Medium vs Hard Balsa

[Nigel Heather]

Hi,

Going to be doing my first ever 'build from plans' this year. Confident of my building skills, have built plenty from kits before (including intensive ones like the Flair Puppeteer) but I've never from a plan where I have to select the wood.

This is an electric sports plane, 45" span, weight less than 3lbs - I want to keep the build light.

Intend to work out what balsa and accessories I need and then order online from someone like SLEC.

I notice when selecting balsa it offers Medium or Hard (maybe Soft too but not seen that offered so far).

I'm guessing that most if not all will be Medium. Is there a rule of thumb for when I should select Hard - maybe spars?

Cheers,

Nigel

[Simon Chaddock]

Nigel

It all comes down to the requirement of strength and weight.

Balsa can have a wide range of densities so building skill is knowing which wood, or anything else for that matter, is appropriate for the duty required of it. This of course requires a good understanding of the loads each part of the air frame is likely to bear not only in flight but for resistance to any "unwanted contact with the ground"!

What you have to avoid if possible is "I don't know so I will use hard just to be safe". The danger of this approach is you will not only make the plane heavier than it need be but weight in the wrong place can be even more serious.

It would be hoped the plan you are working from gives an indication of what type of balsa to use and where.

If it does not then hard for spars is a reasonable approach but there are other areas that may need extra strength, typically the nose area (it carries the motor and usually hits the ground first) whereas the tail need to be relatively much lighter as it carries less load and and being a long way back any excess weight then requires several times as much in the nose to compensate.

I hope this gives a bit of background.

Which plane are you proposing to build? It is quite possible someone on here may already have done so and might be able to point out any build tips or pitfalls to avoid,

 

Edited By Simon Chaddock on 03/02/2020 11:54:22

[Martin McIntosh]

Medium on spars maybe but never hard unless you wish to end up with a brick. Almost all construction involves soft, very light balsa; even so you will find it difficult to get down to a designed weight. You need to pick this by hand, do not rely on a shop sending you nice light stuff because they are likely to be only too glad to get rid of the wood nobody else wants. The Balsa Cabin is the exception here. Specify what you want if ordering by mail or better still go in person.

Don`t buy a plan pack or you may end up with the rubbish stated above.

I recently managed to shave 4 3/4lbs on a second version of a 72" model, the first being 18lbs.

PM me if you want more info.

[Nigel Heather]

Hi,

Thanks for the advice so far. It's an Ohmen, 45" sports/scale-like, electric, target weight between 2.5lbs and 3 lbs.

So far, I have only looked at SLEC - just looked again and it does offer Soft/Medium/Hard. The plan doesn't give any clues that I can see but the idea is to build light.

So should I be picking soft and medium. Medium for wing spars and formers. What about the fuselage sides?

Cheers,

Nigel

[Shaun Walsh]

You could always buy the the laser cut short kit from Sarik.

https://www.sarikhobbies.com/product/the-ohmen-47-5-short-kit-set/

[Martin Dance 1]

Balsa is a very variable material even through individual sheets, I once cut a 4 inch by 1/4 sheet into four equal size pieces. There was nearly 10 gms difference between the lightest and heaviest piece!

If you want to be OCD, and why not? But a set of 'drug dealers scales'off ebay so you can weigh each part, Over the top? Not really build a pair off wings without paying attention to the weight of each component can mean one panel weighing much more than the other, this will have a much more significant effect on the models performance than a slightly miss selected piece for a spar.

[Chris Walby]

Plenty of advice here or you could contact Peter Miller (the designer) if you have still not obtained the desired answer. IMHO I think the design is quite accommodating to the various grades of balsa that can be supplied.

SWMBO has just built one and although the wing is quite open it seems very ridge with the wood pack supplied.

PS, worth a visit to SLEC to see Chris selecting your wood for its various requirements at lightning speed from their stock.

[Peter Miller]

Hard for spars. Medium for the sides, soft for the rolled sheet turtle deck and bottom sheet

Make gthe tail surfaces from soft sheet, not built up as shown because A) I needed a little wieght in the tailanyway and the built up surfaces did not like tumbles.

Don't forget the triangular stock inht ecorner between F-1 and the sides.

Mine was built thaat wat and is deal light

[kc]

The Ohmen is a Peter Miller plan so if you post again in that section I am pretty sure you will get a reply from the designer himself.......... but I think he normally specifies the type of balsa in his article or on his build section here. Usually for his designs he says hard for spars ( contrary to what Martin has advised! ) and medium elsewhere.

[Peter Miller]

Posted by Martin McIntosh on 03/02/2020 12:04:10:

Medium on spars maybe but never hard unless you wish to end up with a brick. Almost all construction involves soft, very light balsa; even so you will find it difficult to get down to a designed weight. You need to pick this by hand, do not rely on a shop sending you nice light stuff because they are likely to be only too glad to get rid of the wood nobody else wants. The Balsa Cabin is the exception here. Specify what you want if ordering by mail or better still go in person.

Don`t buy a plan pack or you may end up with the rubbish stated above.

I recently managed to shave 4 3/4lbs on a second version of a 72" model, the first being 18lbs.

 

PM me if you want more info.

How on earth did you manage to build a 72" span model to 18 lbs!!!! The mind boggles!!! The wing loading must have been insane

Edited By Peter Miller on 03/02/2020 13:54:21

[Martin McIntosh]

Hi Peter,

It was a TN Spitfire and I decided to try the precut pack. It presumably came from SLEC and you would not believe how awful it was. The fus is made from several pieces and one was so hard that I could not bend it by hand to join at the rear. I have put this on my build blog for the model already. The so called light ply ribs were from splintery old tea chests and weighed a ton. I decided to go ahead anyway and and that was the weight it ended up as, but it still flies well. Most other versions of this which I have heard of are between 16 and 18lbs. Landing has to be very gentle or it is yet another retract rebuild.

I dispensed with all of the 1/4" birch ply parts on the new one and hand picked the wood from BC.

Now over powered with a Laser 180 so down sized to a 155 which is just right.

Nigel, you may wish to look at my build blog on the Ohmen to supplement Peter`s instructions.

[Nigel R]

rules of thumb for me:

spars - hard balsa

wing skins (also tailplane skins, if built that way) or sheeting (D-box, turtle decks, etc) - soft / light

anything made out of big lumps of block - soft / light

everything else - medium

if something needs lots of strength (U/C mounting areas, for instance) then reinforce with some doublers, typically 1/32 or 1/16 ply

[Peter Miller]

Hi Martin.

SLEC have cut sets of parts for me, No complaints.

I don't know the TN Spitfire but there must be an awful lot of large section wood in it.

I use hard spars. I work on the full size practice of Primary, secondary and tertiary structures. By doing that one gets maximum strength with minimum weight.

[kc]

The best way to select balsa is to weigh it on a digital scale and compare various sheets. Often 2 sheets will look the same but be very different weights. Treat your kitchen to a set of digital scales and get one that can be zeroed ( tare ) when used with a suitable object like a plastic box to lift the balsa or complete plane clear of the worktop. There are charts to show how much a sheet of any given size should weigh for hard, medium, soft or contest grade.

Selecting sheets for the job in hand is a bit of an art. It's worth ordering a few extra sheets as postage is dear. Trying potential fuselage sides to see if they bend the same is worthwhile. Using the lightest possible sheet for the tail parts will obviously be best. Save any sheets that will bend easily across the grain for curved fus sheeting, and for the LE sheeting. Ribs can be cut more economically by interleaving LE with TE and it does not seem to matter much if by doing so the grain is at a slight angle.

There are several things of note in the Ohmen articles

1 Triangular reinforcement to F1 is needed but not shown on plan.

2. the tail parts could be made from sheet if suitable lightweight sheet is avail.

3 the wing ribs are actually on a corner of the plan for the Coquette model in the free/ pullout plan!

4. Peter advised that the ply doubler could do with being one inch longer.

 

Edited By kc on 03/02/2020 15:31:51

[Martin McIntosh]

Hi again Peter,

I kept the same wood thickness throughout with the exception of the formers and wing spars, which were originally 3/8" sq spruce x 8, replaced with 1/4x1/2 medium balsa. Not knocking the original design in any way because you have to start somewhere on a highly stressed model. I even left out the braces on the now three piece wing and relied on the glass cloth covering. Despite trying I have not managed to break it yet. Nigel seems to have the right ideas as to wood choice from what I can tell other than the fus sides which in my opinion should be very light since they account for most of the weight at the rear end.

[Martin McIntosh]

PS,

Just checked and mine is 2 3/4lbs ready to fly.

[Nigel R]

Light would work for fuselage sides - the normal ply doubler up front mops up all the structural needs.

I wouldn't worry about it too much either way though. The tailplane weight dominates the balance issue, not the fuselage sides.

[Peter Miller]

My Ohmen weighs 2 lbs 6.8 ounces. including about 10 grams of lead under the tailplane

[Nigel Heather]

Posted by Peter Miller on 03/02/2020 18:14:47:

My Ohmen weighs 2 lbs 6.8 ounces. including about 10 grams of lead under the tailplane

The designer of the Ohmen has been really helpful and provided lots of guidance. He also suggested making the tail, fin, elevator and rudder from solid sheet so as to use the inevitable balance weight to add robustness to the rear end.

Cheers,

Nigel

[Peter Miller]

I have been reccomending this modification ever since a rather abrupt landing made the tailplane break and then the fin gt a bit er...flexible

[Martin Harris - Moderator]

Posted by Peter Miller on 03/02/2020 13:52:01:

Posted by Martin McIntosh on 03/02/2020 12:04:10:

I recently managed to shave 4 3/4lbs on a second version of a 72" model, the first being 18lbs.

How on earth did you manage to build a 72" span model to 18 lbs!!!! The mind boggles!!! The wing loading must have been insane

I can believe that original model may have flown OK as my fairly well detailed [read that as heavily built!] Miles Atwood Special came in at a similar weight at 73" span - although it was a large model overall at 1/3 scale. There was a great deal of lead concealed just behind the spinner. It flew - marginally - on an SC 120FS and soon after, quite well on an ASP 180FS, both propped to comply with the 82dB limit. The only downsides to the rather high wing loading were a tendency to flick on a hurried spin recovery and to be unforgiving if energy management at low airspeed was neglected.

Incidentally, this was the cleanest IC model I've ever built - the exhaust extension was flush with the hole in the underside of the cowl and there was only ever the merest hint of oil staining just behind it after extensive flying - and none anywhere else on the airframe! The same engine in a Yak 54 on the same fuel left the airframe dripping with oil!

Edited By Martin Harris on 04/02/2020 12:31:04

[eflightray]

Posted by Martin McIntosh on 03/02/2020 14:37:19:

Hi Peter,

It was a TN Spitfire and I decided to try the precut pack. It presumably came from SLEC and you would not believe how awful it was. The fus is made from several pieces and one was so hard that I could not bend it by hand to join at the rear. I have put this on my build blog for the model already. The so called light ply ribs were from splintery old tea chests and weighed a ton. I decided to go ahead anyway and and that was the weight it ended up as, but it still flies well. Most other versions of this which I have heard of are between 16 and 18lbs. Landing has to be very gentle or it is yet another retract rebuild.

I dispensed with all of the 1/4" birch ply parts on the new one and hand picked the wood from BC.

Now over powered with a Laser 180 so down sized to a 155 which is just right.

 

Nigel, you may wish to look at my build blog on the Ohmen to supplement Peter`s instructions.

Now I built the TN 72" span Spitfire Mk V, (free plan 2010) specifically for electric power, minimum of balsa, (used light weight), with Depron sheet in place of much of the balsa.

Mine flies at 7 lbs - 2 oz, (that includes, flaps, retracts, and a 6s Lipo).

I know some people like heavy models for IC power, I just like lighter builds when it comes to electric power.

Luckily I have a model shop not too far away where I can select balsa. I have found by weighing some sheets, a good lightweight sheet ( 4" x 36" ) , can weigh the same as a medium/hard sheet of half the thickness. Selection is well worthwhile if you are a weight conscious builder.

The model is nearly 8 years old, still flies great, and my favorite flier. Building light is just another technique.

Ray.

Edited By eflightray on 04/02/2020 14:46:56

[Martin Harris - Moderator]

Here's an interesting quote regarding the weight of the 76" Tempest, which is my current long term project:

When the model was designed the FAI weight limit was 5kg and IB used an OS 120FS in the prototype. His later models were fitted with Super Tigre 3000 (30ccm), weighed 7kg and flew much better!! Ian actually test flew a Tempest built by a customer who had a Quadra 42 petrol fitted with an all up weight of 10kg. Ian said that it flew better than his at 7kg!!

[Don Fry]

Can't help thinking a Super Tiger two stroke trying to lug that lot about is not going to be great. A bigger prop helps, hence the quadra scores.

But to post, someone mentioned heavy IC models. I have never seen the point in putting any additional weight on a model. And all an IC model needs is a slightly stiffer nose to damp the extra vibration down. And by the time it's all installed, I don't think there is any difference in weight. They all break when it hits the ground. The elusive trick is avoiding hitting it.

Edited By Don Fry on 04/02/2020 16:40:26

[kc]

Lighter models crash lighter and they also crash less often!