BMFA A-certificate 1kg minimum model weight

[Bearair]

Posted by Darlo0161 on 11/03/2013 19:15:24:
Silly question maybe but I have to ask. If I have a 750g trainer. Can I load or with a bit of ballast to make it over 1kg ?

Or is it the officially listed weight ?

I know this is an old thread but is anyone still following it ?

Absolutely you could. The only criteria now is weight. There are no official listings of weights for models. For one thing you could fly your own design.Its interesting that since the 1kg rule was introduced because of the improvements in materials and techniques models are being produced in foam that have even better flight characteristics than the "shockie/foamy" model and at over 1kg. Take something like a top models Sputnik, weighs over 1kg but much easier to fly than the original "shokie" model! My top recommendation for anyone looking to take their B test.But be warned some examiners may try to stop you taking your A or B with a Sputnik,

Roger

[Biggles' Elder Brother - Moderator]

Nice idea that L-test Martin - we might look at nicking,...opps I mean of course "adopting" - that idea

Just to clarify a point made above re the fact that your 34% heavier foamie might fly slower - actually I think that it will either fly considerably faster - or it will not fly at all!

You see you're going to need 34% more lift, now given that you haven't increased the wing size at all the only place you are going to get that extra lift from is more airspeed! High wing loaded models always have to fly fast.

So, will it fly? Well it depends if you have enough engine power in hand to get the speed up high enough against the drag in order to generate enough lift. One thing is likely (though not certain!) it probably won't be very pleasant to fly like that and it will make the test harder than it would be with a trainer designed to be over 1Kg.

So, while I agree its likely to be admissible within the rules, I'd think twice before recommending it as a plan!

BEB

Edited By Biggles' Elder Brother - Moderator on 11/03/2013 21:58:26

[Simon Chaddock]

Martin

Most interested to hear your club use an "L" certificate and is exactly the sort of thing that shows a tangible recognition of a beginners ability on a light machine.

The 1kg limit for a test to show competence up to 7kg is reasonable but without a lower weight achievement scheme to go with it does seem to prejudice against a whole category of planes that many beginners are likely to have.

[Bearair]

Posted by Martin Harris on 11/03/2013 21:37:56:

Roger,

I quite agree that there are more aspects than simple weight of a model but the fact remains that the average < 1kg model that is likely to be presented for an A test will have a relatively low wing loading.

The A test is taken, in almost all cases that I've witnessed, with a 50 to 70 inch span model with a wing loading commensurate with a model suitable for training purposes. This model won't reproduce the flight characteristics of a 7kg warbird but does represent a good starting point towards flying more advanced models whereas a typical foamy will handle very differently and have little inertia.

We've recognised the gap within our club and introduced an "L" test - which is designed to acommodate those not wishing to fly larger models in the near future. It's based on the BMFA test with the exception of compulsory undercarriages and minimum weight and we expect similar competence and knowledge of safety procedures and rules although any of our B rated instructors can perform the testing. Once in possession of an "L" the member can operate their small models with the same freedom from supervision of any "A" cert holder. Obviously it wouldn't be valid outside our club but we see it as a way to encourage those with different aspirations.

If they want to progress, they only need to practice with a qualifying model under supervision and repeat the test with an examiner - something that should hold no terrors for them having already learnt the basics..

P.S. An overloaded foamy will probably provide quite a good demonstration of flying skill, Darlo, so if you want to take the A on it, I can't see any problem using it - just take your scales along with you!

Edited By Martin Harris on 11/03/2013 21:42:46

In 2012 I took 3 A tests, each one of them borrowed a Wot 4 foam E to take the test with. All of them were elderly gentlemen who have no intention of moving on to heavy models, they are old school aeromodelers who Know a bit about the value of low wing loading. So my experience as a examiner is different to yours.

Ive heard stories of people passing their test with a Vapour. IMHO anybody who can see(let alone land)from about 200 feet with engine at idle with a Vapour then good luck to them because they are a better pilot than I have ever seen!

Roger

[Biggles' Elder Brother - Moderator]

Oh - a challenge, smashing! - the "Land a Vapour from 200 Foot in a breeze" competition. Would be a good 'un for Greenacres that - especially after a few cans!

BEB

[Slickriff]

I fly at the same club as BEB, in fact we are both on the committee. I remember when the 1kg rule came in, and the committee actually viewed it as a relief. As BEB said, in our situation the A-cert is basically a competence to fly alone in a public park where safety is paramount. It is actually a fantastic place to fly especially since we had the strip levelled and we want to keep it that way. Although some very sporty models are under 1kg (eg delta's) they are not the kind of model people tend to learn on. There is a HUGE DIFFERENCE between how a light (say 0.5kg) high wing foamy flies and anything of a weight of above 1kg, or better still above 2kg. There is not just the difference in response to wind, but the big difference some momentum makes. Also don't forget that scaling the aircraft larger or smaller does not scale the air it flies in, which again means that responce is different. Now the issue here is not whether flying lighter or heavier models is harder, it is that they are different. The other thing that is different is the damage done to an object or person on impact, and for us, and I would have thought most clubs, that is important. I have seen someone hit by a sub 1kg model (not at the club), I don't think I would want to see the results of someone hit by a 6kg model. If you are used to a heavier model, then it is less likely to end in tears, and if the difference of a lighter model catches you out then damage will be minimal. If you are only used to lighter models and the difference of a heavier model catches you out then the results could be more serious. I fly models from well below 1kg, to just under 7kg as variety is the spice of life....but boy are they different. I, like the rest of our club, viewed this rule as a valid safety measure.

[Bearair]

Posted by Biggles' Elder Brother - Moderator on 11/03/2013 21:56:32:

Nice idea that L-test Martin - we might look at nicking,...opps I mean of course "adopting" - that idea

Just to clarify a point made above re the fact that your 34% heavier foamie might fly slower - actually I think that it will either fly considerably faster - or it will not fly at all!

You see you're going to need 34% more lift, now given that you haven't increased the wing size at all the only place you are going to get that extra lift from is more airspeed! High wing loaded models always have to fly fast.

So, will it fly? Well it depends if you have enough engine power in hand to get the speed up high enough against the drag in order to generate enough lift. One thing is likely (though not certain!) it probably won't be very pleasant to fly like that and it will make the test harder than it would be with a trainer designed to be over 1Kg.

So, while I agree its likely to be admissible within the rules, I'd think twice before recommending it as a plan!

BEB

Edited By Biggles' Elder Brother - Moderator on 11/03/2013 21:58:26

What is important is by how much you increase your wing loading not your weight. A 6ftmodel glider with a very low wing loading will take a 34% increase in weight much easier than 2 foot wingspan model glider of the same weight .The critical factor on the models performance is the increase in wing loading NOT WEIGHT. A heavier model will not fly faster it will have an increase in stalling speed. This may make it harder to fly but it would be very negligeable on a model with a low wing loading in the first place. As you state "high wing loaded models always fly faster" thats not about weight. Why then put a weight limit on permisable models and not the much more sensible" wing loading" if indeed you have to have a limit at all?.

Roger

[Biggles' Elder Brother - Moderator]

I thought that was exactly the point I was making Bearair?

BEB

PS Hi Ant!

Edited By Biggles' Elder Brother - Moderator on 11/03/2013 22:46:32

[Martin Harris - Moderator]

Posted by Bearair on 11/03/2013 22:10:37:

In 2012 I took 3 A tests, each one of them borrowed a Wot 4 foam E to take the test with. All of them were elderly gentlemen who have no intention of moving on to heavy models, they are old school aeromodelers who Know a bit about the value of low wing loading. So my experience as a examiner is different to yours.

Roger

I'm struggling to understand this point Roger, as I wouldn't have any qualms examining them for an A test with a borrowed model, even if they intended to fly lightweight models normally.

I'd imagine that having seen or heard about the idea of borrowing the Wot4 the other two found it a good idea to follow suit. Most people still seem to take the medium size trainer route when they join a club to learn to fly but we're talking about those who choose not to follow this path.

The related point I was trying to make, in agreement with BEB's much more succinct explanation, was that taking the test on a model designed to fly at a very much lower wing loading would be a good test of flying skill and energy management.

[Bearair]

Posted by Slickriff on 11/03/2013 22:33:12:

I fly at the same club as BEB, in fact we are both on the committee. I remember when the 1kg rule came in, and the committee actually viewed it as a relief. As BEB said, in our situation the A-cert is basically a competence to fly alone in a public park where safety is paramount. It is actually a fantastic place to fly especially since we had the strip levelled and we want to keep it that way. Although some very sporty models are under 1kg (eg delta's) they are not the kind of model people tend to learn on. There is a HUGE DIFFERENCE between how a light (say 0.5kg) high wing foamy flies and anything of a weight of above 1kg, or better still above 2kg. There is not just the difference in response to wind, but the big difference some momentum makes. Also don't forget that scaling the aircraft larger or smaller does not scale the air it flies in, which again means that responce is different. Now the issue here is not whether flying lighter or heavier models is harder, it is that they are different. The other thing that is different is the damage done to an object or person on impact, and for us, and I would have thought most clubs, that is important. I have seen someone hit by a sub 1kg model (not at the club), I don't think I would want to see the results of someone hit by a 6kg model. If you are used to a heavier model, then it is less likely to end in tears, and if the difference of a lighter model catches you out then damage will be minimal. If you are only used to lighter models and the difference of a heavier model catches you out then the results could be more serious. I fly models from well below 1kg, to just under 7kg as variety is the spice of life....but boy are they different. I, like the rest of our club, viewed this rule as a valid safety measure.

I learnt to fly 40 years ago at Hengrove Park in Bristol, I did not have an engine above 15 size for the first 10 years of flying I dont think I had a model much above 1kg. Believe me the difference between my trainer and my pylon racer was absolutly huge but it had nothing to do with weight.

There is NOT A HUGE DIFFERENCE between a model of 0.5 kg and a model of 200000kg if the aerodynamics and wing loading are the same(ok reynolds numbers may make a bit of difference but if anything I would of thought it would favour the larger craft) Weight has got nothing to do with it.

[Martin Harris - Moderator]

How much landing roll is there for the 0.5 kg model against the roll for the 200 tonner? Not a huge difference? In general terms, the heavier model with the same wing loading will have more inertia and is therefore different in some respects. The scheme attempts to apply simple rules to accomodate the majority and it doesn't seem too far off the mark to me.

It would be interesting to see if the achievement sceme might see some merit in a formal  "L" test eqivalent - perhaps if some more clubs adopted something similar it might prompt them to adopt something along these lines?

Edited By Martin Harris on 11/03/2013 23:41:52

[Bearair]

Posted by Biggles' Elder Brother - Moderator on 11/03/2013 22:46:07:

I thought that was exactly the point I was making Bearair?

BEB

PS Hi Ant!

Edited By Biggles' Elder Brother - Moderator on 11/03/2013 22:46:32

You state a model 34% heavier will fly faster. No it won't weight has nothing to do with how fast a model flies.

You state your going to need 34% lift to fly. Why? Are you saying that a Jumbo Jet full of people needs 34% more lift to fly and it gets it by flying faster?

Roger

[Martin Harris - Moderator]

A generation of glider pilots loading water ballast must be wrong then? Increase the wing loading and you must fly faster.

[Slickriff]

There is NOT A HUGE DIFFERENCE between a model of 0.5 kg and a model of 200000kg if the aerodynamics and wing loading are the same(ok reynolds numbers may make a bit of difference but if anything I would of thought it would favour the larger craft) Weight has got nothing to do with it.

There is sufficient difference between a 0.5kg high wing foamy and something like an Arising Star or Boomerang to be very noticeable, in exactly the same way as there is a noticeable difference between an Arising Star and a nearly 7kg Cub or Cessna. Wing loading is not the only factor, as I said, momentum changes response as does the non scaleability of air. This is not always about an aircraft flying steadily, it is how it responds with sudden inputs which happen both deliberately and as a response to things not quite going to plan.

Another factor we often notice is the "bicycle clips needed" effect on peoples ability to fly a new model safely, basically down to nerves. In my experience this is always made worse moving to a bigger (often more expensive) model, but not normally made worse by dropping down to a smaller lighter one.

[Slickriff]

Sorry, didn't know how to "quote"

Posted by Slickriff on 11/03/2013 23:48:53:

There is NOT A HUGE DIFFERENCE between a model of 0.5 kg and a model of 200000kg if the aerodynamics and wing loading are the same(ok reynolds numbers may make a bit of difference but if anything I would of thought it would favour the larger craft) Weight has got nothing to do with it.

There is sufficient difference between a 0.5kg high wing foamy and something like an Arising Star or Boomerang to be very noticeable, in exactly the same way as there is a noticeable difference between an Arising Star and a nearly 7kg Cub or Cessna. Wing loading is not the only factor, as I said, momentum changes response as does the non scaleability of air. This is not always about an aircraft flying steadily, it is how it responds with sudden inputs which happen both deliberately and as a response to things not quite going to plan.

Another factor we often notice is the "bicycle clips needed" effect on peoples ability to fly a new model safely, basically down to nerves. In my experience this is always made worse moving to a bigger (often more expensive) model, but not normally made worse by dropping down to a smaller lighter one.

[Slickriff]

Still got it wrong, the quote was from bearair

[Martin Harris - Moderator]

To get a handle on inertia, try hot air ballooning. An object which can weigh less than the air it displaces has an enormous amount of inertia such that a massive part of learning to fly one is learning to anticipate the inertia - stop burning before a descent starts to slow or it will bob up again like a cork in water - burn too late and you'll make a nasty dent in the earth...

Edited By Martin Harris on 11/03/2013 23:58:08

[PatMc]

Posted by Bearair on 11/03/2013 23:40:16:

You state a model 34% heavier will fly faster. No it won't weight has nothing to do with how fast a model flies.

You state your going to need 34% lift to fly. Why? Are you saying that a Jumbo Jet full of people needs 34% more lift to fly and it gets it by flying faster?

Roger

It will have to fly faster to in order to increase it's lift by 34% maintain level flight.

If the weight is increased by 34% the lift needs to increase by 34% because in level flight lift = weight.

[Tom Wright 2]

There seems to be two views on this subject ,and IMHO neither can be said to be valid when considered within the context of low speed and low inertia . There are many under 1kg models that are in effect missiles capable of considerable damage .Then there are large heavy models with low wing loadings that by virtue of there mass even at low speed would also be capable of severe damage .

The only classification that would perhaps represent a safer option would be a light model with a low wing loading ,that was low powered and was incapable of any significant level speed . Even so such a model pointing earthwards would have the potential to do damage.

One thing I do agree with is the fact that a very light low speed model can be more difficult to fly accurately ,so if a student passes the A with such a model it's a fair bet the student would be controlling the model rather than the model leading the student.

I always use a very light slow model for initial air experiences , and then for at least for the next hour ,simply as it's safer, gives the new pilot thinking time , and is less prone to damage .

I could do a B test using a 12 oz Pipe lagger, but it would be interesting to hear the examiners view on that!

Would the average under 1 kg foamie or typical balsa stick built model be less harmful than a 6 lb 40 powered trainer well in all probability yes .But we know a small chuck glider can very harmful indeed, so maybe the real issue is the degree off safety awareness instilled from square one and how that knowledge is implemented over the long term .

Tom.

Cross posted with John.

Edited By Tom Wright 2 on 12/03/2013 00:19:50

[John Privett]

Posted by Bearair on 11/03/2013 23:24:52:

There is NOT A HUGE DIFFERENCE between a model of 0.5 kg and a model of 200000kg if the aerodynamics and wing loading are the same(ok reynolds numbers may make a bit of difference but if anything I would of thought it would favour the larger craft) Weight has got nothing to do with it.

Sorry, I have to disagree totally with that. The wing loading will be higher on a larger plane compared to a small plane of similar "behaviour". Wing area goes up with the square of the linear dimensions whereas the weight goes up with the cube.

A very quick "back of an envelope" calculation gives a wing loading for my .90 powered 1.6m span Extra 300 at something around 5kg/m^2. So should I expect a rather bigger one - say the full-size aircraft - to have the same wing loading? Well a Cessna 152 has a wing loading of 51kg/m^2 - TEN times that of my model. I wouldn't expect the full-size Extra to be dramatically different. (From figures I can quickly find, the wing loading of the f/size Extra must be something over 60kg/m^2.) Continuing to look at bigger planes, a 747 or A380 has a wing loading of around 700kg/m^2. That's over 100 times the wing loading of the model Extra and about 10 times the loading of the f/size Extra.

So you can't simply ignore weight and say that the wing loading gives you the whole picture. You need to take the weight as well as the wing loading. The 1kg. limit for the A and B test is perhaps a bit simplistic, but it's a realistic check to expect the examiner to make. The alternative would need the examiner to measure the model, calculate the wing loading and check both the weight and loading against a chart. On balance I think the 1kg. limit is a sensible choice.

[PatMc]

Posted by Bearair on 11/03/2013 22:43:08:

What is important is by how much you increase your wing loading not your weight. A 6ftmodel glider with a very low wing loading will take a 34% increase in weight much easier than 2 foot wingspan model glider of the same weight .The critical factor on the models performance is the increase in wing loading NOT WEIGHT.

Roger

An increase in weight of 34% means an increase in wing loading of 34% no matter what size the model is. Therefore however critical any increase in wing loading is on performance, it's directly in response to the increase in weight.

[Martin Harris - Moderator]

I'm pretty much in total agreement with Tom, John and Pat - but you won't get your B with a 12 oz model, Tom as the minimum 1kg limit also applies to that test!

[Simon Chaddock]

I do smetimes wonder if the point is being missed.

Was it not an 'Achievemnt' Scheme' to demonstrate competance? Its what the BMFA call it.

What in effect has happened is that no matter how well you might have mastered flying there is no way you can demonstrate any "achievement" with light models.

Although the 1kg limit may have been a relativelly recent introduction I do wonder if the situation is changing faster than the regulations.

[David E]

Patmc, Roger, (sorry, quote doesn't work in mobile) if the same aircraft gets 34% heavier, it needs 34% more lift, as Patmc says. This is because for stable flight lift is always equal to weight. If they weren't equal the model would be accelerating towards or away from the ground! However it isn't necesssary to fly 34% f aster to increase lift by 34%, as this can be achieved by increasing the angle of attack at the same speed. However, if you increase the angle of attack too much you will stall. This is why the stall speed is higher when you increase the weight.

So, you may have to fly faster, but only if needed to avoid a stall. To put it another way, you won't be able to fly as slowly. The plane will feel faster, but only if there is enough power, as the higher angle of attack means more drag! Putting in extra weight without increasing power will actually reduce the top speed, as you'll need more throttle to fly at the same speed.

Net result, if I haven't confused my physics, is that adding weight but not power reduces the range of speeds you can fly - you must fly faster to stay in the air, but you can't fly as fast.

Does that sound right?

[Olly P]

Sounds right David - minimum speed at same AoA goes up, max speed goes down