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Please help me understand the principles of scaling down a plane

#1
Hi guys,

So, as you might have heard, in other countries, Asia in particular, the 3/16" DTFB does not exist. What we do have is 3mm and 5mm un-papered foam, which I've been tinkering with using Experimental Airlines's tape-covering technique.

Now 3/16 of an inch is 4.7625 millimeters according to Google, and it has indeed been enough of a difference to make it difficult to use the base plans. But we also drive around on scooters here in Asia, so that means I need smaller planes, and can't simply scale up the plan an make a plane even larger.

So what I'm wondering is, what are some of the issues one might run into if they, for example, took a FT Bronco pattern and scaled it -down- to fit 3mm foam, which would be... *quick math* 63% of the original size? Would a 63% Bronco be expected to fly at all, or does it completely toss the lift area to weight ratio out the door? Has anyone ever tried scaling down the full size plans by this much? I've heard of 20% reduction elsewhere on the forums, but this is a 37% decrease. Let's play a quick conjecture game of "Will It Fly?". What obstacles should I anticipate if I try to do this?
 
#3
Are you considering scaling so you can transport or to adjust the A & B folds to fit your foam?
Both. I like the Bronco planform, but I can't transport a plane that big on a scooter-- the wings would probably be ripped off. I also have to work with the foam that I have available.
 

DamoRC

Well-known member
Mentor
#4
I think you can decouple the foam thickness from the scale of the plane you want to build. The A and B folds can be adjusted to suit at any scale. So the question would be what wingspan do you want to build at?

It could be that you still want to build the bronco at 63% scale.

Based on the specs on the plans, the Bronco wingspan is 42.77 inches, the wing area is 323 square inches and the all up weight (with their recommended electronics) is 29.3 ounces. This makes the cubic wing loading (WCL) 8.7 (also on the plans).
If you reduce the wingspan to 27 inches (63%) then to maintain the same WCL the all up weight would need to be 25% (~7 oz). This, theoretically, will give you a plane with the same flying characteristics. That being said, you could build to an all up weight of 10oz and still have a WCL of 12 which should still be pretty flyable.

So I think if you can keep your build as light as possible and use appropriately sized electronics, you should be fine. The final plane will probably be a little harder to fly.
 

Merv

Well-known member
#5
Both. I like the Bronco planform, but I can't transport a plane that big on a scooter-- the wings would probably be ripped off. I also have to work with the foam that I have available.
Yes, the Bronco is going to be difficult to transport on a scooter. I can understand why you would want to scale it down. Yes I believe it can fly at 63%

I have not done exactly what you are attempting. I have scaled planes down, from a 50" plans to a 15" plane for indoor flight. What I found is that everything changes. Wing loading of a 50" wing span 10-15 oz/sqft works well for me. But at 15" span the wing loading needs to be 1-2 oz/sqft. A 10X change. I don't know what the changes will be in your situation. I'd recommend figuring out what your wing span will be. Then look at other planes similar to the size you want to make and look at their specs, wing loading, power, etc. This will give you some idea.

Regarding the A & B folds, It may be best to just not do them. To use more of an old school balsa building style that the FT folding style. Similar to Andrew Newtons Fokker - Fuselage build
 

BS projects inc.

Well-known member
#6
Hi guys,

So, as you might have heard, in other countries, Asia in particular, the 3/16" DTFB does not exist. What we do have is 3mm and 5mm un-papered foam, which I've been tinkering with using Experimental Airlines's tape-covering technique.

Now 3/16 of an inch is 4.7625 millimeters according to Google, and it has indeed been enough of a difference to make it difficult to use the base plans. But we also drive around on scooters here in Asia, so that means I need smaller planes, and can't simply scale up the plan an make a plane even larger.

So what I'm wondering is, what are some of the issues one might run into if they, for example, took a FT Bronco pattern and scaled it -down- to fit 3mm foam, which would be... *quick math* 63% of the original size? Would a 63% Bronco be expected to fly at all, or does it completely toss the lift area to weight ratio out the door? Has anyone ever tried scaling down the full size plans by this much? I've heard of 20% reduction elsewhere on the forums, but this is a 37% decrease. Let's play a quick conjecture game of "Will It Fly?". What obstacles should I anticipate if I try to do this?
I would scale it down for the 3mm foamboard and make sure that the a folds and b folds are all the correct width. That is the main challenged of scaling down a plane. As far as the lift to weight ratio you should be fine, its the thrust to weight ration you need to consider. The electronics that you would typically use on a full size bronco would be very overkill. It like like you could use a twin f pack to power the setup. And yes, scaling the plans to using 63% is the correct scaling size to use.
 
#7
Reallly interesting replies here, particularly the math for wing loading which I haven't seen explained this way before. Yes I was definitely thinking along F-pack style electronics. I'm currently working on a 3mm Mini Sportster for indoor flying, and will be trying the Mini Bronco next!
 

Hai-Lee

Old and Bold RC PILOT
#8
FYI the simple reason for the seemingly massive required reduction in wing loaded as models decrease in size is simply that the air does not scale. The molecules of air travel at approximately 900mph and in the case of a wing they travel from the high pressure to the low pressure in as straight a line as is possible.

On a standard wing there is a sideways component to the airflow over a wing which is determined by the flying speed and the time it takes for the air to travel the span of the wing to remove the induced pressure areas. A half sized wing will take half the time for the air to reach the centre of pressure from the wing tip, (pressure differential has a serious effect upon the speed and direction of the flow.

.Effectively as you scale down the wing not only does the profile generate less lift in the first place but it also bleeds away the pressure centres at a faster rate. It is almost an exponential relationship. For a 60% scale the difference is not extreme but it is noticeable and it is the reason that the landing speeds and take off speeds do not seem to scale along with the planes reduction in dimensions. Often unless you reduce the weight by greater than the cube of the dimension change the landing speed will be somewhat the same as the original and in some cases even higher tin order to fly properly.

For a better explanation do a search or so on Reynolds numbers and what they mean!

Have fun!