]

Hi everyone, If you wanted to build a replica of a typical 60 sized F2b model for a smaller engine (say a 35), by what percentage would you reduce the size? Should you do it by wing area or just make everything 10% smaller for example.
Thank you for your thoughts, Glen Alison (UK)

    Depends on which "60" and which "35", because you have everything from a Merco 61 to a PA 61, and a McCoy 35 Redhead to an Aero Tiger.

   There really isn't a fixed scaling that will result in the same performance. I would generally go by wing area, the wing area ratio scales the linear dimensions by the square root of the area ratio. But I would shade the "tail moment"* to be disproportionately longer than a straight scale-down would suggest.

   Scaling something in the normal range for 60-sized piped airplanes - say, 675 square inches - down to something like an 4-2 break 35, I might scale it to 475 square inches, which yields a linear scaling factor of about 85%. Assuming an 18 "tail moment" on the original, that would give you about a 15" tail moment. I would extend that back out to 16", knowing what I know now (vice what everyone "knew") in 1955. That would give you a good starting point, but it won't fly the same. That you will have to achieve by careful experimentation over several seasons.

   You could probably build that to maybe 30-35 ounces pretty easily, which would make it go very well with a 25LA, once you realized that it is the year 2020, instead of 1965.

    On the other hand, if you were to go from the same original to an Aero-Tiger, 35AX, then, that's a lot bigger airplane. Most 60-sized tuned pipe airplane would fly acceptably well - better than most airplanes before about 1980 - with NO scale-down, an Aero-Tiger 36 would get my regular airplane through patterns most of the time, although you might have to fly it pretty fast.

     In the latter case, I might scale it to 575 square inches, or 92% of the linear dimensions,  and make the "tail moment" 17-17.5". That gives you an airplane in the low-mid 40 ounce range. You would have enough overhead at these dimensions to have substantial control over the way it flew using engine adjustments. This would be a *very potent* combination for competition, you could win just about any contest at any level with something like this, probably only running into trouble against "the usual suspects".

     Brett

[Glen_Alison]

Thanks for the advice Buck. I have both an AeroTiger 36 and a OS 35x Aero by Randy. My normal F2b models are a Yatsenko Classic with Stalker 61 and a Max with RoJett 76 (Muffler)
Glen.

[Brett Buck]

Thanks for the advice Buck. I have both an AeroTiger 36 and a OS 35x Aero by Randy. My normal F2b models are a Yatsenko Classic with Stalker 61 and a Max with RoJett 76 (Muffler)
Glen.

  Just to see, you can probably fly the Yatsenko Classic - just at it comes - with an Aero-Tiger, it will fly and get through patterns in good conditions. You might have to run it pretty hard, but it will do it.  That gives you some idea the kind of performance you can get out of even a less-than-ideal situation.

   I am curious about the 35AX performance myself, just examining it and the spec sheet looks like it has very high potential, like a high-performance version of the 40LA.

    Brett

[Dan McEntee]

   I am curious about the 35AX performance myself, just examining it and the spec sheet looks like it has very high potential, like a high-performance version of the 40LA.

    Brett

      The problem with the AX.35 is FINDING THEM!! I have only ever seen a few listed on eBay and full retail and then they didn't last long. I lucked into one that shtterman had listed as "used" and snatched it up at a decent price. It has the same bolt pattern as a OS.32 but the case is bigger and full ball bearing, bigger bearings than the .32. I have a thread started on it over in the engine section. I finally hit on the correct venturi size/needle valve combination, (which happened to be the same as what I was running in an OS.32 in the same airplane) but need to put it in a bigger model I think. I would put it up at way more than a high performance LA.40! A few others have chimed in with their experiences with it, and one of them said OS has discontinued it!!??? Time will tell.
  Type at you later,
  Dan McEntee

[Brett Buck]

      The problem with the AX.35 is FINDING THEM!! I have only ever seen a few listed on eBay and full retail and then they didn't last long.

https://www.towerhobbies.com/cgi-bin/wti0001p?&I=LXUYR8&P=0

  In stock. If it's a good engine $189 sounds like a good price to me - you sure aren't going to want to wait around for the next generation, because there probably won't be one.

    Brett

[Randy Cuberly]

I recently bought a 35 AX from Tower Hobbies and had a venturi made by Jim Lee (actually two venturis one a bit smaller than the other.  I've so far only bench run the engine but it shows terrific potential.  It's light and very strong with a very stable run.  It turns an 11.5 X 4 prop with aplomb using a ST needle valve.  My best guess is that it will fly anything that an LA46 will handle.

Randy Cuberly

[Dennis Leonhardi]

Back to the original question, Scaling ...

I'd simplify Brett's answer.  Yes, determine wing area, but then look at "moment arms".

If you consider, for example, a series like the Flite Streak, Junior Flite Streak, Baby Flite Streak I think you'll find each wing to pretty much be a % of another, but not so nose and tail lengths.


Dennis

[mccoy40]

I've scaled drawings up. Gone to Office depot and asked them for a 133% increase and then a 150 % increase.

The design is the Ambroid scrapper and I did the scaling based on the length of the airfoil and the length of the nose I wanted from the wing leading edge to the prop.
This is different from your approach in that I was taking a old combat design and trying to make it into a sport/stunt type of plane.
The 150 % version is what I went with, and even at that size the front of the plane is too short. My next change is that the plane will go from a profile fuse to a full body box fuse. This will accomadate the tank I'm planning on using much better than the profile would.
Wing span will stay at 36", but I may also build one at  48" span.

The areas are what really limit this type of thing - my stab and elevator went from around a 12" span to a 18" span and the area on the stab more than doubled - Same with the rudder.

Mine is just a minor experiment and an attempt to build a design that appeals to me and fits the equipment I have.

     

It   

[Tim Wescott]

    Depends on which "60" and which "35", because you have everything from a Merco 61 to a PA 61, and a McCoy 35 Redhead to an Aero Tiger.

Pay attention to this part!  My first effort at scaling assumed that an OS 20S would have the same power per cubic inch as an OS 40FP; I built a very wimpy airplane, that was only saved by the fact that I ended up with an OS 20FP that happened to be a very good fit.

[Tim Wescott]

Pay attention to this part!  My first effort at scaling assumed that an OS 20S would have the same power per cubic inch as an OS 40FP; I built a very wimpy airplane, that was only saved by the fact that I ended up with an OS 20FP that happened to be a very good fit.

Sorry, it's been a while -- I scaled the plane down and put an OS 25S in it, and it was wimpy -- it barely flew on 52' lines.  Then I put an OS 20FP on it and it flew with authority on 60' lines.

You cannot, cannot, cannot just scale by engine size.  Which gets really tedious when you're trying to figure out the best engine for your shiny new stunter, but it is what is.

[Istvan Travnik]

If you want to count in the simpliest way, take not square root, but cubic root of the ratio between 'average' engines of their class.
E.g. you want to go from .60 engine to .15,  take a 60" wingspan model, divide 60 by 1.5874 it gives the usual 38"...
Istvan

[Craig Beswick]

Istavan,
I don't want to be the dumbest kid in the class but how did you get 1.5874?

I opened a cubic root calculator and nothing I put in, from your numbers, came close to 1.5874.
Thanks
Craig

[Craig Beswick]

Do you divide smaller engine into bigger engine from that you get the cube root then divide into wingspan ?

Craig
I feel really dumb now.

[Istvan Travnik]

Dear Craig,
Sorry, I'd explain a bit more in my example.
Here is the missing line: .60 engine's displacement is 4 times bigger than .15; cubic root of 4 is 1.5874.
So we need to divide by 1.5874 the linear dimensions of bigger model.
My example was a little bit extreme, but works.
When we count .60 engine to .35,  60/35 = 1.714, and curt 1.714 = 1,197, approx. 1.2.  So the 60" wingspan goes to 50.
I need to emphasize, this method is correct only when you take "average"  exemplars of their engine class.

[Trostle]

As some have already rightly suggested here, there is a simple solution to the question.

I think some are making this more difficult than needs to be.  To establish the size of the revised design based only on engine displacement does not take into account the different power output and running characteristics of different engines of the same size.  If the power plant is known, there probably is some idea of its characteristics as a stunt motor and the size of model, as in wing area, that it would be comfortable/suitable.  Just compare the desired wing area to the wing area of the model to be copied and change the size accordingly, keeping in mind that the scaling factor is the square root of the ratio of the two wing areas.  That will give you the right proportions.

Then, if you want to substitute airfoils, or think different nose/tail moments are wanted, then you have not copied the original design.

Keith

[Dan McEntee]

https://www.towerhobbies.com/cgi-bin/wti0001p?&I=LXUYR8&P=0

  In stock. If it's a good engine $189 sounds like a good price to me - you sure aren't going to want to wait around for the next generation, because there probably won't be one.

    Brett

     If I had the extra 200 to 600 dollars to burn, I would just call up Randy and order them. I have done this hobby like I have done my whole life on all of my vintage dirt bike stuff  and kart racing when I did that years ago, on a shoe string budget, with lots of work on second hand stuff that other people thought wasn't worth the effort, then beating them with it! I have done the same thing with the model hobby, spending hard earned funds generated on the side so as to not dig into the house hold budget. I have a fairly large engine collection that for the most part I have only paid a few pennies on the dollar looking for bargains. I'm not sure of how many OS.LA.46's I have but I would venture to say that the average price I paid for them is about 30 bucks or so and that includes the NIB engines I have.  I have a good collection of ST. G.51's that I maybe paid a Tower Hobbies price for one or two, but I got the bulk of them from people that couldn't relate to them, and in frustration sold them off cheap. ST.G-51's have served me pretty well. I have done pretty well in all aspects of model competition that I have tried and while I have never been a NATS Champ, I have had a hell of a lot of fun, and very satisfied with how my "stunt career" and other endeavors have gone. The trophy collection in the basement is indicative of it. I have been watching all the different venues for any of the AX.35's. I have even checked Tower from time to time and saw "on back order" . I watch the local Craig's List also for bargains, and all the local swap meets, but have never seen them. I have watched the contest results closely and don't ever recall any one listing one as the engine in their airplane. When you don't see them anywhere in use even in an R/C model, you have to wonder about getting involved with them. But my curiosity kept me looking and I finally found one that was in my budget. If you read the thread I started on this engine, there is not a lot of feed back on people actually using it even in a classic or super 70's model. But my recent results have my curiosity increased and now that I am back to work, I just may have to find a way to finance at least one more, maybe two, since I can now feel that they may be worth the effort and price, and I don't like having only one of any engine that proves to work like it should . I'm basing that on what I am seeing, and feeling by operating it with my own hands and seeing the results with my own eyes, not spending the hard earned money on a whim, just because some one said I should. So, the search for bargains resumes, unless Uncle Whiskers decides to send out some more stimulus money!
   Sorry for the thread drift again!
   Type at you later,
  Dan McEntee