Deciding how far to seperate aerofoil sections.

[Tom Richards]

Hello,

I'm building my very first RC plane with a balsa wood skeletal structures, whilst trying to minimise the weight. Is there a rule of thumb when it comes to spacing these aerofoils so that the structure is rigid and the film is tight.

[Robin Colbourne]

Tom, have a look at a few plans on Outerzone to get an idea for rib spacing. If you give us some idea of the intended wingspan it would help.

Do yourself a favour and get a proven model to learn to fly on, and save your own design for when you can fly without frequent crashes.

[Gary Manuel]

Rule of thumb = If it looks too big, it is. If it looks too small, it is. If it looks about right, it is.

Is that you Harry?

Edited By Gary Manuel on 11/12/2020 00:09:41

[kc]

For wings of about 50 inch total span 2.5 inches is common for outer ribs while centre section ribs are closer and must come over the fuselage sides. But it depends on size of model and type of model.

But the advice to build a proven model and start with a high wing trainer is the right advice. Look at Outerzone and view some of the trainer designs - Super 60, RCM Trainer, Tyro Major,etc   Tyro and Tyro major are also available from DB Sport and Scale

If you must have a smaller model then perhaps Mini Super is good at 48 inch span.   Anything smaller will be tricky to see at the sort of safe height needed for training.   Mini Super and Super 60 use a lot of strip wood which is easier to obtain at the moment than sheet balsa.

 

Edited By kc on 11/12/2020 00:44:31

[Tom Richards]

Posted by Robin Colbourne on 10/12/2020 23:58:36:

Tom, have a look at a few plans on Outerzone to get an idea for rib spacing. If you give us some idea of the intended wingspan it would help.

Do yourself a favour and get a proven model to learn to fly on, and save your own design for when you can fly without frequent crashes.

Total wingspan will be 1.335m long, the reason I am asking as it is for a school project and hence I don't have time to build a trainer model. But I will look on outerzone thanks!

[Tom Richards]

Posted by kc on 11/12/2020 00:31:41:

For wings of about 50 inch total span 2.5 inches is common for outer ribs while centre section ribs are closer and must come over the fuselage sides. But it depends on size of model and type of model.

But the advice to build a proven model and start with a trainer is the right advice. Look at Outerzone and view some of the trainer designs - Super 60, RCM Trainer, Tyro Major,etc

Ok great, we are trying to minimise weight for this aircraft so is there like a way to work out the minimum amount required. The dimensions of the span is 1.335m

[kc]

The answer is years of experience! Check out one of the designs shown or anothe recommended design. These designs have been developed the hard way - crashing to destruction - over decades and have proven structures just strong enough not to fail in flight but light enough to fly well. Designing your own is a waste of time and waste of precious balsa!

Over the decades of RC flight the areas needing reinforcement have been realised and only those parts strengthened - varies for each design.   This is critical now as a structural failure may well result in damage or injury and a lawsuit.   Therefore insurance is important.    Also a legal requirement for CAA registration for anything but tiny models.

Edited By kc on 11/12/2020 00:59:36

[Nigel R]

Hi Tom

Welcome to the forum

What's your experience of rc? Have you flown before? What will this creation be used for?

[Peter Miller]

The advice to look at existing designs is excellent. This is called "Research!"

Generally 2 1/2"rib spacing is good for most models in the range that you are looking at. Spar design is far more important as the ribs merely hold the coveing in an airfoil shape, the spars are the members that take the loads.

[Tom Richards]

Posted by kc on 11/12/2020 00:50:48:

The answer is years of experience! Check out one of the designs shown or anothe recommended design. These designs have been developed the hard way - crashing to destruction - over decades and have proven structures just strong enough not to fail in flight but light enough to fly well. Designing your own is a waste of time and waste of precious balsa!

Over the decades of RC flight the areas needing reinforcement have been realised and only those parts strengthened - varies for each design. This is critical now as a structural failure may well result in damage or injury and a lawsuit. Therefore insurance is important. Also a legal requirement for CAA registration for anything but tiny models.

Edited By kc on 11/12/2020 00:59:36

Its less than 7kg, designing the plane itself is part of the project aim, there is no way to avoid it..

[Tom Richards]

Posted by Nigel R on 11/12/2020 06:08:01:

Hi Tom

Welcome to the forum

What's your experience of rc? Have you flown before? What will this creation be used for?

Hello!

First time building a plane, I wont be flying the final result and the aircraft has to be able to carry a water payload

[Tom Richards]

Posted by Peter Miller on 11/12/2020 09:02:32:

The advice to look at existing designs is excellent. This is called "Research!"

Generally 2 1/2"rib spacing is good for most models in the range that you are looking at. Spar design is far more important as the ribs merely hold the coveing in an airfoil shape, the spars are the members that take the loads.

Perfect, thanks. I will try and focus more on the spar loads then

[kc]

Tom, you referred to 7kilos so perhaps you seem to have misunderstood my comment about CAA registration. In Britain there is compulsary registration of Operators of model planes over 250 grams Pilots also have to register and to do this they need to pass a theory test or hold a BMFA certificate etc..

If the design is to carry a payload it is even more important to have studied other proven designs especially in the wing spar design. It is also necessary to use proven methods of wing attachment, elevator and aileron hinging ( sufficient hinges and pinned ) etc,etc to avoid disastrous structural failure in the air.

Anybody can draw a pretty picture of a plane but to design a plane light enough to fly but strong enough to withstand the loads imposed requires expert knowledge. Each plane that flies is just a development of a a previous proven design.

[Tom Richards]

Posted by kc on 11/12/2020 13:12:48:

Tom, you referred to 7kilos so perhaps you seem to have misunderstood my comment about CAA registration. In Britain there is compulsary registration of Operators of model planes over 250 grams Pilots also have to register and to do this they need to pass a theory test or hold a BMFA certificate etc..

If the design is to carry a payload it is even more important to have studied other proven designs especially in the wing spar design. It is also necessary to use proven methods of wing attachment, elevator and aileron hinging ( sufficient hinges and pinned ) etc,etc to avoid disastrous structural failure in the air.

Anybody can draw a pretty picture of a plane but to design a plane light enough to fly but strong enough to withstand the loads imposed requires expert knowledge. Each plane that flies is just a development of a a previous proven design.

Ok great. The aircraft is being flown by a Professional at a flying club and Ive been told they will help as as well.

[Tom Richards]

I have looked online and I was wondering what are some typical values for Tail Volume Coefficients. I'm currently using 0.5 for the horizontal an 0.04 for the vertical. Are these values ok

[Nick Cripps]

Hi Tom,

Can't help with any values but suggest that you obtain some literature such as "Basic Aeronautics for Modellers" by Alasdair Sutherland from a well-known online site.

[Peter Miller]

Normally we look at the tailplane as percentage of the wing area.

This is also coupled to the position of the centre of gravity (CG) which is measured back from the leading edge of the wing as a percenatge of the chord (Width) of the wing.

For maximum stablity 25% of the wing area is very safe

The balance point between 25 and 30% of the chord is normal.

A very valuable book is Flight Without Formulae by Kermode. Easy to read and so good that, although first written in the 1930s it is still in print

[Tom Richards]

Posted by Peter Miller on 11/12/2020 18:20:30:

Normally we look at the tailplane as percentage of the wing area.

This is also coupled to the position of the centre of gravity (CG) which is measured back from the leading edge of the wing as a percenatge of the chord (Width) of the wing.

For maximum stablity 25% of the wing area is very safe

The balance point between 25 and 30% of the chord is normal.

A very valuable book is Flight Without Formulae by Kermode. Easy to read and so good that, although first written in the 1930s it is still in print

Ok perfect those are the exact values we got!

[Robin Colbourne]

Posted by Tom Richards on 11/12/2020 00:37:58:

Total wingspan will be 1.335m long, the reason I am asking as it is for a school project and hence I don't have time to build a trainer model. But I will look on outerzone thanks!

Tom, I see where you are coming from now.

It would be a good idea to Google "BMFA Heavy Lift Challenge" and "BMFA Payload Challenge" as these were similar competitions for university students.

Also read the rules you have been given, very carefully. The wingspan (1335mm) and the weight (7kg) are only realistic with a fairly large engine or a catapult launch.

Don't get obsessed with the airframe weight in relation to the payload. Light is nice, but controllability and robustness are equally important. A lot of entries for this type of competition are either uncontrollable or break on the first attempt and are out of the competition before they've started.

If there is an engine or motor limit you may be better off not aiming for the full payload, but just what is realistic for the power and wing area.

If you can let us see all the rules (photo or PDF) it would be a big help.

You could consider an unorthodox design such as the Wainfan Facetmobile. It maximises lifting area for the width (wingspan) as well as being quite robust.

Edited By Robin Colbourne on 11/12/2020 18:53:21

[Simon Chaddock]

A tail volume coefficient is a reasonable for the horizontal tail but it is just a ratio figure.

The actual dimensions and what gives the best combination of aerodynamic effectiveness, stiffness and strength is a matter of experience or plagiarism.

This is why the advice is start with a design that is known to work and 'adjust' it to suit the particular circumstances.

If your project is carrying a payload the plane's structural design (its strength to weight) is likely to be as important, possibly even more so, than the quality of its aerodynamics.

[Tom Richards]

Posted by Robin Colbourne on 11/12/2020 18:52:04:

Posted by Tom Richards on 11/12/2020 00:37:58:

Total wingspan will be 1.335m long, the reason I am asking as it is for a school project and hence I don't have time to build a trainer model. But I will look on outerzone thanks!

Tom, I see where you are coming from now.

It would be a good idea to Google "BMFA Heavy Lift Challenge" and "BMFA Payload Challenge" as these were similar competitions for university students.

Also read the rules you have been given, very carefully. The wingspan (1335mm) and the weight (7kg) are only realistic with a fairly large engine or a catapult launch.

Don't get obsessed with the airframe weight in relation to the payload. Light is nice, but controllability and robustness are equally important. A lot of entries for this type of competition are either uncontrollable or break on the first attempt and are out of the competition before they've started.

If there is an engine or motor limit you may be better off not aiming for the full payload, but just what is realistic for the power and wing area.

If you can let us see all the rules (photo or PDF) it would be a big help.

You could consider an unorthodox design such as the Wainfan Facetmobile. It maximises lifting area for the width (wingspan) as well as being quite robust.

Edited By Robin Colbourne on 11/12/2020 18:53:21

I think this is what the task is based on. We are trying to carry 1.5kg payload and hope our aircraft to be about 3kg. So it seems all good, and yes there is a set motor hence why we havent gone for the max weight. Thanks for the all the advice

[Tom Richards]

Posted by Robin Colbourne on 11/12/2020 18:52:04:

Posted by Tom Richards on 11/12/2020 00:37:58:

Total wingspan will be 1.335m long, the reason I am asking as it is for a school project and hence I don't have time to build a trainer model. But I will look on outerzone thanks!

Tom, I see where you are coming from now.

It would be a good idea to Google "BMFA Heavy Lift Challenge" and "BMFA Payload Challenge" as these were similar competitions for university students.

Also read the rules you have been given, very carefully. The wingspan (1335mm) and the weight (7kg) are only realistic with a fairly large engine or a catapult launch.

Don't get obsessed with the airframe weight in relation to the payload. Light is nice, but controllability and robustness are equally important. A lot of entries for this type of competition are either uncontrollable or break on the first attempt and are out of the competition before they've started.

If there is an engine or motor limit you may be better off not aiming for the full payload, but just what is realistic for the power and wing area.

If you can let us see all the rules (photo or PDF) it would be a big help.

You could consider an unorthodox design such as the Wainfan Facetmobile. It maximises lifting area for the width (wingspan) as well as being quite robust.

Edited By Robin Colbourne on 11/12/2020 18:53:21

I think this is what the task is based on. We are trying to carry 1.5kg payload and hope our aircraft to be about 3kg. So it seems all good, and yes there is a set motor hence why we havent gone for the max weight. Thanks for the all the advice

[Tom Richards]

Posted by Simon Chaddock on 11/12/2020 19:34:17:

A tail volume coefficient is a reasonable for the horizontal tail but it is just a ratio figure.

The actual dimensions and what gives the best combination of aerodynamic effectiveness, stiffness and strength is a matter of experience or plagiarism.

This is why the advice is start with a design that is known to work and 'adjust' it to suit the particular circumstances.

If your project is carrying a payload the plane's structural design (its strength to weight) is likely to be as important, possibly even more so, than the quality of its aerodynamics.

Ok perfect, that all makes sense. Thanks a lot

[Robin Colbourne]

Posted by Tom Richards on 11/12/2020 22:14:14:

I think this is what the task is based on. We are trying to carry 1.5kg payload and hope our aircraft to be about 3kg. So it seems all good, and yes there is a set motor hence why we havent gone for the max weight. Thanks for the all the advice

Ah!!! That's a much more achievable target, Tom.

Having lost a model in a water-carrying competition once, due to it getting in a servo, if you are likely to build your design, put all the R/C and electrics as far away as possible from where the water is carried.

[perttime]

Posted by Peter Miller on 11/12/2020 09:02:32:

The advice to look at existing designs is excellent. ...

For a First Model, you might also consider something that doesn't make you cry too much when you break it.

Something with a minimum of blood, sweat, tears, time, and money in it - before First Flight.