Another method to get the plane to balance

[Hai-Lee]

There are many posts about issues related to getting the latest creation to balance at the proper CG point and a set of standard responses but each one has its own issues or penalties.

Shifting the Servos often leaves a hole or two in the fuselage that needs to be patched.

The shifting of the battery away from its design location does distribute the weight or mass of the aircraft and can effect the control responses.

Adding weight even if it is just an increase in the size and weight of the battery increases the wing loading and detracts from the design handling.

Some recommend sliding the wing either forwards or rearwards but often this is far from simple and even easy.

So if your pet design has serious trouble making the correct balance and you want to keep it light and nimble how do you achieve proper balance?

There is a rarely used and very effect method which is relatively easy to do and clean in execution. It has even been used on real or full sized aircraft where the fully built and fitted out fuselage would not balance correctly, (without a complete rebuild). The method is to separate the wings and introduce a sweep angle. It the plane was tail heavy introduce a slight rearwards sweep and if nose heavy a slight sweep forwards.

All you need to do is to know where the balance point should be and where it is. Go to an online CG calculator and play with the numbers until the wing balance point is exactly where you want it and then trim and rejoin the wings at the required angle.

Just a bit on rarely considered info which is useful for that unique design you may be working on!

Have fun!

 

[quorneng]

Whilst sweep can shift the CofP to better coincide with a specific CofG doing so can also bring in new problems with sweep forward being the most likely to create significant structural issues.
By far the best solution to achieving a specific CofG is to avoid the problem in the first place by using appropriate design features.
Easy to say but much harder to achieve.

 

[Hai-Lee]

Whilst sweep can shift the CofP to better coincide with a specific CofG doing so can also bring in new problems with sweep forward being the most likely to create significant structural issues.
By far the best solution to achieving a specific CofG is to avoid the problem in the first place by using appropriate design features.
Easy to say but much harder to achieve.

It definitely can be hard to achieve especially when people use different grades and weights of materials to construct with generally using the same set of plans,
Structural integrity is something I tend to build into all of my designs and so I normally expect that others are similarly aware.
As for forward Vs rearward sweep angle both are equally effective though I will admit that the sweep angle I refer to is quite moderate up to around 5 degrees because beyond that the CoL could be moved away from the wing saddle area and cause the wing to tend rotate, forward or rearwards thereby changing wing incidence angle stability.

Yes the best solution is to avoid the issue in the first place but due to different materials used it is not always possible for the beginning home constructor.

It would be interesting to find out how many home design projects where abandoned simply because proper balance could not be achieved without large amounts of dead weight being needed to get balance.

Anyway the original post was made to remind constructors that there is another path to get the fruit of their endeavors into the air without adding a large weight penalty. It is always going to be better to fly light, (with a little sweep angle), than to fly heavy due to added ballast. Sweeping the wing is not a solution for everyone but it is a valid approach to achieving proper balance.

Have fun!

 

[FoamyDM]

Thank you for the reminder of all the great ways to find that magic balance point! Boy I wish you were at my build table/flight line as I have 7 designs I need to successfully get airborne.
Most have been attempted and failed in part.

Fire Fox plane
VF-9
LPL- Submarine
Kalinin K-12 (Firebird)
Bloody Black Widow
Bloody Komet
Sam-7

What value does there seem to be to building a simpler glider version?

 

[Hai-Lee]

My latest weapon in getting that special design into the air safely is the bungee described in another thread. Our club had a surprisingly large number of planes that people were unable to get into the air successfully. Now our club has three bungees with more on the way and we are seeing a lot of planes which have been hidden in cupboards for ages suddenly given a new lease on life and suddenly flying whereas before all they seemed to do was crash. Whilst we only have the three bungees we have about 10 bungee launch users and the number is growing each week.

There are a number of new aircraft, (mostly EDF with tiny wheels and retracts), which are now launched and landed without use of their landing gear to save ripping out the nose wheels.

As for the glider version, doing a bungee launch without motor power or thrust and doing a straight low level glide is the first step with the bungee setup to provide a moderate launch speed.

Next project is a ballistic parachute system for RC model aircraft, (under $10), Just looking for a suitable aircraft to do the inflight deployment testing!

have fun!

 

[AcBates]

Just a heads up that changes in wing sweep for non-flat wings is a change to the wing's attack angle.

For flat wings, the change is generally just a shift in the spread of centers of pressure across the wing, pitch stability being the only thing changed really.

For dihedral and polyhedral though, it's a change to the attack angle of the airfoil. Adding rearward sweep, with dihedral/polyhedral will decrease the effective attack angle... and forward sweep will increase the attack angle. So for changes to these wings; elevator neutral will be different, as is the relative thrust line to the wing attack angle. Dihedral/polyhedral being a way to induce a roll from a yaw input, this will likely also be tweaked

Some minor notes; adding rear sweep to dihedral wing will potentially make it more stable and harder to tip-stall. You'll need more up elevator trim for neutral to get the wing back into the same lifting attack... due to thrust line change, it may mean quickly adding throttle it may climb as a result.

Forward sweep with dihedral wing, will make it a little less stable in turns, may make it more likely to tip stall. neutral trim may need a little more down elevator than normal as the wing's attack is now more positive... quickly adding throttle may tuck the nose.

Regretfully for not-flat wings, simply changing the sweep isn't a trivial change to the design at all.

 

[Hai-Lee]

If I was recommending a sweep angle of 20 degrees or more all that was posted against it would be true. It actually amazes me that when you mention wing sweep angle many people automatically have a mental snap to somewhat extreme angles and do not consider that the aerodynamic effects of minor angles of sweep.

There is a full size aircraft example which was produced in quantity which was a twin engined flying boat that had the wings swept backwards in a desperate to save the project when the plane balance was incorrect. With an engine in each wing the production aircraft had engine thrust lines as per the original design, (with respect to the original wing angle), and so production aircraft had the 2 engines angles outwards at a small angle. the wings were swept backwards and all of the doom and gloom effects mentioned by the critics NEVER eventuated.

As for minor sweep angles I have seen various FB examples here with dihedral and minor construction errors which have caused a minor degree of inadvertent sweep and they all fly quite well, though as stated with polyhedral wings a degree of yaw instability can occur as the outer wing panels dihedral joint does not align with the airflow and the tip panels can effectively either be toe in or toe out.

Sure you can rebuild or re-engineer your project and turn it into something resembling swiss cheese trying to get balance. You can add a lead ingot or a huge battery and stagger into the air becoming a bomb looking for somewhere to impact. If you are terrified of a degree or 2 of sweep then do not try it but rather than a complete re-engineering exercise a breaking and rejoining of the wings may allow you to get into the air and fly in short time.

Some consider it as possibly dangerous, (mainly because of extreme examples and their lack of experiment), but there is little more dangerous than a model grossly overweight and underpowered struggling to keep flying as it passes overhead.

If it is not for you or you do not understand it then it is best that you do not try it for the remainder it is a mathod to acheive balance and to keep your model at its orignal flying weight.

Have fun!

 

[AcBates]

I'm not against the idea, and not trying to argue, to the contrary I reckon people should get out and try _all_ the things!

I was just typing up some information for information's sake, which is what this is all about; helping the peoples get edumacated about the flying things. Changing the attack angles may have an effect people hadn't considered, so it's handy to have some info there that may explain why... and what to consider to adjust next. Some models it's easy to tweak the attack angle, so people could both change sweep and correct an attack angle change.

If balancing a model turns it from something happy into something grossly overweight and under powered, something has gone seriously wrong with the build (or using equipment way out of spec)

There has been some crazy things done in full scale aviation, no doubt about that !

 

[Hai-Lee]

It is OK, No upset here! just I have seen and heard of some of the extreme things people have tried to get their model airborne and sadly it often leads to disaster. Here the FB I use is rather heavy and so making the FT models balance at minimum weight is something I do every build.

The one point you did not mention in relation to wing sweep is that, (at larger sweep angles), the effective wing cord is increased and the vortice induced drag is lessened.

There are other benefits and detrimental issues but taking it down to a highly technical level is way beyond most of our audience.

No offence taken! Just a straw for those who clutch at straws when facing an insurmountable problem.

have fun!

 

[AcBates]

it's a real shame that the foam board is hard to get overseas... I keep telling Dad that he should make these things, but the equivalent board just isn't there (Australia). It's cheaper to get depron there, which is just nutty, and still more expensive than it should be.

 

[Hai-Lee]

it's a real shame that the foam board is hard to get overseas... I keep telling Dad that he should make these things, but the equivalent board just isn't there (Australia). It's cheaper to get depron there, which is just nutty, and still more expensive than it should be.

Try your local picture framing business for FB. The sheets are twice the size and stronger though a bit heavier.

With practice and a few tricks any designs can be successfully made and flown. Whilst a litle more expensive it is still affordable.

You could also check with HobbyKing to see if they still sell their cheap FB though you will need to pay freight.

Have fun!