Fuselage Design Length

[Mastercrashman]

Hello Forum

I have a spare KFM3 depron tapered wing, 42” span / 10” root chord / 8.63” tip chord / 9.31” MAC / 390 in² Area / 5:1 Aspect Ratio / CG is about 3.1” / No Ailerons.

I am looking to fit a simple and quick fuselage to the wing. However I am a little confused about the recommended fuselage proportions that I’ve found on the web.

One basic drawing shows the fuselage split into three parts

· Prop driver to wing leading edge = 1 to 1.5 x chord

· Root Chord

· Trailing Edge to Rear end of Fuselage = 2 to 3 x chord

So if I use my wing root chord measurements and use the smallest chord multiplier then the fuselage length would be (1 x 10) + (10) + (2 x 10) = 40” length.

The second basic drawing I’ve found just shows the fuselage as being about 75% of the Wingspan which would mean the fuselage length would be 42” x 0.75% = 31.5” long which is quite a difference.

I would be grateful for your advice.

MC

[Jim Newberry - Nuviation Ltd.]

Well you can find lots of mathematical formulae that will tell you how long your fuselage should be and what the distance from prop to L.E. and then from T.E. to tail should be - or you can look at a few pictures of similar models and design a fuselage based on what looks about right. This is by far the simplest way of doing things and you are unlikely to go far wrong.

[Mastercrashman]

Thanks for that.

MC

[Simon Chaddock]

I would strongly recommend Jim's approach.

You will find many variations. It is the tail moment that is required. This can be achieved by either a long fuselage and small tail surfaces or a shorter fuselage with bigger ones.

However if you get the tail too close to the wing the airflow will be disturbed and give unpredictable results. Structural weakness and CofG position usually limit on how far back it is practical to place the tail.

Of course the area of the fuselage ahead of the CofG detracts from the tail moment so has to be taken into account.

Finally the prop itself also generates some effective side area.

Its so much easier to copy what has been shown to already work!

[Djay]

There was a really good series of articles in RCME not so long ago that dealt with how to design your own plane, in easy to understand format.

Try looking in the digital issues or index to find it, it may help you.

Darryl

[Mastercrashman]

I downloaded a copy of Design Calc by Lee Van Tassle (Excel spreadsheet) for the wing information, which is a nice little program, however the fuselage dimensions didn't seem true (to me) so I looked elsewhere, hence my confusion. Since Jim's post I have already started looking around at rudder / elevator models to get more of a feel for the more gentler pitching proportions.

I am building a second Piwakiwaki at the moment, in foam this time, powered by the same motor as my first model, a Suppo 2217K-9 and extended the nose slightly for (i) a little more room and (ii) to soften the pitching (the first model really did keep me on my toes). I then had problems with balance and bandsawed the front off and rebuilt to the plan. So with this spare wing I was trying to get it right first time (or there abouts).

I will look back through the RCME archives for that series.

Thank you for the replies.

MC