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Biest

What is the "Surface Area" in FAI-Rules ?

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Biest

Hallo Friends,

while going with my Ballast to FAI-Max at 75g/dm² i was always only looking for my projected Wing-area + the projected Tail-Area.

Now I read in the FAI-Rules https://www.fai.org/sites/default/files/documents/sc4_vol_ciam_general_rules_18v1.1.pdf

Quote

Page 23

B.4.22 Surface area

The surface area includes the total surface of the wings and that of the horizontal or oblique stabilising surface or surfaces. The surfaces taken for calculation are the orthogonal projection on to a horizontal plane of the surfaces in question with each surface at zero incidences.

When wings or stabilising surfaces are built into the fuselage of the model aircraft the surface taken into account shall include that area contained within the normal contours of the flight surfaces extended so as to meet at the plane of symmetry of the model.

What i understand with my german school-English is, that the size of the whole Wings (0° Wingjoiner) and the complete projection of the Model defines the surface Area.

So the Surface-Area will be: Wing Area + projected Tail-Area + projected Fuse-Area, isn't it ?

As you are native English Speakers: Please can you tell me how to read this section correct ?

Thank you in advance and Kind regards

Tobias

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Phil.Taylor
26 minutes ago, Biest said:

Hallo Friends,

So the Surface-Area will be: Wingarea + projected Tail-Area + projected Fuse-Area, isn´t it ?

Tobias

My understanding is:

Surface-Area will be: Wingarea + projected Tail-Area + (the bit of fuselage between the wings) + (an even smaller bit of fuselage between the tails!)

Think of it as if the wings are 3-piece bolt-on - like the older Acacia etc - and that centre piece of "wing" actually exists (hope that makes sense?) - maybe its a piece of history from when wings did indeed bolt on to the top of the fuselage?

Phil.

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Witch_1
23 hours ago, Biest said:

The surfaces taken for calculation are the orthogonal projection

Yes, as Phil says, but wing area is projected as well, at the normal assembled dihedral.   It allows reasonable comparison of models with large dihedral, or polyhedral, gull wings, wide fuselages, pylon-mounted wings, etc. 

Imagine placing the fully assembled model with the sun vertically above, then drawing round the shadow of the wing and tailplane only.  Then take the model away and, where the fuselage was, extend the leading edges and trailing edges to the centreline.  The two areas drawn are the FAI total surface area.

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Biest

Thank you for the Feedback.

 

Cheers

Tobias

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Peter G

As far as I am aware, the fuselage area is not normally counted.

I don't know if this is a formal convention as the fuse doesn't contribute much to lift or just what happens in practice but it would have the effect of allowing some tolerance in the calculated figure.

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oipigface

There is an excellent list of all the rules in the FAI sporting code. This formed the basis  of an article in BMFA News some time last year which not only repeats the FAI rules, but gives some pointers as to how the FAI surface area can be calculated in practice. Peter G is wrong: the area of fus subtended by the wings (and, separately, the tailplane) should be included. Phil’s account is fairly close, but he fails to include the word ‘projected’ before wing area. 

Anyway, I can recommend the BMFA News article to you: https://edition.pagesuite-professional.co.uk//launch.aspx?eid=cd7f64ad-6cf4-41ba-bf1a-cff35f0cf468

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Peter G

This is how things are shown on F3X vault:

201797707_ScreenHunter_176Jun_1122_00.jpg.89fe66a5e1ab1f5e0c4b901473e77b7b.jpg

So I may be wrong in terms of the FAI definition but I think it is the 'convention' not to include fuse area.

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mikef

To avoid confusion, The 'fuselage area' is most certainly included.  Nothing on the F3X vault page above contradicts this, in fact the numbers quoted support it.  Aspect ratio = span squared over area.  31x31/57.5=16.71

 The printed  rule is below,  as quoted at the top of the thread.

(Incidentally, the contribution of the fuselage to lift is significant.  The reduced pressure above the wing also tends to act on the section of fuselage between the wing roots.)

 

image.jpeg

image.jpeg

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oipigface
8 hours ago, Peter G said:

This is how things are shown on F3X vault:

201797707_ScreenHunter_176Jun_1122_00.jpg.89fe66a5e1ab1f5e0c4b901473e77b7b.jpg

So I may be wrong in terms of the FAI definition but I think it is the 'convention' not to include fuse area.

F3X Vault seems to have got it wrong as well (although it may be that "Plane Wing Area" and "Tail Wing Area" include the relevant parts of the fuselage) and should be encouraged to correct, (or clarify) their site. The Sporting Code seems very clear to me, and is the only convention that counts in competition. I think in my article, I suggest erring on the low side when calculating wing area. This is because not all scales give the same measurements. I wouldn't want my world record F3F run to be disqualified  (after I do it)  just because of a technical problem with the scales!

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Peter G

I don't doubt what it says in the FAI rules.

John's point about all scales not measuring the same and tolerance being sensible is exactly the reason I think they just use the wing and tail area. To allow some tolerance in the numbers.

Having traced various 3 views in CAD, they concur with what I have said and what is given on the F3X vault. The numbers I have measured from my Jedi (in CAD), are what is listed on the F3X vault i.e. not including the fuselage element.

I will carry on sourcing my information from f3X vault / and CAD and erring on the cautious side. The manufacturers should really quote the FAI area on their 3 view drawings.

Where do you get your area information from John? I suspect if you are flying 50g heavier than that listed on F3X vault (because you have calculated it correctly) you may run into some difficulties with your world record!

 

 

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oipigface
1 hour ago, Peter G said:

Where do you get your area information from John? I suspect if you are flying 50g heavier than that listed on F3X vault (because you have calculated it correctly) you may run into some difficulties with your world record!

That will be the day!

On 11/06/2018 at 10:56, oipigface said:

I go into some details of what I do in the BMFA article. Drawing around the surface onto a piece of paper, and then drawing squares all over it does a good job. Smaller squares are best towards the tips. The projection adjustment is to divide by the cosine of the dihedral angle. It makes little difference for most wings because the the cosine of a small angle is close to one ; of course, it makes a big difference when you are looking at a V-tail, and none at all for X-tails with no dihedral. 

There are a number of ways suggested of making sure that what comes out at the end is an underestimate.

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Stefan Bertschi

It was all very clear until:

" The projection adjustment is to divide by the cosine of thedihedral angle." 🤓

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mikef

He meant multiply by the cosine.

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oipigface

Well spotted, Stefan, and correctly corrected Mike F.

It’s right in the BMFA News article.

 

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