Scaling up scaled down aircraft

[Paul Catanach]

I have an Isaacs Fury which is 7/10ths of a Hawker Fury (if you squint a bit in low light). What would be the pitfalls, hoops, problems in scaling the drawings back up and building on the same size as the original?

[DaveWhite]

Don't consider scaling back up from the replica drawings - build from the original drawings or a pattern aircraft.

The former route will be to put compromise onto compromise, and has a significant risk of perpetuating both any errors and (more likely) deliberate design decisions appropriate to the smaller version but not the original.

My thoughts, anyway.

[Rob Swain]

I seem to recall that the justification for the 90% scale of the Spitfire Mk26B was that to build it 100% size would mean it would cost several times the price because so much needs to be beefed up to take the extra loads the extra wing area, weight etc imposes on the structure.

I'm reasonably certain of the above...
...less so of what follows, but I'll chuck it in as a discussion point as we are in Hangar Chat!

Please don't ask me for figures: my limited understanding is it's to do with area values squaring and volumes (and therefore weights) cubing when you increase the length of something.
Don't even want to go near what scaling does to engine sizing, output and efficiency.

Simply put: it's applied Physics, also known as Engineering!

A good example of the weirdness of all this is if you scale down a Piper Cub to 'hand launch'able model size then you stand a good chance of being able to get it to climb almost vertically and do a good impression of a helicopter. We all know a full size one doesn't do that!

[Brian Hope]

An aesthetic problem with scaling down is that you can't scale down the pilot, so compromises have to be made so that a full sized pilot can fit into the reduced size aircraft. If you then scale back up you end up with a full size aircraft that also has those compromises enlarged too, and possibly results in a pretty odd, out of proportion aircraft. The smaller the original downsize - say a WAR replica type - the odder scaling it back up will look.
As has already been said, there will also be many issues with the strength of the larger structure, additional weight etc, so the engineering of the design becomes a complete redesign/re-stressing exercise.

[Nigel Hitchman]

Perhaps you should investigate if plans were made for the full size replica Hawker Fury, built by Viv Bellamy (Westward Airways) G-BKBB, which is now in the US with Jerry Yeagan's collection the Military Aircraft Museum at Virginia Beach. It was on a Permit to fly, but perhaps at that time a CAA Permit rather than LAA permit. I believe John Isaacs may have had something to do with it, although not sure.
It has a RR Kestrel engine.

[Paul Catanach]

Thanks Nigel.

My question was based on curiosity, I certainly wouldn't contemplate building a full-size Fury (The Boss says )

[Trevor Harvey]

I believe there is a function called the Reynolds Number for scaling full size down to models for accurate replication of performance etc.
I have no idea what this is but out of curiosity would love to know?

[Paul Catanach]

Here y'go, Reynolds number definition (bet you're sorry you asked )

Oh, and don't ask me to explain it. I haven't a clue.


https://en.wikipedia.org/wiki/Reynolds_number

[Chris Martyr]

Reynolds Number ! Flippin 'eck , even engineering instructors recoil in horror at it's mention .

Trevor,,,,,,,,you could have scored a first on LAA forum here mate.

[Trevor Harvey]

Oh my good god giddy aunt!!!
On the same subject. I once put it to an instructor that model aircraft fly in relatively much denser air than the full size version, the operative word being (relatively).
I was rubbished, don't be ridiculous, it's the same bloody air!!
What about those programmes, Thunderbirds etc, model ships look like they are sailing through thick oil with globs of "water" coming off the bow, compared to the fine spray that I used to get soaked with.
Same thing innit? Fizzix an all that.

[Joseph Cullen]

If I stretch my memory back nearly five decades, I recall that the Reynolds number could be thought of as a "dimensionless velocity" and is very useful for understanding flow characteristics in particular the change from laminar to turbulent flow. Another dimensionless group is the Prandtl number which contains the viscosity and thermal conductivity but no length term. The Prandtl number, therefore unlike the Reynolds number, cannot be scaled which is why models, especially boats, never look quite right. Hope this helps!