Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

cubic versus pounds per square foot ? Differences ?

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Old 01-05-2013, 12:56 PM
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cyclops2
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Default cubic versus pounds per square foot ? Differences ?


What does each affect more ? Why consider Cubic ?
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Old 01-05-2013, 03:57 PM
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Default RE: cubic versus pounds per square foot ? Differences ?

Well, with the regular wing loading the optimum target for any general type of design varies with the size. A wingloading that is considered feather light on a 6 foot span model would be considered as being in the lead sled category if that same design were scaled down to 3 foot.

The whole cubic loading calculation is one way to take the sizing into account. If you scale the model so that the cubic loading remains constant then as you cut the 6 foot model in size to a 3 foot version and run the calculation for cubic you'll find that the weight needed to achieve the same loading is automatically lowered to a suitable amount.

So for some folks that like it it allows for a quick check of what a scaled up or down design should weigh if they are not familiar with that sort of model in the new size.

For those that fly or look up data on similar models at the new size it's easy enough to find data on what represents a suitable wing loading for that size and style of model.

Niether method is "better" than the other. Just different.
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Old 01-05-2013, 06:10 PM
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Default RE: cubic versus pounds per square foot ? Differences ?

And both methods offer a subjective valuation of their loading figures. So neither really can be 'wrong.'

I've often wondered if the aeronautical engineering community has ever offered an opinion of how well the cubic system works in practice. Wondered how alike a small model flies compared to the larger one when they both have the same volume. Like stall speed, load carrying etc.

BMatthews might know about that????
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Old 01-06-2013, 09:55 AM
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Default RE: cubic versus pounds per square foot ? Differences ?

Quote:
ORIGINAL: da Rock

And both methods offer a subjective valuation of their loading figures. So neither really can be 'wrong.'

I've often wondered if the aeronautical engineering community has ever offered an opinion of how well the cubic system works in practice. Wondered how alike a small model flies compared to the larger one when they both have the same volume. Like stall speed, load carrying etc.

BMatthews might know about that????

I'm not BMatthews (never even played him on TV) but as a retired NASA aero engineer who worked in part with scaled down models in aerodynamics research, I'll weigh in.

The short answer is yes, this issue has been studied and discussed at length. And the second short answer is no, there aren't any scaling methods that exactly duplicate the full size counterpart in every respect. But for research purposes, it is sufficient to understand the differences and account for them. For model flying, the issues are different. Do you want the airplane to fly at the same speed, or do you want it to fly at what APPEARS to be the same speed ? Do you want the time for a 360° roll to be the same or do you want the distance travelled during a 360° roll to be the same ? Do you want the diameter of a 60° banked turn to be the same, or scaled to the size of the model ?

There are always differences when you scale down a plane. You can adjust the weight (density) to achieve the effect that you want, but something will be different - speed, angular rotation rates, distances travelled, etc.

The only scale size that makes all parameters come out exactly the same is one-to-one. That's not to condemn scale models; you just have to accept the differences and optimize them for the image that you wish to project.

Dick Fischer
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Old 01-06-2013, 03:13 PM
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Default RE: cubic versus pounds per square foot ? Differences ?

Well, I certainly don't have the background or credentials that otrcman has. I just learned as i went along in this hobby. And much of what I type is parroted back from other folks and sources.

As indicated the wing loading or cubic loading of two models of the same design other than size is an indicator of what it will be like to fly each. But since it's a subjective thing and the cubic loading equation has all the earmarks of being only quasi scientific there is room for some interpretation in the results. It also depends on what factor you're looking to match as suggested by Dick in his reply.

If the replies so far don't answer the question then you may need to supply more details.
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Old 01-06-2013, 05:28 PM
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Default RE: cubic versus pounds per square foot ? Differences ?

Thanks guys.
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Old 01-07-2013, 04:30 AM
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Default RE: cubic versus pounds per square foot ? Differences ?


I am satisfied with the differences.


Rich
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Old 01-07-2013, 05:10 AM
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Default RE: cubic versus pounds per square foot ? Differences ?

Do you consider other factors in scaling a model plane to a different size, such as Power Loading, ie. Weight in ounces, divided by engine displacement in cubic inches? Are there any other important factors that need to be considered, in scaling models up or down? Thanks for sharing your experience.
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Old 01-07-2013, 08:22 PM
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Default RE: cubic versus pounds per square foot ? Differences ?


Quote:
ORIGINAL: edsplane

Do you consider other factors in scaling a model plane to a different size, such as Power Loading, ie. Weight in ounces, divided by engine displacement in cubic inches? Are there any other important factors that need to be considered, in scaling models up or down? Thanks for sharing your experience.
BMatthews is probably best equipped to answer this question. It sounds like you are more interested in going from a large model to a small model or vice versa. Everything I was involved with was unpowered and involved more stability and controllability issues than performance.

Dick
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Old 01-07-2013, 09:10 PM
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Default RE: cubic versus pounds per square foot ? Differences ?

Dick,

I'm interested in dynamic scaling, some of which is obvious, but most is not. For instance power is to the ratio of scale to the 3.5 power. And scaling time (scale ratio) to the half power. I've seen some NASA documents on the subject, pictures of spin tunnels and the like. I like the analog puzzle of aerodynamics in the digital world. I hope to design an single place homebuilt after I finish my RV-4, but would like to build a half scale model to test first.
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