Initial tip weight determination?

How do you guys guess as to where to start with tip weight?

Typically, I start with an ounce, but with this Oriental, the wings are not equal so I'm sort of at a loss.

Maybe 3/4 ounce for starters?

>How do you guys guess as to where to start with tip weight?
>
>Typically, I start with an ounce, but with this Oriental,
>the wings are not equal so I'm sort of at a loss.
>
>Maybe 3/4 ounce for starters?
Hold the plane with one hand on the spinner and the other on the bottom of the vertical fin and the outboard wing should slowly drop. Fly it and adjust from there.....

place a set of lines on a reel about halfway out on the inboard wing and an empty reel in the same spot on the outboard wing. Balance as Rob describes. Fly and adjust as needed, better to start with too much than too little.

Hi

I estimate the spanwise position for centre of lift and ballance
the model on that point using 1/2 the weight of the lines on the
inboard wing as an average line weight effect between vetrical and
horizontal flight.

I put the calculations into an excel spreadsheet to make the process easier. The basis of the calcs are as follows.

Divide the winspan into strips and using the center of each strip,
calculate the lift (relative to mean lift force) for straight and
level. This is an estimation method only so pretty easy. It is
based on an assumed eliptical lift distribution across the span. We
can assume this distribution since (assuming no beta twist or stall)
lift force distribution is close to eliptical regardless of plan form.
Note: I am not talking Lift Coefficient here, that does vary markedly
with chord. Precise calcs for the spanwise distribution of lift
forces are very complex and for this purpose, not necessary since the
difference in answers will only be a mm or two.

Next, I calculate the velocity at the centre of each strip. This is
again a value relative to the mean velocity at the centre of the span.
As lift force is proportional to the square of velocity, for each
strip I multipla the relative lift by the square of the relative
velocity.

Using these values, I calculate the relative moment for each strip
about the centre of span.

Adding all these moments together gives the sum effect of lift moment
about the centre of span and from this I calculate the center of lift
offset from the centre of span.

That is my starting point for spanwise CG and engine thrust line.

To date, this has proved quite close, within a few mm of final
trimming.

I doubt there is a really good way to calculate the required tipweight. The result is determined by too many things such as asymmetry and relative width of the flaps.

I use 1/2" asymmetry, outboard flap 1/8" wider at the tip. I build in about 1 1/4 ozs of tipweight and add another 1/2 oz before first flight. Too much is the best place to start trimming. Tipweight is the easiest of all trimming adjustments. With too much tip weight, the airplane will "hinge" in corners. This is unsightly but safe. Reduce tipweight 1/4 oz/flt until the airplane quits hinging. For best line tension, carry maximum tipweight.

After you get the airplane in near final trim with leadout location, rudder offset/movement, nose/tail weight, engine, fuel, prop size, line length and lap time pretty settled, then try tipweight again to see if you can squeeze on a bit more.

OK I made some pretty bold and hard to believe statements up there
so lets try and run some numbers. If you tell me your wing span and
line length from the centre of circle to centre of wing span, I'll
run the calc.

Then we can compare my results with your CG location.
Lets try some different plan forms and see what we get.
Trueth is I did this as an expereiment and while not expected to
be spot on, it seems to be close on the few planes I used it on
but I'd like to do this compasison just to see how universal the
calc is or isn't.

There are some previsos though like no differential flaps. In fact
given that flap chord distribution can change beta along the wing
introducing serious aerodynamic twist, lets leave flapped wings
right out of the test. The wing should be built with a symetrical
airfoil right through the whole span and no twist. Can be a
symetrical or asymetrical plan form though.

Here's how I determined initial tip weight after I refurbished my airplane. I had to put a new bottom in the weight box. The old one had deteriorated. I put as much of the weight that had been in the box before the refurb as would fit. If I need any more tip weight, I'll have to sand the left wing.

Howard (Is depleted uranium as nasty as they say?) Rush

>Howard (Is depleted uranium as nasty as they say?) Rush

If memory serves (hopefully ) some of it is. If the dinosaurs buried some, it would still be "hot".

Ken,

If you integrate lift by span as you described, you will get AC, which is valid in level flight and with direct incoming air. There are three problems leading to significant difference:

1/ Incoming air is not direct. Our models have air coming from left side, because the model has in normal flight little yaw. It could be 1-1.5 deg depending mostly on trim and fuselage length. This will move AC little left or your result is too much tip weight. If you know that yaw, you can rearrange your strips that way, you will get better result.

2/ Unequal panel lengths make unequal flap percentage – or you can build in unequal flap chord at tip. This will not modify your level result, but it WILL modify what the model does in high lift maneuvers where minor flap chord difference changes ratio between straight flap and deflected flap. Result is that AC in level is different from AC in corner. The same happens if AC of elevator does not match AC of wing sidewise. (Brett wrote it several times)

3/ You can calculate load of lines to left tip. If you do a maneuver, they do not follow model immediately. It depends on its natural frequency and corner radius (means corner time), but in any case there is typically little overweight … until not balanced (and should be) like in point 2/.

But in any case, if you do it like you described, you typically have good starting point on too heavy = safe side and it is easy to go slowly down while trimming.

>But in any case, if you do it like you described, you
>typically have good starting point on too heavy = safe side
>and it is easy to go slowly down while trimming.

Yes it is only intended to be a starting point.
which is why I reduced it to only require line length
and wingspan as the input values. Seems to work pretty
good so far but I have not been able to compare results
with many real world set ups so I don't know how good or
bad it is. I'd like some examples to check my results against.

As for moving flaps, they can be problematic and move the AC
if not designed correctly. If the flap is kept at a constant
% of local chord, they will keep the geometric AOA and beta
constant along the whole span. While changing the magnitude
of mean lift, the distribution of lift force relative to mean
lift will not change. Therefore the spanwise AC will be stable
through all AOA and flap deployment angles.

Changing the flap % along the span will change local AOA and beta.
That will change the distribution of lift forces and destabilise
the AC. My program is not designed to cater for that. However I
did a test program where I kept lift distribution constant along
the whole span (it never is) just to see how sensative the result
would be to lift distribution. I can't remember the exact results
now but I felt at the time it indicated lift distribution would not
greatly influence the final result provided it was designed to be symetrical in srtaight and level flight.