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Flight characteristics of scaled down design


hambush
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I should have asked this question before I started my project...

 

I'm curious about how the aerodynamic performance of a design changes as you scale it down. Or rather, how do you go about maintaining a similar performance as you scale down? The relevant factors that I can come up with are:

 

  1. Wing loading
  2. Reynolds number

 

My specific case is that I'm scaling down an existing slope soarer design to 50% of the original. I know that the wing loading will remain roughly the same, as the weight is going to be close to 25% of the original (and I can add ballast if necessary). If I want it to fly at roughly the same speeds as the original, then the Reynolds number will be 50% of the original. So, if the aerofoil used in the original has a similar performance across that range of Reynolds number, or I can find a different aerofoil that has a similar performance to the original but for the lower Reynolds number, then the wing should behave similarly.

 

I feel I must be missing some important factors, but I've no idea what. Or it the problem that it is difficult to find an aerofoil that has similar performance at the smaller scale?

 

Thanks!

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Your 50 per cent model will fly more than  50 percent faster, than  than the 100 per cent model it was based on. That’s what Reynold told us. it needs more relative speed, to get a good number, or it stalls. 

That is why, flyers of big models tell us they fly better. Better flight envelope. 

KC is being a bit over simplistic. Reynolds numbers still work at any scale, but, if a midge, the midge has some strange adaptations, to enable flight. But from the midges viewpoint, it flies well, Reynolds might wish to recalculate. 


 

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If you reduce the wingspan by half, and the weight to a quarter, the two models won’t fly the same. Same wing loading, but it’s going to buzz about, it’s big brother will seem to float.

if you want the same flight characteristics, I think it needs a lot more expertise than we have on this forum. 
NASA might know, they have the computers, a new wing section can compensate. Not being sarcastic. That is a complicated question.

Welcome.

Edited by Don Fry
Politeness
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Frank is correct, cubic wing loading or Wing Cube Loading (WCL), [the same thing], is what you should consider.

If the model & full-size WCL are approx. the same, scale model performance will be approx. be similar.

If not then you can end up with scale 'supersonic' Spitfire, Piper Cub or ASW22 models!

Here's another calculator that also allows the use of metric measurement inputs:

 

http://www.ef-uk.net/data/wcl.htm 

 

 

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Welcome to the forum hambush. You say you want to scale down a slope soarer design by 50% but essentially achieve the 'same performance' by perhaps using a different wing loading or aerofoil section. So what exactly do you mean by 'performance'? Speed range? Ability to penetrate? Soaring qualities? Stability? Aerobatic agility? 

I suspect that you may emulate one or two facets of its larger stablemate's handling but changing one parameter will effect another to its detriment. However the new smaller model that you have created may still be a delight to fly, just a little bit different to its larger brethren. 

An interesting project and the best of luck. ?

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In reference to wing loading, you'll find that in scaling down a model, you also need to scale down the wing loading. Imagine a large scale sailplane, like Icares' 6m Ventus flies on a wing loading of 30oz/ft2 but that would be FAR too much on a 2m version. That 1/3 scale model if the scale model, with 1/3 the wing loading, would be 10oz/ft2, which would work really well at 2m span.

Remember that speed doesn't scale with size, hence a 1/10th scale spitfire doesn't fly at a scale 30mph unless extremely light, whereas a 1/3 scale spitfire might fly at a scale speed of 100mph. 

So, in order to get a 1/2 scale slope soarer to have equivalent performance, I would aim for a wing loading of 1/2 that of the original.

 

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hambush

The lift to drag is not just keeping the same wing section but relative viscosity (skin drag) has to be included and that works distinctly against you as you scale down!

You can counter some of the effect of viscosity by using thinner wing sections but that does alter the flight characteristics significantly.

 

Reynold number is not proportional to dimensions as the dynamic flow characteristics are included in the equation. These do not scale at all well at low Reynold numbers.

Even a 1/4 scale glider cannot begin to match the performance of the full size.   

 

 

 

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Thanks everyone for your helpful suggestions. I've been locked out of my forum account for the past week, which is why I haven't responded until now.

 

I wish I'd known about WCL earlier! I'm too far down the road with this current project to do much about it. Maybe I'll do a version 2...

 

However, in the process of reading up on WCL I remembered that it is possible to modify the models in the PicaSim simulator, or create new ones, so maybe I could use it to predict the performance. Luckily the sim can already simulate the full size model (Le Fish), so I created a scaled down version. The good news is that I can fly it (for some definition of "fly"); the bad news is that it is a bit exciting, as you all predicted!

 

I'm hopeful that the simulator approach will be reasonably accurate. I've found the sim to be realistic for the Weasel (which I've flown in real life); I also found that a mini Le Fish where the weight was scaled according to WCL performed much closer to the full size model, which gives me some confidence in the simulation.

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Hi, No technical response here, but from experience after a major mishap with a prized model I enquired about having a set of foam veneer wings made (the original were factory jig constructed so little chance of replicating that) and was greeted with what profile would I like to choose from. Apparently just for this model at least 4 designers had selected different wing profiles and washout depending on what they wanted to achieve. 

 

Way above my head, but clearly even designers of small aircraft specify various and completely different wing profiles from the original full size to achieve more desirable flight characteristics. Really not sure copying full size is the best idea although no doubt you will find out when it comes to the maiden how well mannered it really is.

 

Good luck and please keep us all updated

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5 hours ago, hambush said:

Thanks everyone for your helpful suggestions. I've been locked out of my forum account for the past week, which is why I haven't responded until now.

 

I wish I'd known about WCL earlier! I'm too far down the road with this current project to do much about it. Maybe I'll do a version 2...

 

However, in the process of reading up on WCL I remembered that it is possible to modify the models in the PicaSim simulator, or create new ones, so maybe I could use it to predict the performance. Luckily the sim can already simulate the full size model (Le Fish), so I created a scaled down version. The good news is that I can fly it (for some definition of "fly"); the bad news is that it is a bit exciting, as you all predicted!

 

I'm hopeful that the simulator approach will be reasonably accurate. I've found the sim to be realistic for the Weasel (which I've flown in real life); I also found that a mini Le Fish where the weight was scaled according to WCL performed much closer to the full size model, which gives me some confidence in the simulation.

I don't think that WCL is based on any actual science beyond TLAR (That Looks About Right).

The scale areas are calculated by either the square or square root law whilst scale weights & powers are calculated by either the cube or cube root law depending if the scale is up or down from the original. That's all WCL does. Trouble is the results don't always provide weight & power figures that are practical so these have to be adjusted by a factor. But this factor varies depending on the chosen scale plus some characteristics of the original aircraft. The factor is then chosen by TLAR.   

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  • 8 months later...

As I see it, the smaller and lighter the model the more it “flits about”. 

 

Air density and and wind currents have a bigger effect on the small models than greater ones. I’ve even seen bumble bees blown backwards by strong winds, you’re not going to see that happen with a 747.

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