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Scaling down aircraft

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Old 10-03-2012, 06:06 PM
  #1
partisan
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Default Scaling down aircraft

I have plans for a quickie 500 that I built and it flies great. I was wondering if I scaled it down by 30% to 50% if the engine size would scale down evenly?

I would like to have about 250 sq. inches of wing area.

Thanks for the help
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Old 10-03-2012, 06:32 PM
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Default RE: Scaling down aircraft

the Spickler Q-500 was one of the first kits available. He actually did a Q-200 that obviously was 200 Sq In and was powered with TD .051. One of the local clubs tried a 350 Sq In version. If memory serves me they had a 40" span and were powered with .25 engines.
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Old 10-03-2012, 07:47 PM
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Default RE: Scaling down aircraft

We're the smaller ones just a clipped wing or did they change the chord also.


P.s. this one will be electric powered
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Old 10-03-2012, 11:17 PM
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Default RE: Scaling down aircraft


Quote:
ORIGINAL: partisan

I have plans for a quickie 500 that I built and it flies great. I was wondering if I scaled it down by 30% to 50% if the engine size would scale down evenly?

I would like to have about 250 sq. inches of wing area.

Thanks for the help
If you want it to fly similar to the full size, then you want to retain same the proportions.
For a fast or really "sporty"plane you can get a feel after the reduced size drawing is made for what size engine, tank and radio gear will be a good fit.
The other method is to already have your equipment in hand and then design the plane around it. This is my favorite path.
In the case of your electric power I think you will find a greater variety of choices and plenty of expert [sales] advice once you have arrived at what the size and projected weight will be of your project.
Bear in mind that there is nothing wrong with borrowing the structural methods and details that have already been worked out for whatever "scale" you decide to build.
The methods and materials will vary some what for a given size plane so that it is built strong enough to withstand the forces of flight without taking on extra, fun robbing and performance robbing weight.
Spend as much time as you need studying plans that are out there scattered here and there if you do a little bit of searching. Some of the RCU forums are a jackpot for this stuff.
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Old 10-04-2012, 04:00 AM
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Default RE: Scaling down aircraft

If you want to scale a plane up or down and have it fly similar to the original design, make sure that the wing loading and power loading match the original design. The wing loading is the total model weight(tank empty) in ounces, divided by the total wing area in square feet. To convert square inches into square feet, divide the wing area in square inches, by 144 sq. in. per sq. ft. So, for a plane with 600 sq. in(4.167 sq. ft.) wing and a weight of 5 pounds(80 ounces), the wing loading would be WL = 80 / (4.167) = 19.19 oz/sq ft.

The other scaling factor, power loading, is the total plane weight(tank empty) in ounces, divided by the cubic inch displacement of the engine.
So, for a 40 size engine(0.40 cu. in.) (CID)and the 80 ounce plane, the power loading would be PL = 80 / 0.40 = 200.

Now, for the new design, just work the above equations backwards to find the wing area and CID of the plane, starting with a wing loading of 19.19 oz/sq ft and power loading of 200.

If you have an electric, you could redefine the power loading factor as wing loading divided by power in watts, that is, voltage times average currentfor example. I'm not into electrics so you should check the electrics documentation to see if they use another definition. You may also find other helpful information at www.rcaeronauts.com.
Best of luck.
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Old 10-04-2012, 05:29 AM
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Default RE: Scaling down aircraft

No, not wing loading, it is wing volume loading that you need to keep the same. The attached graph show this. Keep the "Wing Volume Loading" the same for each size and the flight characteristics will be similar.
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Old 10-04-2012, 01:47 PM
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Default RE: Scaling down aircraft


Quote:
ORIGINAL: Rodney

No, not wing loading, it is wing volume loading that you need to keep the same. The attached graph show this. Keep the ''Wing Volume Loading'' the same for each size and the flight characteristics will be similar.
Does wing volume loading take into account the fact that you can't scale down air molecules or gravity..?
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Old 10-04-2012, 05:25 PM
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Default RE: Scaling down aircraft

Ok... My quickie 500 weighs 3.9 pounds and has a o.s. .46ax. That is a power loading of about 136. I think I can build the smaller one right at 2 pounds. Using a .15 or similar will net around the same power loading.

My plan is to build the frame, making it wide enough for the battery which seems to be the biggest item. After I know how much the frame weighs I can adjust the mAh of the battery to get to 2 pounds.

My next question is if I just scale down the plans, can I skip half the wing ribs to save weight. I can't see needing something like 22 ribs in a 35 inch wing.

Also should I have the plans printed at 70% original so I can copy the stab/rudder design or could I do some math and get it close.
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Old 10-04-2012, 07:30 PM
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Default RE: Scaling down aircraft

A .15 powered plane will fly pretty good at 32 ozs [and with a reasonable wingloading]...but shoot for 28 ozs if you want unlimited vertical and any sort of 3D aerobatic capability when you get fatigued from boring huge holes in the sky..
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Old 10-05-2012, 04:11 AM
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Default RE: Scaling down aircraft

The Wing Volume Loading, also referred to as Wing Cube Loading, is touted in some of the literature as an ideal factor for scaling airplanes and was originally defined by Francis Reynolds in an article in 1989. There is some info on it, and a WCL calculator at 
http://www.theampeer.org/e-basics/e-basics.htm#WCL The original article doesn't seem to be available on line and I haven't found anything that proves that it is better than wing loading. If one of you guys can find the reasons and some proof that WCL is better to use than wing loading, please let us know. We all want to get the facts on this one. Thanks.
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Old 10-05-2012, 05:45 AM
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Default RE: Scaling down aircraft

I fly all sizes of models from the little 1/2A to the 1/4 scale and have found that the WVL factor relates very well, much better than wing loading, to determine what type of performance a model will give. You can check this out yourself quite easily, just plug in some of the different planes you fly and note where they fall on the chart in one of the above comments. You will find that anything with a WVL of less than 7 is a real floater while anything above a WVL of 12 starts to approach the performance of a lead sled. If you have flown both the 1/2 A and the 1/4 scale, you will see that the 1/4 scale with a wing loading in the high 20's or even 30 ounces per square foot fly rather nicely but a 1/2 A with that wing loading will not fly at all.
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Old 10-05-2012, 06:24 AM
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Default RE: Scaling down aircraft

When you get to the small indoor stuf - you will be looking at wing loadings of 1-4 oz in sizes of 60 sq in to 300sq in.
The problem is not "what is the correct loading but rather - how can I get it lighter
I have one 100sq in setup which weighs 30 grams - needs to shed 10 grams - just can't get there .
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Old 10-05-2012, 06:42 AM
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Default RE: Scaling down aircraft

Quote:
ORIGINAL: partisan

Ok... My quickie 500 weighs 3.9 pounds and has a o.s. .46ax. That is a power loading of about 136. I think I can build the smaller one right at 2 pounds. Using a .15 or similar will net around the same power loading.

My plan is to build the frame, making it wide enough for the battery which seems to be the biggest item. After I know how much the frame weighs I can adjust the mAh of the battery to get to 2 pounds.

My next question is if I just scale down the plans, can I skip half the wing ribs to save weight. I can't see needing something like 22 ribs in a 35 inch wing.

Also should I have the plans printed at 70% original so I can copy the stab/rudder design or could I do some math and get it close.
I think a rib every 3 inches gets the job done OK with this size plane. A 70% size copy of your plans ought to give you what you want. If you want speed then figure you don't want the wing any fatter than 3/4 inch, so this makes building the wing more challenging as you go for "thin-ness" Assuming a 7 inch chord, if you really want some speed a 5/8" thick airfoil is more like it.
I've built a P-51 like this with a flat bottom airfoil, full depth hard balsa spar that was built directly over a 1/16" balsa bottom sheet. The wing's leading and trailing edges were slivers of hardwood and the "riblets" were fairly thick balsa scrap. The whole works gets sanded with a "long board" before it gets the 1/16" top sheeting. The idea is to shape / sand what looks like a nice, smooth airfoil that doesn't have any dips or flat spots. The high point of the airfoil [where you place your spar] should be about 2.5 inches from the leading edge.
1/16" balsa is capable of slight compound curvage, but I sheeted the top with 2 seperate pieces. Your Q350 wing will be constant chord, so 1 sheet for the top will work...but it's more hectic to make all those little glue joints in one fell swoop. It's easier I think to sheet the top in 2 seperate operations.

I've found that 350 mah NIMH packs and HS81 servos work well with ".15 size" planes. A 3 oz Hayes tank is a nice size that allows a fairly slim fuselage with some extra space for padding.
Otherwise, a 2 oz Hayes tank will allow you to stuff the battery way up front if you end up with a design that has a short nose. This is a slight "pitfall" with scaling down some designs because the engines get disproportionally lighter as you drop down in size. The tendency is for tail heaviness to creep in.
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Old 10-05-2012, 08:32 AM
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Default RE: Scaling down aircraft


Quote:
ORIGINAL: combatpigg

Quote:
ORIGINAL: partisan

Ok... My quickie 500 weighs 3.9 pounds and has a o.s. .46ax. That is a power loading of about 136. I think I can build the smaller one right at 2 pounds. Using a .15 or similar will net around the same power loading.

My plan is to build the frame, making it wide enough for the battery which seems to be the biggest item. After I know how much the frame weighs I can adjust the mAh of the battery to get to 2 pounds.

My next question is if I just scale down the plans, can I skip half the wing ribs to save weight. I can't see needing something like 22 ribs in a 35 inch wing.

Also should I have the plans printed at 70% original so I can copy the stab/rudder design or could I do some math and get it close.
I think a rib every 3 inches gets the job done OK with this size plane. A 70% size copy of your plans ought to give you what you want. If you want speed then figure you don't want the wing any fatter than 3/4 inch, so this makes building the wing more challenging as you go for ''thin-ness'' Assuming a 7 inch chord, if you really want some speed a 5/8'' thick airfoil is more like it.
I've built a P-51 like this with a flat bottom airfoil, full depth hard balsa spar that was built directly over a 1/16'' balsa bottom sheet. The wing's leading and trailing edges were slivers of hardwood and the ''riblets'' were fairly thick balsa scrap. The whole works gets sanded with a ''long board'' before it gets the 1/16'' top sheeting. 1/16'' balsa is capable of slight compound curvage, but I sheeted the top with 2 seperate pieces. Your Q350 wing will be constant chord, so 1 sheet for the top will work...but it's more hectic to make all those little glue joints in one fell swoop. It's easier I think to sheet the top in 2 seperate operations.

I've found that 350 mah NIMH packs and HS81 servos work well with ''.15 size'' planes. A 3 oz Hayes tank is a nice size that allows a fairly slim fuselage with some extra space for padding.
Otherwise, a 2 oz Hayes tank will allow you to stuff the battery way up front if you end up with a design that has a short nose. This is a slight ''pitfall'' with scaling down some designs because the engines get disproportionally lighter as you drop down in size. The tendency is for tail heaviness to creep in.
I don't know what type radios you ar e using but a 350 ma pack -in NIMH may be real trouble using 2.4 radios
The reason being that a number of small micro servos can pull upwards of 200ma under load ( I load test all my stuff)
so IF someone is using a 1C rated 350 ma pack - it is easy to depress voltage to UNDER 3 volts -using that setup.

On 72.xx radios this is no big deal - they just slow down-but on computer radios, they all shut down at aprox 3 volts - all of em- no exceptions
they will restart once the load is removed IF the voltage rises to above 3.5 but will again shut down if the servo is still under a command to move.
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Old 10-05-2012, 12:15 PM
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Default RE: Scaling down aircraft

You're right about 350ma being marginal. IIRC they are packaged as 2/3AAA cells. I can get 2 or 3 flights per pack driving 2, 3 or even 4 mini / micro servos. I've learned to only charge them on the over night rate at home, they don't tolerate field charging too well. The weight and size of these packs is too alluring for me not to use them for the little planes...but you really need to watch them like a hawk.
My only 2.4 radio is a Spektrum parkflyer job...so it's possible that the RX isn't as thirsty as a standard RX..?
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Old 10-05-2012, 03:18 PM
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Default RE: Scaling down aircraft

do a voltage depression check on the cells - -
I use a Horizon meter which allows .5 or 1 or 2 amps load
On 2 amps a NEW Eneloop 2000ma pack (which some guys are buying as rx packs), shows 5.5 volts freshly charged BUT with only a 2 amp load - it sinks to 3.9 volts- which means it has very minimal ability to deal wit h even small micro servos which are the popular digitals of 40 in ounch torque - these draw 300ma each just in casual use
I am certain the vast majority of unexplained "failures ar e simply voltage depression.
FWIW
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Old 10-06-2012, 10:57 AM
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Default RE: Scaling down aircraft

I plan on using an esc/bec with a 5v 2a rating so I don't need to carry another battery.

Should I use a balsa slab or try building up surfaces with sticks and sheeting for the rudder and stab? I want it to be strong, but really light.

Could carbon fiber sticks be used to built it up, but sheet it with balsa?
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Old 10-06-2012, 11:56 AM
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Default RE: Scaling down aircraft

If you can find light-to-medium dense balsa slabs 3/16" to 1/4" thick..it is hardly work the effort to build up the tail feathers on a small racer type model.
Figure your 70% Quickie wont have very much wood back there to begin with.
Take a gram scale with you to the hobby store when you select your wood. This will teach you how to recognize by eyeball which sheets will be light / medium / heavy. For practical purposes, I don't like to use contest grade [ultra light] balsa for planes like this unless you are carving out a thick block. I don't like heavy balsa unless it is for spars, stringers [longerons] and other structural items.
Medium weight is my favorite for tail surfaces with this general size of plane. You have some warp resistance and enough stiffness for a 14 inch span stab / elevator, plus the wood is just right for slitting hinge slots. By the time you "airfoil" a 1/4" thick tail surface, you've removed about 1/2 the weight but still have enough of the original integrity to withstand the forces of flight.
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Old 10-06-2012, 04:28 PM
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Default RE: Scaling down aircraft

Quote:
ORIGINAL: rmh
do a voltage depression check on the cells - - I use a Horizon meter which allows .5 or 1 or 2 amps load
On 2 amps a NEW Eneloop 2000ma pack (which some guys are buying as rx packs), shows 5.5 volts freshly charged BUT with only a 2 amp load - it sinks to 3.9 volts- which means it has very minimal ability to deal wit h even small micro servos which are the popular digitals of 40 in ounch torque - these draw 300ma each just in casual use
I am certain the vast majority of unexplained "failures ar e simply voltage depression.
+1 I lost a few planes this way when Ifirst migrated to 2.4 gear. Spectrum got the message and redid their software to dramatically shorten the reconnect time when a low voltage shutdown happens. The really simple backup when using the smaller packs is to plug a capacitor into the receiver - It handles the short load spikes and weighs only a few grams. I use them on all of my small pack planes.

HS-81s don't draw very high, but the HS-65s are pigs, and I typically get 3 or 4 flights from a 350mah pack with HS-81s.

EG


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Old 10-06-2012, 04:46 PM
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Default RE: Scaling down aircraft


The aDVICE FROM cOMBATPIGG IN POST #4 IS CORRECT.

A good plane flys first on paper & a motor & prop/battery calculator. There are plenty of good power calculators on the web.
I find I can spend a couple of hours on a power & flight calculator & be VERY satisfied with the completed planes.
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Old 10-06-2012, 06:00 PM
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Default RE: Scaling down aircraft

Quote:
One of the local clubs tried a 350 Sq In version. If memory serves me they had a 40" span and were powered with .25 engines.
Shawn, I was the guy that tried to promote the .25 sized quickie at the Pioneer RC club. They had 38" wingspan and 300 sq in of wing area. About a dozen or so were built and raced. While they flew well, they were a bit on the hot side for landing for the average flyier. They were a pretty good F1 trainer in terms of landing speeds. Even then (around 1983 or 84) not to many people liked to scratch build. I think we tried AT-6's for racing next. Both of those airplanes that I built back then still survive, so the racing series for each airframe did not last very long. But an OS .25 will spin a 7-6 prop and pull that size of an airframe to a pretty good speed.
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