sig hog bipe CG problems
 

Hi i have a newly completed Sig Hog Bipe with a brand new os .91fs (non Pumped) running. I have been running the engine very rich to start out with. I tested every thing before i had some one with a hog bipe at my club fly it. The CG was exactly were it was supposed to be, 3 1/4 inches back from the leading edge. The plane taxied fine and seemed to take off fine as well. Every thing after that was not though. The plane was flying extremely tail heavy causing the plane to snap and stall very easily. THe guy test flying told me it was flying very tail heavy and needed more weight in the nose but the manual says 3 1/4 from the top wing leading edge. Has any one had this problem. Where should the CG be? I can't believe that the lowest setting in the manual would be that tail heavy! It even suggests to go further back to over 4 inches for more advanced aerobatic flight. Any insight would be great thanks
-Matt

RE: sig hog bipe CG problems
 

How do you have to hold the controls? Are you holding a lot of 'up' elevator just to sustain level flight? You may not be 'tail heavy', but underpowered, and flying too slowly. Usually, if the airplane's not flying well, and tends to wallow around tail-low, people will say it's "tail heavy". Not so. That usually means an airspeed problem. If the model was 'tail heavy', it would tend to be very sensitive on the pitch axis.

You didn't say what propeller you're using, and what RPM the engine's turning. If you have a prop with too little pitch, and the engine's rich enough that the RPM isn't where it belongs, then you will not get enough airspeed to get the model to fly well. A 13 x 8 APC prop should turn in the mid- to high-9,000's, and should provide plenty of airspeed to fly the airplane well.

One way to check balance when the airplane's flyable, is to take the model up high and put it into a dive. Release the controls. If the model tends to pull up, then it's nose-heavy. If it tends to steepen the dive, it's tail-heavy. If it only changes a moderate amount, it's about right.

RE: sig hog bipe CG problems
 

ah ha, i had a 14/6 on there and i thought i had a 14/8. thats why i did not think about the pitch because there was (or i thought there was) plenty.

RE: sig hog bipe CG problems
 

i told the guy who flew it the response about the air speed and this is what he said back.
Matt,

I think the engine had sufficient power and we had plenty of airspeed (except for the turn where it stalled and fell out of the turn). After that stall, I kept more airspeed than I thought the plane needed and did a very "hot" landing. I was actually adding down elevator to level the plane and get airspeed, but we weren't up in the air long enough to "trim" it out so I couldn't say for certain if it was an "off trim" down or if down trim was required to fly level. It was stable enough that we probably should have taken it higher and played with it a little to evaluate the balance and trim. Since it was a first flight I felt we needed to bring it down and look for problems first. With a 4-stroke a higher pitch prop would probably be better since they turn at lower RPM, so I would recommend the 14/8. I have a tach that we can use to check the RPM.

It is possible that the plane is nose heavy vs tail heavy - either condition can cause instability, however since I was adding down elevator to get it level it seemed to me like it was tail heavy. Planes that are nose heavy are generally more forgiving than tail heavy ones. If your directions are the same as mine, I would move the CG back to spec and try again. We might have my son, Ron, fly it and give his opinion, he likes to balance his planes tail heavy to get "snapper" maneuvers and is therefore used to flying planes that are less stable.

Their "steep dive" maneuver assumes the plane has been trimmed for straight and level flight. If normal trim requires down elevator (to hold up a heavy tail) then an increase in speed will cause the nose to drop due to the down trim. Conversely up trim will cause the nose to rise with increase in speed.

In short - we weren't "wallowing around" at stall speed so I still think it is a "balance/trim" problem.

Chuck

what do you guys think could this be true?
-matt

RE: sig hog bipe CG problems
 

Try double checking the top wing incidense vs the lower wing and make sure they match...worth taking a look.

RE: sig hog bipe CG problems
 

I have been flying the exact set-up you describe. Best flying plane i have had (over 30+ year). I can fly it very slowly with minimal or no risk of snappy stall. I am running a 14x8 APC. I did not feel that 13" prop felt right. Also, check the relationship between the wings and the overall incidence with respect to fuse. My CG is at 4 1/8" (per plan).

Check out this link regarding hogs.

http://www.fubarhill.com/

RE: sig hog bipe CG problems
 

Hello,

I am new to the forum but I picked up a built SIG Hog Bipe from one of my flying club members last fall and flew the plane with no problems. I have since added a pilot and I would like to check the CG.
Since buying this plane used the original owner couldn't find the assembly manual. After reading in this post I have read two different measurements for the CG.
Could someone post the information from the manual for me? Or if you could email me I would apprecaite it. Or is there anywhere I can download the manual?
One final thing. I have just finished balancing my SIG Something Extra and I was extremly happy with the Great Planes CG machine.
Do you balancing a biplane different from a regular plane?

Thanks very much,

Jeff

RE: sig hog bipe CG problems
 

Biplanes are often balanced using the top wing. The most recent ones I've done have all had instructions that reference the top wing.

That often gives the commercial balancers fits, btw.

Remember that the CG you're trying to locate is somewhere in the fuselage on most bipes. And whatever device or rig you use to indicate the CG will actually not be holding the wing/airplane "on the CG". The CG will be hanging directly below the line the balancing device has found for you. For example: when "the manual says 3 1/4 from the top wing leading edge" and your balancer winds up holding the top wing 3 1/4" back from the LE and the airplane is teetering there perfectly horizontal with the floor, the CG is somewhere below that line on the top wing.

RE: sig hog bipe CG problems
 

I think funkworks is correct, you have an incidence problem if you CG is where the plans say it should be. If that upper wing gets to positive with respect to the lower wing, the plane will be very squirley, double check to be sure that it is per plan. If not, start decreasing the upper wing incidence and you will find a point where it tames down and flys well. Also check your thrust line, make sure you have the proper right and down thrust.

RE: sig hog bipe CG problems
 

How much throw do you have in your elevator?

RE: sig hog bipe CG problems
 

Just curious about how much weight you had to add to the nose to balance the aircraft? I've built 2 Hog-Bipes and they each took 4 or 5 ounces to put the CG at 4 1/8" behind the leading edge. They both fly great on OS 91 4strokes with a 14 x 6 turning 9500.

RE: sig hog bipe CG problems
 

mine has no weight added to balance.

RE: sig hog bipe CG problems
 

With same flight set-up as Augie11, mine has no weight at all. I actually needed to move the batteies to the back end of of control compartment, even with the wing bolt mounts.

RE: sig hog bipe CG problems
 

Not trying to insult you, but was the tank emty when you checked the balance?

RE: sig hog bipe CG problems
 

I have teenagers, it will take alot more than that to insult me. Yes, the tank was empty. Again, I have flown a ton of differnt airplanes and can truely say that the Hog Bipe is my favorite for good old pure fun.

RE: sig hog bipe CG problems
 

Thanks guys for the help. After racing cars for the past 22 years I have all my terminologies wrong. I should have said balancing and not CG. But I will be checking both.