Experimental build - 1.7m wingspan F8F Bearcat

[Lipo Man]

Just starting an experimental build of a Grumman F8F Bearcat. If it gets finished it will be 1.7m span and very approximate in terms of scale detail. The payback will be that it should be a very quick and cheap build.
Material will be a mix of Hobbycraft foamboard and B&Q home insulation foam sheets. Plans are being done extrapolating from a 24in span balsa control line design, blown up to 270%. 
Tomorrow I’ll try building one wing and see if the design works. If it doesn’t I may park the idea. 
Here goes nothing!

[PDB]

Very interested to see what techniques you come up with for this mix of materials. 👍

[Lipo Man]

10 hours ago, PDB said:

Very interested to see what techniques you come up with for this mix of materials. 👍

The wing will have a foamboard box spar, and then insulation foam ribs and skin. There’s a sort of building jig from the rib “offcuts” to hold them in place while the skins are glued on - that’s an attempt to build in some washout and make both wings the same. Not sure if it will work - we will see!

[Graham Davies 3]

Great project, and delighted to see another chap raising the foam/ foamboard bar.

 

You've probably seen my 6th scale spitfire (74", or 1880mm). This is built from B&Q insulation, some balsa, lite ply and ply. If it helps; this is 9was) my approach:

 

• 1/6 scale was chose due to the availability of a canopy from Mick Reeves; I actually made use of some other accessories from Mick, so this was a good start
• I found a suitable plan (or 3 view; not fussy) which turned out to be Brian Taylor's Mk1. I scaled this and got it printed at my local print shop.
• I then scribbled all over it with the structure I felt would work
• I started the wings by thinking about strength in the middle section out to the retracts. The structure I settled on was based on a conventional structure using 6x6 pine (from B&Q!) spars with 1/16" balsa sheer webs. In the centre, A solid 1/8" ply dihedral brace allowed a solid mounting point for the retracts
• Remaining structure was basic; foam ribs and balsa false trailing edges for the ailerons so I could use robart hinges. Leading edge is 3 laminations of 6mm foam.
• Whole lot is skinned with 6mm foam, flaps cut from same and reinforced with strips of 1/16 balsa to replicate the scale details
• Fuselage is built on a cruciform with a profile and plan taken from the outline, and the whole lot is built up onto this. Lots of formers to support an outer skin. Doublers from liteply for wing seat through to F1, and the centre foam vertical reinforced with 1/32 ply. The whole lot is then clad in 3mm foam, carefully curved and glued with UHU as a contact. You need a lot of patience here to tease it down without cracking it.
• Cowl is made by laminating 3 or 4 thicknesses of some polystyrene I got free with a fridge! Carved to shape and with a girt big 'ole in the middle to clear the motor. I then gluded it to F1, and finally carved/ filled to get the shape right before stiffening it with 3 or 4 layers of 25gsm glass cloth applied with B&Q water based varnish.
• Then 25gsm glass cloth over the lot, emulsion paint, and a load of grinning...

 

It's come out very well, weighs a little over 8pounds, and flies beautifully. Good luck Lipoman, and feel free to contact me if you get stuck.

 

Graham

[Tosh McCaber]

Nice project for publishing in RCM&E, Graham!!

[Simon Chaddock]

My only concern with your proposed technique is the foam board box spar is not ideal structurally. The box spar would be better made as deep as possible within the wing section particularly at the root where the bending forces are a maximum. 

Cladding a wing in a more flexible foam makes sense but it does rather ignore the potential strength of the foam itself. Some of it alter all will be at the maximum possible wing depth on what looks to be quite a 'generous' wing section. This does raise the question as to whether the box spar needs to go right to the wing tip.

The B&Q XPS foam sheets do have some strength and like most of this type of material more stiffness and strength in one direction than the other.

It is possible to build a complete wing out of just 5mm B&Q foam.

The LH wing of an EDF Sea Hawk.

I try to work on the principle that sheet foam is a 'thick' material and thus needs far less support than conventional thin wing skins.

I will follow your build with interest.

[Eric Robson]

Simon is right the main spar should go top to bottom, I have used foamboard for spars by putting two close together full depth root to tip and feeding servo wires and retract wires through. I  tend to make a hard balsa spar for something I am going to throw around the sky.

 

This picture is how I done the Westland Whirlwind wing no ribs just foam board spars, this is the outer panel 1/16" ply braces were used to join at the dihedral angle

Good luck with the project,  I will be following. 

Edited March 30 by Eric Robson
Photo added

[Mike Chantler]

Nice project, interesting! looking forward to seeing how it works out at this size 🙂 

[Lipo Man]

7 hours ago, Graham Davies 3 said:

Great project, and delighted to see another chap raising the foam/ foamboard bar.

 

You've probably seen my 6th scale spitfire (74", or 1880mm). This is built from B&Q insulation, some balsa, lite ply and ply. If it helps; this is 9was) my approach:

 

• 1/6 scale was chose due to the availability of a canopy from Mick Reeves; I actually made use of some other accessories from Mick, so this was a good start
• I found a suitable plan (or 3 view; not fussy) which turned out to be Brian Taylor's Mk1. I scaled this and got it printed at my local print shop.
• I then scribbled all over it with the structure I felt would work
• I started the wings by thinking about strength in the middle section out to the retracts. The structure I settled on was based on a conventional structure using 6x6 pine (from B&Q!) spars with 1/16" balsa sheer webs. In the centre, A solid 1/8" ply dihedral brace allowed a solid mounting point for the retracts
• Remaining structure was basic; foam ribs and balsa false trailing edges for the ailerons so I could use robart hinges. Leading edge is 3 laminations of 6mm foam.
• Whole lot is skinned with 6mm foam, flaps cut from same and reinforced with strips of 1/16 balsa to replicate the scale details
• Fuselage is built on a cruciform with a profile and plan taken from the outline, and the whole lot is built up onto this. Lots of formers to support an outer skin. Doublers from liteply for wing seat through to F1, and the centre foam vertical reinforced with 1/32 ply. The whole lot is then clad in 3mm foam, carefully curved and glued with UHU as a contact. You need a lot of patience here to tease it down without cracking it.
• Cowl is made by laminating 3 or 4 thicknesses of some polystyrene I got free with a fridge! Carved to shape and with a girt big 'ole in the middle to clear the motor. I then gluded it to F1, and finally carved/ filled to get the shape right before stiffening it with 3 or 4 layers of 25gsm glass cloth applied with B&Q water based varnish.
• Then 25gsm glass cloth over the lot, emulsion paint, and a load of grinning...

 

It's come out very well, weighs a little over 8pounds, and flies beautifully. Good luck Lipoman, and feel free to contact me if you get stuck.

 

Graham

Wow. That’s a stunner. This build is going to be more in the Flitetest realm - not going for much in the way of realistic scale detail, and weight target is about half what your Spitfire is flying at. However at that weight could be a nice easy flyer. 
Congrats on the Spitfire - lovely build!

[Lipo Man]

4 hours ago, Simon Chaddock said:

My only concern with your proposed technique is the foam board box spar is not ideal structurally. The box spar would be better made as deep as possible within the wing section particularly at the root where the bending forces are a maximum. 

Cladding a wing in a more flexible foam makes sense but it does rather ignore the potential strength of the foam itself. Some of it alter all will be at the maximum possible wing depth on what looks to be quite a 'generous' wing section. This does raise the question as to whether the box spar needs to go right to the wing tip.

The B&Q XPS foam sheets do have some strength and like most of this type of material more stiffness and strength in one direction than the other.

It is possible to build a complete wing out of just 5mm B&Q foam.

The LH wing of an EDF Sea Hawk.

I try to work on the principle that sheet foam is a 'thick' material and thus needs far less support than conventional thin wing skins.

I will follow your build with interest.

I’m sort of along those lines - but rethinking a bit based on the feedback here. I’ll deepen the spar to add some strength. 

[Lipo Man]

2 hours ago, Eric Robson said:

Simon is right the main spar should go top to bottom, I have used foamboard for spars by putting two close together full depth root to tip and feeding servo wires and retract wires through. I  tend to make a hard balsa spar for something I am going to throw around the sky.

 

This picture is how I done the Westland Whirlwind wing no ribs just foam board spars, this is the outer panel 1/16" ply braces were used to join at the dihedral angle

Good luck with the project,  I will be following. 

Having a rethink based on your feedback - will make the box full depth. 

[Lipo Man]

Back to the build this evening. Full depth foamboard spar feels more than sufficiently robust for the kind of flight loads I’m anticipating. Feeling optimistic!

 

[Graham Davies 3]

That should do it L-man

 

Graham

[Lipo Man]

Simple jig constructed from the rib “off cuts” to hold the intended washout. Skinned with pre-shaped 6mm foam sheet glued with foaming gorilla glue. I’ve left it all overnight to see how it works - fingers crossed!

 

[Lipo Man]

Glue looked dry-ish so decided to take a look. Very excited about this - looks promising. Very rigid and only 160g - pretty good!

 

[RICHARD WILLS]

This is what I like about Foam Board/ depron .  

You can just try something with very little outlay and if you get bits wrong , just chop them out . 

I'm very interested to see how this turns out LM .

[Simon Chaddock]

Lipo Man

That is very neat.

However with a 6mm foam skin I do wonder how much work the spar will be doing apart from stopping the skin sagging between the ribs when it is under tension and buckling when under compression at the point of maximum stress. Alternatively you could argue that if the box spar is the primary source of strength than what is all that skin foam doing?

Technically a box spar is used where torsional rigidity is required as well as bending strength however it is quite likely the "all over" 6 mm foam skin will actually provide all the torsional rigidity required thus making the box element of the spar redundant.

I do appreciate there are construction benefits in having a box to work from but with such a thick skin I always worry how much stiffer or alternatively  lighter any spar would be if it was flush with the outer surface of the foam skin.

You can see where this sort of thinking lead me with foam wing construction.

I too will be interested in how it pans out.    

[Lipo Man]

41 minutes ago, Simon Chaddock said:

Lipo Man

That is very neat.

However with a 6mm foam skin I do wonder how much work the spar will be doing apart from stopping the skin sagging between the ribs when it is under tension and buckling when under compression at the point of maximum stress. Alternatively you could argue that if the box spar is the primary source of strength than what is all that skin foam doing?

Technically a box spar is used where torsional rigidity is required as well as bending strength however it is quite likely the "all over" 6 mm foam skin will actually provide all the torsional rigidity required thus making the box element of the spar redundant.

I do appreciate there are construction benefits in having a box to work from but with such a thick skin I always worry how much stiffer or alternatively  lighter any spar would be if it was flush with the outer surface of the foam skin.

You can see where this sort of thinking lead me with foam wing construction.

I too will be interested in how it pans out.    

The spar felt very stiff and the intention was that this takes the bending loads. It feels easily capable of that. The hope was that the skin will add the torsional stiffness - and it does feel pretty resistant to twisting. With the skin glued onto the other side it now weighs 250g - so that’s 500g for a 1.7m span wing before servos are added etc. That seems pretty good to me?

[Simon Chaddock]

250g is not bad at all for such a big span and chord wing.

 

[Lipo Man]

6 minutes ago, Simon Chaddock said:

250g is not bad at all for such a big span and chord wing.

 

To be honest I wasn’t intending to build something quite this big, but it didn’t look as bad on the laptop screen…

😳

[Simon Chaddock]

I can only advise that you put equal thought into keeping down the weight of the Bearcat's pretty "tubby" fuselage. The lighter the overall weight the less power you need and the lower the stress on the wing.

A quick strength test.

When complete and including the battery try picking it up by just its wing tips. If this can be done without the structure making to many 'stress' noises it will put a bending load on the wing equivalent to pulling 4g or doing a 'proper' circular loop. If it feels like the wing is going to break before the plane is fully lifted then stop. The plane can still be flown but steer clear of any rapid or substantial elevator stick movements!

 

I have physically tested all my planes this way except the Depron 2.1m span Antonov AN124. With a thrust to weight of just 0.4 : 1 it simply does not have the push for aerobatics but then neither does the full size.😉   

  

[Lipo Man]

1 hour ago, Simon Chaddock said:

I can only advise that you put equal thought into keeping down the weight of the Bearcat's pretty "tubby" fuselage. The lighter the overall weight the less power you need and the lower the stress on the wing.

A quick strength test.

When complete and including the battery try picking it up by just its wing tips. If this can be done without the structure making to many 'stress' noises it will put a bending load on the wing equivalent to pulling 4g or doing a 'proper' circular loop. If it feels like the wing is going to break before the plane is fully lifted then stop. The plane can still be flown but steer clear of any rapid or substantial elevator stick movements!

 

I have physically tested all my planes this way except the Depron 2.1m span Antonov AN124. With a thrust to weight of just 0.4 : 1 it simply does not have the push for aerobatics but then neither does the full size.😉   

  

Starting the fuselage today. Simple foamboard box as a core, with formers and skin of B&Q insulation foam. Hoping it’ll be pretty light, but the motor is a heavy lump so there’s a limit to how light I can get it!

[Lipo Man]

4 hours ago, Simon Chaddock said:

I can only advise that you put equal thought into keeping down the weight of the Bearcat's pretty "tubby" fuselage. The lighter the overall weight the less power you need and the lower the stress on the wing.

A quick strength test.

When complete and including the battery try picking it up by just its wing tips. If this can be done without the structure making to many 'stress' noises it will put a bending load on the wing equivalent to pulling 4g or doing a 'proper' circular loop. If it feels like the wing is going to break before the plane is fully lifted then stop. The plane can still be flown but steer clear of any rapid or substantial elevator stick movements!

 

I have physically tested all my planes this way except the Depron 2.1m span Antonov AN124. With a thrust to weight of just 0.4 : 1 it simply does not have the push for aerobatics but then neither does the full size.😉   

  

Structure for the fuselage is underway. It’s big…

Formers are 9mm insulation foam from B&Q, and the skin will be 6mm thickness of the same stuff. The central box structure feels very stiff - I’m going to cut some holes in the rear half to help with the CG. It feels strong enough that I think I can afford to lose some material. 
The orange in the photo is for scale - we’d run out of bananas…

 

[RICHARD WILLS]

Yes , but what scale is the Orange ?

[Lipo Man]

2 hours ago, RICHARD WILLS said:

Yes , but what scale is the Orange ?

Ah yes. A good point well made. It’s 1/6th obviously, as the good lord intended. I made it to go in my FW190…