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stretchedpacer
08-28-2018, 11:03 PM
What have folks set the aft CG at for their producers and bushmasters? I know that it likely depends in large part on the length of the stretch and the length of the engine mount and what is installed on the plane but it seems the wing should have some basic margins. In looking at the type certificate for most of the piper ragwings it is obvious the forward and aft CG are all over the board. The reason I ask is that (for experimentals) we have to set this range before the airworthiness certificate is issued even though we have to test it during phase 1 flight testing which may produce different results. I am 30 hours into my phase 1 flight testing (bushmaster) and am about to start doing the portion where various weight configurations are tested. As a result, I would like to know what others have either set their aft CG at or what they have learned through flight testing. To the extent it matters to those responding, my plane is 24 inches longer than a pacer and the engine mount is 3 inches extended with a 160 hp O320 and light catto prop on the nose. I am just looking for general aft CG ranges as I know what the CG is for various loadings on my plane. Thanks.

Stephen
08-29-2018, 04:24 AM
My Bushmaster is 22.5 inches longer and I have a stock Pacer engine mount. I used the STC specified envelope and it works fine. In fact my planes flies much better on the step at much lower speeds. I cruise at 2250 very nicely. Stalls are lower. The length of the fuselage and engine mount are not the whole story. The Center of lift to CG also changes in the Bushmaster.

stretchedpacer
08-29-2018, 10:15 PM
Thanks Stephen. You hit the nail on the head with what I am struggling with. I dont know how to figure the center of lift and its relationship to the CG. Any insight on that? In the alternative, can you share the aft CG set forth in the STC you used? As I recall, you have the plans for sale as well which I could buy if you preferred that I go that route. My plane is finished but I am sure there would be good info in them.

Stephen
08-29-2018, 11:49 PM
No need to get plans now in your case. I've got the data at the airport and will get it and post.

PM me to remind me if I don't get it posted right away.

Center of lift is usually about 1/3 aft of the leading edge. It does not change since Piper used the same airfoil on it's ragwing planes. But, longer wing means greater center of lift force which can off set the longer tail moment.

Stephen
09-01-2018, 07:03 PM
My Bushmaster has an Empty Weight CG of 15.8" aft at 1186 lbs.
My Pacer had an Empty Weight CG of 11.4" aft at 1105 lbs.

The envelope of the Bushmaster and Pacer at gross weight are the same range of +17.5" to +23.0"

The GW of the Bushmaster is 2100 lbs. The Pacer was 2000 lbs.

The envelope at 1800 or less for the Bushmaster is now +15.0" to +23.0" (no change for the aft loading).

Center of lift is about 20" aft. Lift is increased with the longer wings.

wyandot jim
09-01-2018, 07:23 PM
From the VERY little I have read what you really want to know is the aft CG location in relation to the Airplane Neutral Point. If the aft CG becomes to close to the Neutral Point it is possible to enter a deep stall/flat spin that you will not get out of.
There are several examples on how to locate the NP and the aft CG relation to it.
Here is one article.
https://www.flyingmag.com/search-neutral-point

The position of the neutral point is found by taking the algebraic net moment of all horizontal surfaces, measured from the nose of the aircraft, in the same manner as the c.g. is determined, i.e. the sum of all such moments divided by their total area. The stabilizer and elevator dominate this result, but it is necessary to account for all surfaces such as fuselage, landing gear, prop-normal, etc. It is also necessary to take account of the center of pressure of the wing, which can move a good deal fore and aft as angle of attack of a flat-bottom wing section (Clark Y) changes, or not at all in the case of self-stabilizing sections such as the M6.
The neutral point in conventional aircraft is a short distance behind the c.g. ("The feathers of the arrow must be at the back"); but in unconventional aircraft such as canards and those with dual-wings, such as the Quickie, this will not be so. The overall rule stated above must hold, i.e. the neutral point must be aft of the c.g., wherever that may be.

Southern Aero
09-01-2018, 11:44 PM
http://chrusion.com/BJ7/SuperCalc7.html

Stephen
09-03-2018, 10:42 AM
There are several measures of central tendencies that describe flight. One of them, "neutral point" takes into account the whole aircraft's pressure forces as stated. But, importantly it measures the tail's effect of countering longitudinal instability of the wing. In flight when you release the controls you do not want the nose to rise. Lengthening the wing without moving the tail aft increases longitudinal instability. This is why lengthening the fuselage is important when building a Bushmaster. Prior to building my Bushmaster I had a Ferguson wing tip STC with squared off wings on my Pacer. The wing span was 31 feet which increased lift, it also increased instability. When my plane was aft loaded at GW and full flaps the nose had an uncomfortable tendency to rise.

Some longitudinal instability is OK, especially when you want to make a full stall landing.

stretchedpacer
09-05-2018, 10:26 PM
Stephen,
Thanks for the information. That is exactly what I was looking for. Interestingly, my empty CG is almost identical to yours. Thanks again!