What are the rules?

[Steve Dunne]

Thanks, David.

Both wing areas are from the suppliers "blurbs", not measured by me. I have just re-checked both, and the list matches the quoted specs. Both the Hurricane and the FW190 are from YT/ESM.

The FW190 is definitely overweight, partly from over-enthusiastice scale detail additions and partly from the extensive repairs required when I bought the badly crashed airframe....

I'll try to do a hand measure when I can...

Steve.

[Martin Harris - Moderator]

Posted by David Mellor on 02/03/2018 15:15:33:

OK, Martin, I have done the calculations.

 

Firstly, this is one funny full sized plane. I can't actually confirm the real weight, and since it was a racer in a sub-770 pound class, I wonder what its true weight actually was?

Anyway, your model (at 1/3 scale) would have a wingspan of 67 inches, a wing area of 6.9 square feet and a calculated weight of either 18 pounds (if the full size weight was 770 pounds) or 23 pounds (if the full size weight was 994 pounds).

 

I have to say I was shocked to see such a tiny full sized plane with such a high cubic wing loading (24.9 OPCF if it weighs 770lb or 32.2 OPCF if it weighs 994lb).

 

These figures are way higher than, say, a Spitfire or Tempest!! So it must have been a funny little - and very fast - plane.

 

I checked the power loading, though, and it would fly. The 770lb version had 180 Watts per pound (full size) and the heavier one still had an impressive 140 Watts per pound.

 

Whatever the original full sized one actually did weigh, it must have been like a rocket with a long, fast take off and a really 'hot' landing. A speed merchant for sure!

So.... what did your model actually weigh?

First off, my poor little brain has been hurting trying to work out the figures I quoted as I've just realised that 16' 8" divided by 3 is just under 67" and the 1/3 scale plan was for 73.5" span! However, the fuselage comes in at very close to 1/3 scale give or take a little latitude with the commercial spinner at around 68" (16' 9" according to Wikepedia).

Wikipedia mistake? Well, my copy of The Golden Age of Air Racing quotes the same dimensions - which are also identical to the very similar Brown B1 from the same designer (quoted wing loading 19.2 lbs per square feet). The weight of 994 lbs was a quote from the pilot/owner. That source quotes the record as being for sub 700 lb aircraft - the supercharged 185 hp Menasco C4S engine contributed 305 lbs to the total. Landing speed was quoted as 55 mph.

A different (much smaller) model on Outer Zone also shows wings which are noticeably longer than the fuselage! Was this a model designer's fudge in order to improve the flying characteristics?

Mine had a wing loading of 40 oz per square foot at its flying weight of 18 lb which gave me serious doubts before flying it the first time with similar wingspan (but smaller scale) warbirds being reported as a handful at much over 25-30 oz per square feet. Propped to comply with our noise limits, it flew rather asthmatically on a 120FS (15 x 10 APC), was almost lost when I tried a 16 x 8 Master Airscrew and much happier on a 180FS with a 16 x 12 APC.

The wing area I gave you was scaled up from a rough estimation of the model's wing which had slightly tapered and curved tips.

Some notes from the time which I posted on another forum might give an idea of its overall handling:

 

Anyway, what's it like to fly at 40 oz/sq ft? Well, it's actually very straight forward. The stall is a non-event - only deliberately turning while fully stalled would provoke a reasonably gentle wing drop. I took the precaution of setting a little up aileron on both sides to introduce a little washout which may help in this respect.

Take off run is fairly long - with the 120 it needed 100 yards but the landing approach is very comfortable balancing with a little power and a good rate of descent when throttled back.

The spin is superb - proper entry, stable and fully stalled inner wing, no aileron is needed and it recovers promptly with opposite rudder and a little down elevator but look out for flicking on the recovery where you must allow it to pick up speed properly before easing out of the dive. Finding this out caused a few more grey hairs!

Just remember that it handles more like full size than most models and allow it to build energy and you won't go far wrong.

 

I would have thought my portly 18 lb (others quoted around 14) model was near the top limit for safe/pleasant model flying. Interestingly, both the designer and another builder stated that theirs might benefit from being heavier! e.g."quite a handful on windy days due to the relatively light weight and large size"...

I'm not sure how much the anomalies above affect the validity of your calculations but the evidence from the quotes and the figures for the Brown B1 would lean towards the higher weight figure of just under 1000 lbs.

Edited By Martin Harris on 02/03/2018 18:45:58

[Former Member]

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[eflightray]

And to think it all started with the OP's request for - 'What are the rules of design? ', now it's reached page 12.

That must be one of the biggest thumbs I have come across

Ray.

[Former Member]

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[Former Member]

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[Martin Harris - Moderator]

The scary thing is that the figure quoted for the B1 suggests that the aircraft were actually more highly loaded than your assessment of the M&A's flight characteristics was based on! Not too many of the 1930's racers seemed to survive into the 1940s...

Edited By Martin Harris on 02/03/2018 19:14:23

[Steve Dunne]

I have had my trusty tape measure out on the Hurricane and the FW190 - verrrry interrrresting...

The Hurricane measures 6.6 square feet excluding fuselage bit - the radiator etc. underneath precludes any lift assistance from the fuselage. About as described.

The FW190 measures 5.17 square feet outside the fuselage . Massively less than the 6.1 quoted. The wing area under the fuselage includes the wheel wells et., so not really very useful.
Also, the wing span is 70.5" rather than the stated 74" - never noticed that! (bear in mind I acquired this as a wreck, not a new purchase.).
This would give a wing loading of 55 oz/sq foot, and Cubic of 24.3 - or am I off base somehow? I have re-checked the measurements twice!

The aircraft flies fast, and lands fast, but is quite maneouverable, though it was quite exciting when one cylinder went out in flight a couple of times (Laser 200 V-twin).
Does anyone else have a YT FW190 that I could compare notes with?

Steve.

[Former Member]

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[Steve Dunne]

Thanks for the reassurance, David.

I have usually not included the fuselage covered area of the wing unless the aircraft is a low winger with a clean underside. The Hurri and the FW190 have very obstructed underside of the wing under the Fuselage.
Mid wing and shoulder wing models also fall into this grey area.
I have taken this view mainly when calculating CG points and average chords of tapered wings (as per Gordon Whitehead) and found it helpful. I appreciate there may be differences when the measurements are used for scaling rather than aerodynamic purposes.
Thanks again for the help,

Steve.

[PatMc]

Posted by David Mellor on 02/03/2018 08:53:20:

To pick you up on one point, a straight line on a graph can pass through either axis and very few straight lines in general observational data will pass through the origin (x=0, y=0). The point where the straight line hits the axis is called the intercept (or the constant 'c' in the general form of the linear equation y=mx + c).

And you would definitely not expect this line to pass through the origin under any circumstances.

My question was not about graphs in general, it was about the particular ones you have posted.
Since they are supposed to be referable to all scales the hypothetical scale of Zero x full size must be the starting point. At zero inches span it's only logical that the % cu wing loading "C" is also zero. Therefore the line starting at zero, cutting the points of the model examples you've given then terminating at the the full scale point can not be a straight line. i.e. it's not a linear equation.

[Former Member]

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[kc]

Yet again David has patiently explained the theory so it makes sense to us! Excellent and many thanks!

[Former Member]

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[Former Member]

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[PatMc]

Posted by kc on 03/03/2018 10:41:15:

Yet again David has patiently explained the theory so it makes sense to us! Excellent and many thanks!

But it's actually as wrong as a wrong thing.

[Former Member]

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[Rick Tee]

Thank you for all this info and the Vulcan thread. Been running some numbers on my current models now need to go check some weights. Cracking stuff.

[Former Member]

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[eflightray]

Posted by David Mellor on 02/03/2018 19:10:25:

By the way... is that a new avatar, Ray?

No, it's been my avatar for quite a while on this forum. (I think).

It my scratch built Sunderland, 90" span, 7Lb - 2oz flying, now 7 years old, you can probably guess what it's made from

Ray.

[Former Member]

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[kc]

Supertigrefan. That was a message on 19th Feb! An hour or two later I got your e-mail address and so I sent the files on 19th Feb and assumed you received them OK.

Did you receive them and find them useful?

[Steve Dunne]

I have now updated my spreadsheet to set the Hurricane and FW190 to use the full wingspan in the calculations. (I have also corrected a careless error I made when ascertaining the average chord ).

This gives more reasonable figures for these two!
I think (hope!) that the remaining figures are pretty accurate...

Thanks again David for the guidelines.

[PatMc]

Posted by David Mellor on 01/03/2018 11:37:07:

What is going on is that, whilst the model scales (becomes smaller than the real aircraft), the air molecules do not.

But there is an incredibly simple - and exceptionally reliable - way round this.

And to the great delight of (hopefully) everyone, the way round it is a TLAR solution involving no maths and no mention of fluid mechanics (or Reynolds numbers).

We simply need to look at hundreds of existing models, see how they fly, calculate their actual cubic wing loadings and derive a simple correction to the full sized cubic wing loading.

So why bother with any calculations at all ?

Simply make a table of the scale & weight of each the "hundreds of existing models".

Sorry to rain on your bonfire David but what you're advocating isn't a design tool, it's merely a method of predicting, after a fashion, how a model that has already been built should perform.
The first few flights should fulfil that purpose.

Anyone building to their own design will design the structure to what they think will take the loads [often cribbing from known, or their own previous designs], take steps whilst building to ensure that the model doesn't weigh more than necessary and either select a power plant they judge to be suitable or they might tailor the scale of the model around an existing power unit from the outset.

[The Wright Stuff]

Posted by PatMc on 05/03/2018 23:00:55:

So why bother with any calculations at all ?

Simply make a table of the scale & weight of each the "hundreds of existing models".

Sorry to rain on your bonfire David but what you're advocating isn't a design tool, it's merely a method of predicting, after a fashion, how a model that has already been built should perform.

If you read through the thread, PatMc, you'll see I had similar reservations to you. The cubic wing load rule is really just a fancy way of linearising the relationship, such that we can draw a straight line instead of the complicated curve that would result is we simply plotted weight against wingspan.

However, what 'swung' it for me was the observation that the intercept as a percentage of the full size cubic wing loading remains approximately constant regardless of the aircraft type. Now, this is based on very few models, but if it remains true, then it is actually quite a powerful rule. What it allows you to do, is to estimate the ideal (target) weight of any model of a full size prototype (even if no models currently exist).

If the first few flights fulfil the purpose of showing it doesn't fly well because it is too heavy - well - isn't it a bit late by then? So yes, this 'rule' has limitations and doesn't tell us anything particularly fundamental about flight. But if it works, it's a both a rule and a tool!