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What are the rules?

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Former Member19/02/2018 14:07:11
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Biggles' Elder Brother - Moderator19/02/2018 14:09:37
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Its has to able to generate more lift than it weighs. That's it, no other rules!

BEB

Masher19/02/2018 14:11:14
1104 forum posts
79 photos

Can highly recommend this book from Peter Miller, available elsewhere too

 

 

Edited By Masher on 19/02/2018 14:12:06

Former Member19/02/2018 14:16:22
724 forum posts

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Former Member19/02/2018 14:41:56
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Nigel R19/02/2018 14:52:10
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"Its has to able to generate more lift than it weighs. That's it, no other rules!"

I'd suggest we could have another one involving structural integrity.

Anything else is probably a 'nice to have'.

Former Member19/02/2018 15:04:08
724 forum posts

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Martin Harris19/02/2018 15:27:32
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8949 forum posts
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There's always the "empirical" school of design - if it looks right, it should fly right...

Most aircraft are built to a reasonably standardised layout and duplicating this within reasonable limits with your own outline should produce a flying model. Pay attention to rigging angles - define a datum line and set the thrust line, wing incidence (say around 1 degree positive) and tailplane relative to this. Downthrust is dependent on the layout - more will be required for a high wing, for instance but if you get it wrong most designs can allow for adjustment of the engine position or you can simply use a throttle to elevator mix to trim out undesirable pitch changes with power.

It's not so long ago that many full size aircraft used to be adorned with all sorts of tabs and adornments to correct design weaknesses so you will be in good company if you find some fine tuning necessary!

It's important to "add lightness" wherever possible but don't under-engineer structural components - development this way can be a long and frustrating road!

Former Member19/02/2018 16:10:05
724 forum posts

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Former Member19/02/2018 16:12:10
1322 forum posts

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Former Member19/02/2018 16:34:15
1322 forum posts

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Former Member19/02/2018 16:49:03
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Simon Chaddock19/02/2018 17:15:44
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2878 photos

supertigerfan

Don't get too hung up on figures and parameters. Most of model 'own design' comes from experience rather than a set of rules.

The truth of the matter is there are so many variables that there is no absolute right or wrong but just various degrees of attributes that suit your personal preference. A plane that you may like to build and fly may not suit another pilot.

Just a point but 'own designing' a full size scale subject does have the advantage that all the major dimensions are already determined by somebody else! You just set the size.

One way to gain own 'design' experience is to build a scale kit and fly it. Then apply the same construction techniques to a different but broadly similar size scale subject. You learn a lot about scaling dimensions and adjusting structures to suit. You also get to find out how to set it up and whether it actually flies better or worse.

I suspect this is how most own designers started - I certainly did back in the Keil Kraft days and I am still learning. wink 2

Nigel R19/02/2018 17:20:20
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" it is by no means necessary and some designs will stay in the air with no lift"

Should we mention helicopters and/or multirotors at this point? cheeky

Anyway. David is right on the money with the WCL calculation, it is remarkably simple in its application and worthwhile to understand.

"Most aircraft are built to a reasonably standardised layout"

That they are. "My" average aircraft...

Wing aspect ratio between 5 and 6.

Fuselage around 80% wingspan. About 2/3 of which is behind CG, 1/3 in front.

Tail between 20% and 25% of wing area. Less area equals longer fuselage needed.

Fin/rudder around 40% tailplane.

Ailerons 10% of wing area. Elevator 25% of tailplane. Rudder about 50% (or more) of fin/rudder area.

Wing section is not critical for power models. A "semi symmetrical" NACA 2415 (Pete M's favourite I believe) for sport types, or a NACA 0012 for "proper aerobatic stuff". Both of which hail from the 1930's I think - there's nothing much new in the world of aerodynamics at our scales and speeds*.

* fashion choices of various contest classes aside.

Biggles' Elder Brother - Moderator19/02/2018 17:53:02
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Posted by David Mellor on 19/02/2018 16:49:03:

And....whilst it can be a good idea to create lift equal to the weight, it is by no means necessary and some designs will stay in the air with no lift and no forward speed. But they, of course, rely on pure thrust (3-D planes and flat LAR wings working in post-stalled condition, often at ludicrously high alpha).

In that case the thrust is not thrust - its lift. Heavier than air aircraft cannot stay in the air with no lift. Period.

The lift may not be from a classical wing - but if it is the force opposing the weight then it is the lift - whereever its coming from!

In the case of a helicopter/MR - it still has lift - via a rotary wing that's all.

BEB

kc19/02/2018 18:11:48
6079 forum posts
169 photos

There are no rules! however there have been several articles which show the 'ideal' proportions for own design sports models.

Articles by Chuck Cunningham and Ken Willard published in the late lamented RC Modeler magazine ( USA ) seem what you need. A sketch shows proportions relative to the span i.e tailplane is one third wingspan and tailplane chord is 1/3 tailplane span etc etc. The articles used to be online but no longer. However send me a PM with your e-mail address and i will let you have the relevant pdf file.

Peter Miller19/02/2018 18:18:03
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10317 forum posts
1231 photos
10 articles

I suspect the supertigrefan is a bit more confused than he was after this lot

Former Member19/02/2018 18:54:34
724 forum posts

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Former Member19/02/2018 18:54:38
1322 forum posts

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kc19/02/2018 19:19:01
6079 forum posts
169 photos

The short answer and a way to avoid all the calculations is to find an existing design to restyle to your own requirements. Keeping the same proportions and building to a similar weight to the original could produce a model that flys nearly as well. However you are unlikely to improve on the best designs!

It would appear that the best designs have got the balance of all the different parts just right. It will take a lot of experimenting to improve on them. No doubt lots of experiments has got the control movements just right too. However you might manage to improve on the actual construction using modern materials and modern power systems.

Still awaiting a Personal Message with your e-mail addressSupertigrefan if you want the pdf files.......

 

Edited By kc on 19/02/2018 19:22:34

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