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Author Topic: Scale up/ down and power requirements  (Read 391 times)
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Fourfingers
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« on: April 06, 2019, 05:58:02 AM »

Recently built a Scarab (A Hatful 1947) free flight.  Scaled it up a bit -25% - for modern 049 engine.  But heres the question:
25% increase on span is +50% on area and double the volume.  Assuming weight follows volume, more or less, then what power might we need?
Redfin 049 vs original specified Amco has double the power.  Another popular motor backalong was the E D Bee but it has a third of the power of a PAW 1cc, so a modern engine will help.
My Scarab power to weight ratio is exactly as the original, by chance rather than design and flies very well!
If anyone has any more sophisticated calculations, I would be most interested, thanks.
OnYouTube is a giant Scarab - 84 inchspan with an OS 70 on board...... looks overpowered to me.
jpc
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lincoln
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« Reply #1 on: June 16, 2019, 10:43:02 PM »

If I'm not mistaken, power required should be proportional to wing loading^1.5 x wing area. Of course parasitic drag coefficients will be somewhat lower with more size and speed, so it's not quite as simple, but it's probably a good approximation.
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Fourfingers
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« Reply #2 on: June 17, 2019, 01:32:39 PM »

Thanks Lincoln
So power requirements increase in direct proportion to wing loading?
Not quite sure about the 1.5 factor ...
john
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TimWescott
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« Reply #3 on: June 17, 2019, 03:20:45 PM »

Thanks Lincoln
So power requirements increase in direct proportion to wing loading?
Not quite sure about the 1.5 factor ...
john

When Lincoln says (wing loading)^1.5 he means wing loading raised to a power of 1.5.  So if you double wing loading, that factor would change by about 2.8 (because 2^1.5 is 2.8 or so -- work it out on your calculator).

The more experienced I get, the less value I see in rules of thumb like that.  No matter what you do, you're going to either luck out (which you did -- congratulations!) or you'll need to fiddle and fuss with the result.

If I were going to put forth one easy rule of thumb, it's that you should keep the power to weight ratio roughly constant -- and then adjust from there.
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Fourfingers
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« Reply #4 on: June 17, 2019, 04:08:49 PM »

Thanks Tim
I get it now.  Raised to the power etc.  Ina previous life  I attempted to enlighten students on growth and development of animals.  There is a concept known as metabolic liveweight which equates to energy needs, output etc.
Calculated from liveweight to the power 0.74.
Explains lots eg why a mouse feels cold before an elephant.  Surface area vs volume.
Sorry.
john
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Greg Langelius
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« Reply #5 on: June 18, 2019, 11:16:37 AM »

I was thinking that linear dimensional changes drive area computations by the square (two dimensions) of the increase in liner dimension; and volume computations by the cube (three dimensions) of the increase in linear dimensions.

But my education was roughly 50-55 years ago, so I'm including long years and memory degradation in that statement.

For power requirements, I take our gallery plans and divide 48 by the w/s in order to get a plan scaling factor that's more likely to be adequately powered by my sole surviving Mac .35.

That's not in anyway a demonstration of my above thinking, but I do think it renders a reasonable working plan that can make good use of the old Mac's power production.



Another project I'm working up is plans for a series of 29.5-ish" span plans for WWII ETO fighters,etc.; which would be flown as a form of WWII semi-scale duration under P-30 rules, not unlike the Sundog Spitfire P-30 that is enjoying considerable recent interest on these forums. I have a few basic scaling factors, including one for Peanuts being scaled by roughly 230% to get the required W/S (actual measurements T/B confirmed).

The tiled plans are looking very buildable with some exceptions made regarding stock sizes and mechanicals, like prop systems and dethermalizers.

For now, no effort is being made to redraw with the sorta-profile fuselages ala the Sundog Spitfire, but that could be used for an alternative class TBD on the future, with scale alterations along the lines of The Sundog Spitfire (like nose length increase to derive a fuse length to about 30" etc.).

One of the key design emphases in the initial designs will be to reduce tail structure weight, and a preference for longer nose designs, allowing Rubber Motors to be longer and to reduce/eliminate nose ballast weight, which is just a pointless handicap.

The Bf-109F, Hawker, Hurricane, P-40, FW-190, P-51, and the Fairey Battle are among a few more in the works. Scale details would be on the more rudimentary side and would be judged from a distance, and used primarily to identify the aircraft type.

Greg
« Last Edit: June 18, 2019, 12:08:59 PM by Greg Langelius » Logged

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Fourfingers
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« Reply #6 on: June 18, 2019, 11:34:58 AM »

Greg
You are right, I am sure.  These principles do not change in 50+ years!
But I think my original calculation stands...
john
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Greg Langelius
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« Reply #7 on: June 18, 2019, 12:06:21 PM »

Greg
You are right, I am sure.  These principles do not change in 50+ years!
But I think my original calculation stands...
john

I agree completely, I'm just giving example of my own mind experiments.

I also think a standoff scale/semi-scale P-30 classification could be a lot of fun for many of the folks on this site.

...And..., the same concepts/principle could easily be done to a 24" max dimension rules set. Peanuts scaled at 185% could be a good standard scaling factor (verify actual dimensions). But peanuts are also far from being the only source of good subjects.

In the above scaling computation, P-30 size craft would use 29.5 or 30 as the base value, and 24's would use 24 on the top of the fraction.

Greg
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Fourfingers
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« Reply #8 on: June 18, 2019, 12:58:15 PM »

Greg,now you have really lost me!
But since you are there,a question if you please?
A kind friend gave me a biplane 36 in span, quite hefty at 850 g, no dihedral and stunt control line aerofoil.
As a crazy project I would like to convert to free flight (!) with an Ollie Tiger on board.
Wing loading (yes, I got there ...) should be OK at 8 oz per sq ft.
Lack of dihedral - crack the upper wing and build some in?
Perhaps I should buy some lines and a helper?
john
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Squirrelnet
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« Reply #9 on: June 18, 2019, 03:24:40 PM »

An Ollie Tiger propelled 36" biplane sounds slightly scarry but stick one of 3 View's stabilisation modules in it and I'm sure it will fly very well...if a little fast.



A 36" bipe with 8ozs sq ft shouldn't need so much power though. A Mills 1.3 should suit it well in my completely unproven and made up guess work methodology . 30 ozs does sound a lot though does it have very stubby fat wings to get the wing loading ?

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Greg Langelius
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« Reply #10 on: June 18, 2019, 07:52:41 PM »

First, remount the propeller backwards, this should reduce power somewhat; it's what my Brothers and I would do for test flying a new design.

Dihedral might go better on the lower wing.

C/L Bipe to F/F. Hmmm... Griffindor...!

If the bipe has positive stagger (I.e. the Upper LE is noticeably ahead of the Lower LE) we can use that stagger to add some natural pitch stability.

First, separate the lower wing panels from the aircraft at the first ribs bay outboard of the fuselage/wing juncture. Remount them with 2"(-ish) of dihedral and decrease wing incidence by 2-3 degrees. This reconfigures the Center of Lift so that when the upper wing stalls, the lower doesn't, then the center of lift shifts rearward, and this induces a mild downward pitching input, which vanishes as soon as the nose drops enough to unstall the upper wind. This results in dynamic pitch stability.

I don't know about that howler of an engine, it may be a bit much. I flew my 36" Bob Mackey Hummingbird on a .15, back in the 1960's, and it was the right amount of power. A 36" bipe should soak up more power, but I'd ideally be looking for a .15 to .19 to mount up front, maybe even smaller.

Test glide over tall grass, test fly with a short fill in the tank and adjust the needle for a four-cycle run; understanding that once the plane picks up speed, the prop will unload, the engine may break into a two cycle run, and Whoa Baby; be prepared to take cover.

If everything still flies too fast, consider flying the airplane with a long, draggy streamer. Police-style crowd control tape may serve, but heck I'm just spitballin' here, guy...

Probably gonna be some fun, but do it where liability concerns are minimized.



About the scaling factors, above. Here's an example of the computation:

Say I have a copy of Bob Peck A6M5 Zero plan that's 18" W/S stored on my system as a .PDF file.

To produce a plan that's 29.5" W/S, I take the intended w/s and divide it by the actual w/s on the plan.

Formula 29.5 / 18 = 1.63888

You can type the "29.5 / 18" directly into the search box on the Windows/Firefox/whatever... browser page, tap enter, and the answer will appear.

So then I get 1.63888...

I round that to 160, and this is my enlargement percentage. Now, I bring up my printer window, select "poster" option, enter 160 as the scale value, and let 'er rip.

I will get a tile paneled plan with a w/s just a bit less than 29.5"; but I would check the assembled plan against the expected w/s value, to ensure that computer fudge factors don't get me over 30" for my now P-30 sized.

Also, when I print tiled plans, I insert a value of .1 (instead of the default .005) for overlap, and when the tiles print, there's some leeway when matching them, trimming them, and taping them together into a single plan sheet.

I then make sure the tiles are seriously taped together so they don't separate unintentionally when I roll them.

...And..., when I roll them, I roll them with the down side inside. This ensures that when unrolled, they lay flat instead of trying to roll back up on me.

Greg
« Last Edit: June 18, 2019, 08:20:29 PM by Greg Langelius » Logged

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Fourfingers
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« Reply #11 on: June 19, 2019, 04:53:24 AM »

Thanks both for your esteemed replies.
Greg, I should point out my techniques are as crude as yours are sophisticated ..... I measure my plan off the palmpilot screen with a vernier and scale accordingly!  Then of course, hand draw the plan.  Tools?  Farmyard stuff.
But your advice on the bipe is much appreciated. Might just try as she is first -you never know!  Reverse prop is a good move I haveused before.
Squirrel, thanks for that.  Not sure what a 3 view stab mode is but sure as hell, Idont have one!
Yes the wings are stubby (I would attach a pic but cant seem to).  Methinks its a bit beefy for a Mills 1.3 but we shall see.  Have an Enya 09 which pulls like a train, maybe.
Love your vids, by the way.
john
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Squirrelnet
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« Reply #12 on: June 19, 2019, 05:55:20 AM »


Free flight stabilisation

https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=23024.0

Glad you like the vids  Grin
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