Our thanks go to Ivan Bradbury for his wisdom as published in our March 2012 Club Newsletter. Read on…..
To Ballast or not to Ballast That is the Question
Throughout all the years I’ve slope soared, there’s always been something of a ‘Black Art ‘ mentality associated with the use of ballast in model gliders ~ Do I need to use ballast ~ If so, when do I need to uses it~ How much should I use? ~ How will ballast affect the flying of my model etc.etc.
This ‘Black Art’ has constantly been refuelled over the years by the so called experts guarding the amount of ballast they themselves use as if it were some Top State Secret. To confuse things even more, many bog standard slope soarer also throw their own opinions into the ballast caldron. As a result; it’s not surprising that many new comers to the hobby are totally bemused when they hear the word ‘Ballast’. Some even think we are talking about nose weight
Putting ballast in models is not new. I remember it was a much discussed topic when I regularly flew in pylon
competitions in the 60’s and 70’s. This was long before F3F events were on the BMFA’s calendar, and, the only time your model ever flew on the back side of a slope was when you’d cocked your landing up.
During any breaks in these proceedings, you would often see some guy furtively slinking up to either Ken Binks’s or Chris Foss’s model, (These were the slope gurus of the day) take a sneaky look around to make sure Messre Binks and Foss were preoccupied, and then lift one of their models a couple of inches off the ground to feel its weight; a particular practice that I never indulged in by the way. — Well Ok!! I admit I did it once or twice but this was only done in the name of essential scientific research. At this particular time, I was carrying out an in depth study into why I could never get better than second place whenever I raced against one of these buggers.
Anyway, enough of my woes! I’ll now try to do the impossible and give the lowdown on how to and why we use ballast in model gliders. I want to stress here that this article is only aimed at those guys in our club who are comparatively new to the hobby. This is because most if not all our elder members will, if pressed, proudly admit that they know everything there is about ballasting model gliders, and like I’ve said many times before, you can never impart knowledge to a man who knows everything.
I should also make it clear at this point that I don’t profess to have any aeronautical or engineering qualifications. All my knowledge about putting chunks of lead in model gliders has been gleaned by careful observations during the hundreds; nay thousands of hours I’ve spent standing on some God forsaken slope risking the compete loss of my private parts to those sub-arctic temperature.
Back to Basics – As we all know, the shape of a wing is designed to generate lift as it moves through the air. The faster
it goes the more lift it generates. BUT!!! The faster it goes the more resistance to the forward speed it creates (Drag) and if we are to believe the real experts; as the speed continues to increase, there eventually comes a point when this lift versus drag battle starts to swing heavily in favour of drag. However, few of us bog standard fliers ever need to be too concerned about reaching these speeds.
Another point that we rarely think about is that no heavier than air machine, whatever the section or speed, can remain airborne without some other additional form of lift. (engine, rocket or in the case of gliders, slope or thermal lift) Without this extra lift a glider will fly on an ever descending flight path until it lands. Therefore, the upward force on a model (lift) must always be equal to or greater than the total downward force. (Models weight) This applies no matter what the conditions. ~ A howling gale, loads of thermal lift or marginal conditions. (See Fig 1)
To enable the glider to penetrate into a wind that is blowing faster than the normal flying speed of the aircraft, we should use ballast. ~ I know what some of you are saying ~ you are saying that you don’t need ballast to penetrate in strong winds because you can make the model penetrate by applying down elevator (down trim). This is true, but it is not the ideal solution. A previously trimmed out model that needs additional down elevator to penetrate in strong winds is not flying to its true potential; far better to use ballast. What is happening when you apply down elevator is that the model speeds up because it is diving. It might still appear to be flying straight and level. This is because the stronger winds are generating more lift than the model is losing in the dive. ~ It’s like someone trying to walk the wrong way down an escalator. To a bystander who can only see the top of the guy’s head, it would appear that he was either walking on the spot or, if the elevator was moving quicker than he’s walking, (stronger lift) it would appear as if he was walking backwards.
If the wing loading is increased (Using ballast) the flying speed would increase without the need for a trim change and the model would be able to take full advantage of the extra lift. It will also maintain its inertia much better throughout the turns.
Any speed merchant will tell you how important ballast is to the speed of a model. To a dedicated F3F fanatic, ballast is more important than sex but even these speed worshipers can’t make their minds up as to what the ideal ballast should be for their own model let alone the range of models we see on our slopes.
This is not surprising because the weather responsible for generating lift is an extremely complex machine. It’s not just wind or thermal strength; there’s a multitude of different conditions that influences lift. Things like, air temperature ~ ground temperature ~ humidity ~ passing clouds or a passing weather front and of course wind direction and strength. And! That’s not just the wind direction on the flying slope ~ A slight change in wind direction on the terrain several miles up wind can cause a plus or a minus effect to the lift on the slope we fly on. It can cause rolling turbulence and or wave lift. These are just a few of the things that can Fig 2
This is just one of the reasons why wind speeds are not always a true indicator of lift. If the wind is not dead square on a straight ridge like Edgetop, the wind sheers across the face of the slope and much of its potential lift is lost
affect the micro conditions of a particular slope. This is the reason why wind speed alone is not always a true guide to the lift strength. (see Fig 2) It is also why a few members of the LMMGA go to places like the Great Orme Llandudno several times a year and drool over the ultra smooth lift these costal slopes can produce. Lift is more relevant to wind speed on these sites unlike many inland slopes.
Questions and Answers::
So!! How much ballast do you need to put in your model??
To be absolutely frank, I haven’t the foggiest idea!! It’s a case of how long is a piece of string. However, there are a few points that some of our newer members should bear in mind.
They say there are horses for courses; this is doubly true for gliders. All gliders have their limitations as far as speed
(penitration) is concerned. It depends on their size, strength, wing section, weight and the frontal cross section of the plane, (profile drag) you can’t expect a foamy costing £80 to perform like an all carbon mouldy costing several hundred pounds.
As for ballasting in strong winds; it’s not the weight of the model that affects its ability to penetrate (fly fast) , it is its wing loading.
e.g. A large span model can weigh several times more than a small one but the smaller model can penetrate much better if it has a heavier wing loading. ~ Two such models that can be seen on our slope are the 3 metre Dragon and the 40” D40.
6 oz of ballast in the D40 will increase its wing loading much more than the same 6oz would in the Dragon. The increase in speed (Penetration) of the Dragon with 6ozs of ballast would hardly be noticed where as 6oz in a D40 would make a significant difference.
A model’s wing loading is worked out by dividing the weight of the model by the area of its wing. With me being an old as-been I work in old money. I use square feet for the wing area and ounces for its weight ~
Let’s see what affect the 6oz of ballast makes to the wing loading
(wl) of the two models
Dragon:> Wing area 5 Sq feet ~ therefore 6oz ÷ 5sq/ft = increase wl by 1.2oz per sq ft. This would have llittle affect on performance/penetration.
D40 :> Wing area 2sq feet ~ therefore 6oz ÷ 2ft = increase wl by 3oz per sq ft ~ This would have a significant effect on its performance
(The wing areas for the models are only approximations)
From my experience, modellers who use ballast fall in one of two main groups
By far the largest group only use ballast through shear necessity. They arrive on the slope ~ find they haven’t a model to suit the unexpected strong wind ~ start to rant about the “Bloody Weather Forcastres” because the steady 10mph westerly the BBC mentioned as turned out to be a 30 gusting 40.
I call this group ‘The Sellotapers’ because they go round the slope begging or borrowing any lead they can lay their hands on and Sellotapeing it somewhere close to where they think the CofG should be. They then hurl the model off more in hope than certianty .
The other group I call the ‘Whistlers’ because they just love the whistling sound a high speed model makes. These guys rairly fly without ballast even in light winds and their all carbon models unballasted can weigh heavier than some similar sized glass planes fully ballasted. The guy who volinteerers to launch these all carbon models when ballasted up would be well advised to wear a truss
foot and for the larger more efficient models quite a bit more. This is assuming of course that the wing loading of the unballested model is around the average wing loading of 16oz per sq ft range
e.g. My Foamy, (Halfpipe) has a wing area of aprox 3sq ft. When I fly it in winds of 25 mph gusting mid 30’s; I Selloptape 10 ozs of lead to it which increases its wing loading by a tad over 3ozs per sq ft. And it makes quite a difference. (Does this make me one of the ‘Sellotapers I wonder?? )
I suppose the next question to answer is :>> Is it possible to over ballast?~ In a word ~ Yes!! I‘ve found on the odd occasion when I’ve piled a touch too much lead on for the available lift, the affect has been very similar to flying when the lift as suddenly dropped off. In these marginal like conditions, we tend to droop the flaps if we have this option and pull back a little on the elevator stick making the model fly with a slight nose up attitude. This increases the angle of attack of the wing which gives the model a little more lift. However, there’s a down side to this trim change. It slows the model down and creates more drag. Less speed means the wing section generates less lift and the Flap/elevator changes plus a nose up attitude makes for quite a bit more drag
In my opinion ; an over ballasted model is less efficient for speed or penetration than one that’s a little under ballasted.
A word to the guys who occasionally use Sellotape to secure their ballast. ( Mainly foamy wings) Make sure the ballast is well secured. Sticky tape is notorious for failing in damp conditions. A chunk of lead falling from a height can cause more than a headache.
Adding ballast should never alter the trim of the model. (It must NOT change the position of the Centre of Gravity) This is particularly
important for planks or flying wings which are extremely sensitive to a C of G shift >>> Ignore this at your peril,
Another point to remember is that a ballasted model lands faster than an unballasted model ~ well you can’t have it both ways.
If you’re a Sellotaper, know the approximate wing area of your model and fix yourself up with a couple of pieces of lead so that you can increase the wing loading by a known amount. This will be easier if you mark the position of the model’s C of G clearly on the model and also mark the position of the C of G (balancing point) on the lead . Then by lining the two marks up you are sure the ballast is in the right place.
Remember when launching in strong winds there’s nearly always a compression zone at the top of a hill. (A very localised increase in wind speed) This is caused by the wind that’s coming up the slope being squeezed as it passes over the top of the hill by the higher winds which are less affected by the slope. (a Venturi affect) The strength of the wind can be significantly less if you walk a few yards further down the slope to launch the model. It is also advisable to get someone else to launch your model in strong winds leaving both hands free to make a quick correction if needed.
At this point I was going to give my thoughts on how the Whistlers could squeeze another couple mph out of their lead ships but I’ve realised that I’ve been rattling on too long already and in any case, for a Sellotaper like myself to be foolish enough to give advice on ballasting to a Whistler would be putting what’s left of my private parts at considerable risk.
P.S. A Thought!!
Most Whistlers (mainly F3F competitors) will admit that they only ballast according to the wind speed (not the strength of lift) Not quite sure why because if they took thermal lift plus slope lift into consideration (particularly in the summer months) I think they could knock seconds off their time
Perhaps it’s because thermal lift is not so predictable as slope lift?
Maybe we’ll get an answer in the next newsletter???? Narr I doubt it!!