CG question for NW

When I talk to PAul Walker about trimming Impacts he says to measure the CG at the root, and the number is given as a distance from the hingleline.

For example PAul mentioned the Saito 56 Impact was 7" (22%) from the hingeline and the OS vf Impact was closer to 6 3/16" (30%).

Now, Doug tells me that the percentage should be calculated on the half span to compare with Ted and Brett. This would make the same numbers coorelate to 19% and 25% on the same Impact.

Could someone please help me?

Also, Paul if you are out there could you please include the numbers you liked and the amount of elevator ratio?

>When I talk to PAul Walker about trimming Impacts he says to
>measure the CG at the root, and the number is given as a
>distance from the hingleline.
>
>snip

I do it that way because it is easier to check the CG (by balancing) at the root, and the flap hinge line provides a convienent place to measure from. Everything here is relative.

I'm just lazy!

snip
>Now, Doug tells me that the percentage should be calculated
>on the half span to compare with Ted and Brett.
snip
>
That is not correct either. I believe you are making reference to the MAC. If the wing is constant chord, then mid-span is correct. If it is tapered, then it will be biased closer to the fuselage side of mid-span. I don't have the equations handy to show this analytically at this moment.

>Could someone please help me?

I bet Brett has these equations burned into his memory!!;)
>
>Also, Paul if you are out there could you please include the
>numbers you liked and the amount of elevator ratio?

My CG was in the neighborhood of 7" forward of the flap hinge line. The elevator ratio was 10 to 15 percent faster than the flaps. (1" pivot on the flap horn, .9 to .85 pivot on the elev horn)

Good luck.

>That is not correct either. I believe you are making
>reference to the MAC. If the wing is constant chord, then
>mid-span is correct. If it is tapered, then it will be
>biased closer to the fuselage side of mid-span. I don't have
>the equations handy to show this analytically at this
>moment.

Couldnt you just find the average chord for the wing and then locate it on the span and find the CG at that point?

>
>I bet Brett has these equations burned into his memory!!;)

It would be nice if he would enlighten us.

>Good luck.
>
>p. Walker

Since I am begging at the feet of the master, could you please describe your prop, venturi, RPM, and clicks from peak RPM at launch?

The Other Walker

Hey Paul, missed you at the Nats. You are one good wind flyer that would have been interesting to watch.

Oh, by the way, Konstantine did, indeed, trump us all by using 0% nitro and making the finals. Hmmm.

>Hey Paul, missed you at the Nats. You are one good wind
>flyer that would have been interesting to watch.
>
>Oh, by the way, Konstantine did, indeed, trump us all by
>using 0% nitro and making the finals. Hmmm.

We also never saw him leave the center of the circle save for pull out on the four leaf!

Andrew Raney (Slodog)gave the geometric derivation for the Mean Aerodynamic Chord (MAC).

It can be calculated as:

MAC = 2(a + b - ab/(a+b))/3

where a = root chord and b = tip chord

As Slodog mentioned, this works for any straight tapered wing. It also works for any straight tapered swept wing.

There is another useful equation when dealing with the MAC which is the distance (along the longitudinal axis) from the root chord leading edge to the leading edge of the MAC which is m in the following:

m = (s * (a + 2b)) / (3 * (a + b))

where s is the distance (along the longitudinal axis) from the root chord leading edge to the leading edge of the tip chord. This also works with any staight tapered wings with any sweep angle.

NACA (the precurser to NASA) nomenclature defines the MAC as "the chord of an imaginary airfoil which would have force vectors throughout the flight range identical to those of the actual wing or wings."

For standard convention when dealing with CG locations on our stunt ships, I think it still best to measure the CG as a percentage of the MAC. This takes care of any wing taper, however severe and any wing sweep.

(Note: I did not preview this original post. There was an error in the equations that had been originally posted. I apologize for the inconvenience. KT)

Keith, I see only:

MAC = 2/3

It looks I do not see some letters in equation

BTW,
That simplified "half span" way is not perfect, but if I calculate and compare both I see there is only minimal difference on OUR USUAL models because of moderate taper and sweepback we use.

>Keith, I see only:
>
>MAC = 2/3
>
>It looks I do not see some letters in equation
>

Apparently, something got imbedded in the message that either gives a strikeout appearance or jibberish (depending on the browser).

Seems it is a software issue. When converting the "bracket" character to HTML, the translation done by the board software hiccups. I went into the message source and replaced the bracket with a parenthesis. It looks a little awkward - and Keith's formatting was flawless, but I think you should be able to read it now.

Now, if Keith will verify that it is still correct... .

Frank Williams was operating his Impact at the Nats with the CG 5.5 inches forward of the flap hinge line. He said he wanted less spring and more damper in the pitch dynamics. He flew it well.

The easy way to calculate the MAC of a wing;
on the drawings, at the tip mark a point at 50%,next mark the 50% position of the root. connect these two lines.
part 2
measure the tip,including the flap, add this distance to the FRONT of the root and make a mark.
measure the root,incuding the flap, add this distance to the BACK of the tip and make a mark. connect these two points. Where the lines cross is the Mean Aerodynamic Chord. This method is for straight tapered (like an Impact) wings.
Andrew R.