Plan builds using balsa, how heavy are yours?

[Chris Walby]

Question for anyone who has ever built a balsa models from a plan....Quite usual for the designer to quote AUW, but has anyone ever built as per plan and come out on target or below with the AUW?

 

My question is bourn from building half a dozen plan models where none of them come out on target weight or less irrespective of designer. I can't blame poor wood selection as that was done by an expert!

 

Builders who have either changed the build from the plan by adding features or cutting additional holes clearly don't count (or is it everyone but me that deviates from plan!).

 

Lastly if anyone builds models that consistently come out under target AUW...could you share your secret please? 

[Stephen Smith 14]

Glue is heavy and excess is no stronger 

[Richard Acland]

I build all of my models from plans. I think without fail I change things along the way. If something looks like it is a weak point I will strengthen it. If I feel that if something is a bit heavy, I will lighten it. Building in lightness without compromising on strength is the answer. Doing this normally enables me to knock a bit off the original design weight. The photo shows a model that I built completely changing the tailplane and fin assembly to lose a lot of weight at the rear end.

[Nightflyer]

From personal experience over the years in most instances the auw often is an indicative figure. Wood selection plays it's part in the weight and in strength too. As someone else has said amount of glue and type of glue also does.  In reality unless it's a total brick, provided you have built true and cofg is correct it should be no problem unless power is marginal. Of course a dead stick could lead to less of a floater.

[Gary Manuel]

That rear end looks really light Richard. Did it avoid the need to add lead up front, because that is the real overall weight saving, not just the actual build.

[Richard Acland]

Yes, no added weight up front. It did bring the CofG forward slightly but it made no difference to the overall flying performance.

[john stones 1 - Moderator]

I think mine are pretty good, I don't obsess about weight, but the servos etc get fitted last, to aid balancing without lead.

[Gary Manuel]

 

16 minutes ago, Richard Acland said:

Yes, no added weight up front. It did bring the CofG forward slightly but it made no difference to the overall flying performance.

I'm pretty sure that it will have made a difference to flying performance. A lighter model needs less lift to offset the weight and therefore needs less forward speed to achieve flight. It flies at slower speeds, which is a very good thing.

15 minutes ago, john stones 1 Moderator said:

I think mine are pretty good, I don't obsess about weight, but the servos etc get fitted last, to aid balancing without lead.

Me too. Lead is good for church roofs, but not for aeroplanes.

[GrumpyGnome]

Like practically all of us, I hate adding weight....... unlike most, I have never actually weighed a model!

GG

[Eric Robson]

Sometimes deviating from the plan is a good thing as designers don't always get it right. some over the early plans for RC were over the top C.A.P. in particular where they called for the area from the LE to the undercarriage mounting to be filled with block balsa, The Brian Taylor Tempest I am currently building calls for the elevators and rudder to be made from two laminations of 3/8" balsa. So any way I can see of loosing weight without compromising the structure I will go for.

Eric r. 

[Jon H]

I have never understood the hate for adding lead. The model has to balance, so add the lead. This assumes all efforts have been made to minimise the amount you have to add, but if you have, then just stick it in there. I also dont go for the 'use a bigger engine' idea either. Lead is cheaper than a bigger engine, and you dont need the power anyway so it is just a waste of money. 

 

When it comes to warbirds i work on the basis of 10% of total weight being lead. Anything less than that is a winner. My 23lb Sea Fury has 2lbs of lead in the cowling. Its also got a laser 360 and the batteries. Servos are well forward, and the only addition to the tail is a lightweight plastic retract unit for the tailwheel. Without that i could probably shave a few oz off the lead at the front, but 22.5 vs 23lbs? who would notice the difference? 

 

As for lead being no good for aeroplanes..sorry Gary, its nonsense. Many aircraft have ballast in them. Just take a look at this Spitfire!

 

http://i536.photobucket.com/albums/ff321/taylortony/Spitfire MKx1x/more Spit/_MG_6785Edgar.jpg

 

In answer to the OP, i never really pay much attention to the final weight. I try to make it as light as i can, but in the end it is what it is. If its 2lbs over weight i cant do much about it once finished so there is not much point in weighing it. 

 

 

 

Edited March 12, 2021 by Jon - Laser Engines

[Bob Cotsford]

Those models that I've weighed tend to be a couple of ounces over the stated AUW, but then I don't tend to skimp on power within reason, whether IC or electric.  I often seem to need a touch of tail weight?.

I recall that when I were a lad the model shops were full of wonderful Solarbo balsa in grades from feather light to rock hard so you could actually find ultra light balsa block for infills, solid tails et al.  Now it all seems medium to hard grades unless you catch the delivery of new stock that still has some light stuff left, or order contest grade.

 

I'm afraid that I don't follow the 'lighter is always better' theory of flight either, at least not for the average  British day.  A LITTLE more weight helps smooth things out in the often turbulent air at low levels.

 

Edited March 12, 2021 by Bob Cotsford

[GrumpyGnome]

Oh yes, lead and proper balance is way better than no lead and a bad balance..... ?

 

GG

[Simon Chaddock]

Jon

Yes that Spitfire has lead in it but it is in the tail and is small when compared to the planes all up weight. 0.5%? The lead in your Sea Fury is 8.6%. Hmmmm.

   

[Cuban8]

An interesting subject with a whole range of possible conclusions given the diversity of model types, performance etc.

Without a whole lot of aerodynamic test data and mathematics to go on (which will be beyond the scope of many of us including me) we can usually only rely on practical experience to make a judgement about any model's potential performance, or maybe learn from others and how they get on....good or bad.

In my experience, some models will carry dead weight better than others and in some cases extra weight and wing loading will be a distinct advantage, ballasting a glider for windy conditions for instance. As has been said many times before, correct CG is everything despite how much ballast might be needed. I agree with Jon's 10% ballast figure, my 14lb BT Spit carries a tad under a pound and a half of lead right up front and she handles nicely once up and away.

If you've got a few models under your belt then you'll have a feeling for what's OK and what's not and mistakes tend to not stick around for long with models.  Putting a hard and fast rule in place will be impossible,  but generally speaking, whether light weight free flight or 1/4 +scale warbird, build efficiently with as little unnecessary material as possible and concentrate on sticking to this rule on structures behind the CG and particularly around the tail. There is a limit, and one will have to use judgement regarding structural integrity....... e.g. do I use balsa for the main spar or go for a hard wood and accept the extra few ounces? Plenty of examples of in-flight failures on YouTube to prove the point including the large delta jet turbine model that loses its tail fin during a knife edge maneuver.

We worry about excess weight affecting flying performance and of course it does to a greater or lesser extent, and what one pilot will find acceptable in terms of diminished handling qualities, another will will not. Your overweight model might be fine if you have  access to a nice open flying site with loads of smooth runway, but if you fly off of a 'postage stamp' surrounded by trees, would it be so enjoyable?

Extra weight will put more strain on undercarriages. Retract installations are depressingly prone to being overworked and ripped out by the odd duff landing as it is, without the mass of a few extra pounds bearing down on them.

 

Edited March 12, 2021 by Cuban8

[Jon H]

1 hour ago, Simon Chaddock said:

Jon

Yes that Spitfire has lead in it but it is in the tail and is small when compared to the planes all up weight. 0.5%? The lead in your Sea Fury is 8.6%. Hmmmm.

   

 

But my sea fury is nowhere near its scale weight, so you cant make that connection. Our models also have very light engines. A full size Sea fury is 4200kg dry, and near 25% of that is the weight of the engine. At the same ratio, the engine in my model would need to weigh 2.6kg but its only 2kg, so half of my 1kg lead block makes that up. The other half can easily offset my very light propeller vs the full size as a 5 bladed metal prop is pretty heavy and my 2 bladed wooden job will be very much lighter. So, this is why i say 10% ballast or less is fine. I cant make the tail any lighter (it was a YT artf), i cant use a much heavier engine, the radio and batteries are well forward, its got a heavy ali spinner...it has to balance so its time for lead. My point re the full size spit was that even at full size, you still have to hang lumps of lead all over it to make it balance. Its very rare for any aircraft to just balance perfectly and the range of acceptable c/g is very small. 

 

 

Edited March 12, 2021 by Jon - Laser Engines

[J D 8]

  Most large piston engine type's also had the weight of a gearbox on the front of the engine as well.

[Nigel R]

Some good points being made. Pick the right materials for the job, the weight is what it is... Not built a brick yet.

 

1 hour ago, Bob Cotsford said:

A LITTLE more weight helps smooth things out in the often turbulent air at low levels

 

I concur.

 

1 hour ago, Jon - Laser Engines said:

As for lead being no good for aeroplanes..

 

In the full size game, light weight of airframe and components is always a goal, as it feeds directly into range and payload. As to weight feeding into handling, well, mainly the CPU interpreting the pilot's commands will be dealing with a good portion of that.

 

In our end of the gene pool we have vastly diminished range and payload considerations - we're mostly concerned with handling.

 

Obviously anyone aiming for maximum flight time will disagree, but even then glider guys (e.g.) may be playing off weight against penetration, etc.

 

 

1 hour ago, Eric Robson said:

designers don't always get it right

 

Quite so!

 

 

[Jon H]

24 minutes ago, J D 8 said:

  Most large piston engine type's also had the weight of a gearbox on the front of the engine as well.

 

Oil and coolant tanks were often floating around the front of the aircraft too. The merlin in a spitfire had a coolant tank round the front of it and oil tank under it. The typhoon had a monster radiator hung under its chin. The mob in Canada working to restore a typhoon mentioned in a video that they will be using composite look alike blades for their tempest as they are cheaper than wooden ones, have longer life, easier to make etc etc. They also stated that they are much lighter and a balance adjustment will need to be made to take that into account. 

 

At a Shuttleworth collection airshow some years ago during a Hawker Cygnet display, the commentator said that Sydney Camm once stated during development of the Hawker Tempest that the propeller for the Tempest weighed more than the first aircraft (the Cygnet) he designed. Looking at the specs, it means the prop on a tempest weighed between 370 and 950lbs as these are the empty and gross weights of the Cygnet. 

 

Anyway, in the case of scale models we just do not have as much heavy stuff at the front when compared to full size. As a result lead is going to be needed. 

 

 

 

 

 

Edited March 12, 2021 by Jon - Laser Engines

[Geoff S]

My as yet unflown 60" ws Sopwith Pup has a huge amount of lead bolted/screwed into the cowl as far forward as I can get it. I deliberately made the spun aluminium cowl attachment very strong to allow for the extra weight.  Those WW1 rotary engines were heavy lumps, especially compared to the brushless rotary 'engine' I have fitted so lead was always going to be needed.

 

I usually weigh my models as I like to know how the power train is likely to perform on the maiden and adjust prop size accordingly.  They usually come out a bit heavier than the spec figure.  I think it depends on the finish and/or the amount of scale detailing.  In the case of the Pup DB only specify the weight of the bare airframe and I'm not sure what that includes. I know it's a lot less than mine weighs ready to fly.

[Jon H]

How heavy is yours Geoff? if its around 8lbs it will be fine. 

 

My dad had a flair pup and it ended up with about 1lb lead in the cowl along with an OS48 4 stroke. Flew lovely and its still hung in the garage so its now...34 years old which is a bit of a worry as i apparently helped carry the box home when it was bought for him. Means im getting old ?

 

 

 

[john stones 1 - Moderator]

No aversion to adding lead to balance a model myself, If you can balance without it, It's a no brainer.

[Eric Robson]

I built a Brian Taylor P40 Warhawk with a 70fs and had to put lead in the tail. The  original was for a 60 two stroke. nice flyer but needed more power.

[Gary Manuel]

Of course if a model is out of balance and everything possible has been done to lighten the overweight end (usually the rear end, especially on scale models) then adding weight may be the only option. Personally, I'd rather put a larger engine in than add lead but that's just my preference. 2 reasons for this preference:

1. Because of the difficulty in adding lead securely right up front, more overall weight may be needed than fitting a larger engine because the engine is naturally right up front where the weight is needed.

2. A bit of extra power can sometimes get you out of trouble. Throttle is infinitely variable so there is no need to use it, but it's there when you do. Also an overpowered engine can sound better and last longer because it's not struggling at the top of it's power range.

Regardless of how you ADD weight, it is going to have a detrimental effect on the flight performance though. The take off and landing speed will inevitably be increased for every ounce of added lead (or engine). This is a fact based on the laws of aerodynamics and can never be described as a good thing.

I've only ever weighed one finished model and that was at the request of the designer of the model.

Edited March 12, 2021 by Gary Manuel
Typo corrected

[Geoff S]

2 hours ago, Jon - Laser Engines said:

How heavy is yours Geoff? if its around 8lbs it will be fine. 

 

My dad had a flair pup and it ended up with about 1lb lead in the cowl along with an OS48 4 stroke. Flew lovely and its still hung in the garage so its now...34 years old which is a bit of a worry as i apparently helped carry the box home when it was bought for him. Means im getting old ?

 

 

 

 

It's getting on for 10lb IIRC but it's a much bigger model than the ws implies.  I was surprised how it makes my 58" ws DB Gypsy Moth seem small.  I've added a lot of scale detail which doesn't help, I suppose but it's still far from Danny Fenton scale and is really very much stand-off a fair way.  My very much smaller Flair SE5a weigh 6.9 lbs and flew well on an OS52 Surpass and weighs the same electrified on 4S LiPo without any lead (but as yet unflown in that format).  I'm fairly confident both will be OK

 

The Flair Puppeteer (which I also have, but as a hanger queen for a few years) despite the same ws  is smaller looking and has a much longer nose to make getting the CoG in the right place easier ... and, of course, nowhere near scale.