Quicksilver MX weight and balance

Did Eipper ever published a weight and balance for the MX?   I don't know, I have been told yes and no.  The Quicksilver web page doesn't have anything! Searches of the internet found nothing.

Lets see if we can work out what the CG limits should be for the Quicksilver MX

If we use "Javafoil"¹ airfoil software to analize the center of lift for the airfoil²


We can see the center of moment curve ranges from 0.45 (45%) of the cord at 0° angle of attack moving forward to 0.22 (22%) at 12°.

The center of lift range is then .22 X 60" to .45 X 60".  or 13.2" behind the leading edge to 27" behind the leading edge.

UPDATE:  I have since found a worksheet on CG range. It shows a range of 24.5" TO 34.5" behind the leading edge.   I am not sure of the source of this worksheet. I don't like the way they are doing the weight with 0 angle of attack the wing has no lift and this method changes the real CG range. If you do the weights with root tube at  a realistic 5° you shift the CG back by about 3". The link to the worksheet is at:   MX weight and balance tif image with more information at: http://groups.yahoo.com/group/quicksilverultralightowners/files/

That is 13.8" change in center of lift. This high under camber airfoil has interesting results.  It is quite a bit different from the airfoils found on most general avation aircraft. The thicker wings without all this under camber don't see this large a shift in center of lift.  If this data IS correct the CG becomes less critical for the MX because changes in airspeed and angle of attack cause such a large change in the center of lift.  You can pick your center of lift by changing your air speed.

If we pick the balance datum at the front axle (a nice location for weighing the MX) the center of lift from the datum range is 48.7" to 62.5"

A reasonable range of CG limits for the MX with the size of the tail would be about 51" to 57" behind the front axle. As the angle of attack goes up the CG will move forward over 4" due to the position of the pilot and lower part of the trike.  At the same time the center of lift moves forward by about 13".

This agrees with the balance I measured for my MX at 54" with pilot and 2 gallons of  fuel, it will fly hands off at 36mph (GPS at 5500') 24mph (Hall's) airspeed.

The location of the fuel tank will cause the CG of the aircraft to move back 1.125 inches as the tank goes from full to empty.

To find the CG for your MX

First weigh each of the wheels with the airplane kept level. This can be done with one scale 3 times or 3 scales, one for each wheel. A more detailed CG measurement can be done by setting the root tube at 3 angles of 5, 10 and 15 degrees and recording the weights for all 3.  These will give you better information at flying angles of attack.  Blocking up the scales for each wheel for wings level and desired root tube angle for each weighing.

Next add the main gear weight and multiply by 62.25" (the location of the rear axle will differ on the MX due to nose wheel setup) to get the moment.

Now add the total weight on all 3 wheels and divide the moment calculated above by this weight.

If the result is more than 51" and less than 57" your MX should be in range. Due to the differnt nose wheel setups on the MX using the nose wheel datum may be confusing.

If it is toward the front the plane will be more stable and take more stick force to keep the nose up.  If the result is toward the rear it will respond faster and take more force to put the nose down.

The faster you go the more nose down stick required. This is great for static and dynamic stability.

Other interesting information about the airfoil:

L./D chart: at 9° the airflow in the computer model gets to be a mess.  This chart has a range of 0 to 15° angle of attack

Flight with an angle of attack indicator shows the highest L/D is at 10° and stall is at 16°. The single surface simply won't fly below about 4° angle of attack.

Lift chart: Cl vs angle of attack. Real flights suggests that this is not accurate above 7°


Note the lift at angles below 3 degrees is negitave.   An angle of attack indicator would be an interesting addition to the MX to learn more.  (UPDATE:   I made and flew an angle of attack indicator.  The computer model is good up to 8°. The best L/D is 10° and stall is 16°.)

The undercamber at the leading edge creates a large negitve pressure under the leading edge at low angles of attack.  This robbs the lift and moves the center of lift way back. 

Chuck Hoelzen Aug 6, 2007
References:
1) javafoil is a program that allows you to design and test airfoils
2)Here is a copy of the ordinant list for this single surface Quicksilver airfoil. With this you can play with the airfoil software yourself.