CG on Bipes

[Dennis Toth]

Got my YakYak just about ready to take to the field this weekend, am doing the final bench checks. Vertical CG is right down the leadouts (about 1 1/2" above the bottom wing on strut). One question is where should the CG lie along the wing cord? This ship has the top wing about 1 1/2" ahead of the bottom. For a normal Yak I would balance back about the 15% of the average cord to start. As it sits now it is at about 5% back on the bottom wing and 20% back on the top. I've flown this ship before but its been about 6 years and I don't remember where it was balanced at. Seem close to me, anyone have info on CG for Bipes?

Best,      DennisT

[Will Hinton]

Hi Dennis, I've had several bipes, but this information is not based on a knowledge of aerodynamics as applied to biplane design, simply knowledge gained by experience.
I've ended up balancing each one about 25 to 30% back from the leading edge of the top wing regardless of the relationship of the top and bottom wings.  All of those were very close to the same relationship top to bottom as yours.
Don't take this as gospel, but I think it would be a good place to start.  (That's where my new bipe, the Helldiver in the building techniques section is balanced.)  I hope to fly it soon.

[Tim Wescott]

That can't work for a staggerwing.  It just can't.  Or at least -- I wouldn't have the courage to try it there.

[proparc]

I've ended up balancing each one about 25 to 30% back from the leading edge of the top wing regardless of the relationship of the top and bottom wings.  All of those were very close to the same relationship top to bottom as yours.

This is where most planes balance. Single or double wing!!!

[Will Hinton]

You have a point, Tim, but think of the excitement generated if someone DID try it that way!

[john e. holliday]

Think of each wing as only one and mark the point for balance on each.  Now draw a line between the two points.   The halfway point should be the place to balance.   Simple common knowledge.   Didn't even need algebra whenI did this years ago.   Yes on a Staggerwing the balance point is well forward of where it would be if the wing was only one wing.

[sleepy gomez]

CG on bipe:  Use birds eye view.  It will give you total chord, le to te.  Balance percent as if this were the chord of one wing.  This has worked on all four of my original biplanes and on my triplane.  Even Howard seemed to agree.

[Keith Miller]

CG on bipe:  Use birds eye view.  It will give you total chord, le to te.  Balance percent as if this were the chord of one wing.  This has worked on all four of my original biplanes and on my triplane.  Even Howard seemed to agree.

A perfect explanation.  I agree.

[Dennis Toth]

Guys,
Found this cool little spreadsheet for calculating CG's. Works out right were Sleepy's birds eye view came out.

Best,       DennisT

[Dennis Toth]

Guys,
It FLIES!!!!

Got to the field today and did a few ground checks, test ran the motor - all good. Out to the circle, spinner flip on the K&B 40, away we go. The ship got off no trouble but the controls are a bit touchy. I had move the tail horn in one hole and just grabbed the first handle I had free, it had a wider line spacing than normal.

Well, all things considered, the first flight for the Yak-Yak in 6 years was a success. Wings appear level, engine run on the 11x5 was on the money, lap time on 60' C-C was 4.9 sec and the glide after the engine quit was flat no ballooning or diving. All up weight was 46 1/2 oz. Seems it is pretty close, I need to slow the controls and reduce the line spacing and it should be ready to get into the OT pattern for the Super Unlimited 52 event.

Best,     DennisT

[Will Hinton]

Thanks for the flight report, Dennis!  That's a nice looking bipe!

[Dennis Toth]

Guys,
OK, ordered the tongue muffler from RSM. I decided to change the tail horn to a longer 1 1/2". I had thought about moving the pushrod in on the bellcrank but it was difficult to get to it so the tail horn should work. I had to do a similar tail horn mod on my OTS Ringmaster that has the same type Sig white bellcrank. I will also reduce the handle line spacing to about 3.5" down from 4".

Not sure how some of these ships get so touchy, I guess as a kid we balanced them on the leading edge and that needed a lot more elevator.

Best,        DennisT

[Avaiojet]

Dennis,

Do you have full size plans for the wings and fuselage?

You shortened one wing, otherwise it's two kits out of the box, so to speak?

Anyone offering the Yak kit?

If I start now, I could probably have one completed in a few years.

Great looking model!

[Tim Wescott]

I had move the tail horn in one hole

Eh?  Moving the pushrod in at the tail horn in will make the model more sensitive to controls, at least if you mean "in towards the hinge line".  Or did you move it in at the bellcrank (which would make the model less sensitive)?

[Dennis Toth]

Tim,
I moved it longer on the tail horn (original tail horn was 7/8", new one is 1 1/4") to slow the movement. I did this on a Ringmaster that I fly in OTS, had the same SIG 3" bellcrank. I had it on the inner hole but it is still to much, put the big tail horn on and it flies fine.

Best,        DennisT

[John Stiles]

You have a point, Tim, but think of the excitement generated if someone DID try it that way!

I had a buddy that showed up down here with a bipe he built....he was really glowing, and the thing did fly well...but upon , landing it flipped over and broke the vert stab off. He went and fixed it, and took her off again....landed the same way, and the stab popped off again. So he left it off and flew it one more time and this time the upper wing went awol, and from that day forward it was a single wing[wing-under] hotrod. Oh...he did put the vert. stab back on (with epoxy)! LOL  

[John Stiles]

Tim,
I moved it longer on the tail horn (original tail horn was 7/8", new one is 1 1/4") to slow the movement. I did this on a Ringmaster that I fly in OTS, had the same SIG 3" bellcrank. I had it on the inner hole but it is still to much, put the big tail horn on and it flies fine.

Best,        DennisT

You ain't gonna show us a pic? Oops sorry I see it now...nice!

[Dennis Toth]

Ok had a good morning with the YakYak. The long horn worked pretty good on calming down the controls. On the second flight I changed the handle spacing by 1/2" tighter, this also worked well to get it to flyable. On the third flight I got her inverted and did some eights. It flew stable but still needs some trim. Inverted it flew a bit high in level flight. Seems the leadouts might need to be raised up a little on the strut. The K&B 40 was great on the thin blade 11x5 (had one little problem with a chuck, yes chuck, of grit that cause it to go lean). Once the dirt was cleaned out it ran solid, inverted the setting was dead on with the center line of the tank right on the center of the venturi. The wings seem pretty close to level. The one area that needs work is the outside turn. It seem to turn down very very fast. So fast that it causes the nose to precess in on a strong down control (like the top of the climb maneuver). Inside maneuvers are smooth but the outsides are quick. Balance seems very good as it glides flat once the engine quits.

My plan is to first replace the 1/4 fiberglass arrow shaft to a new Carbon fiber 3/16" light weight pushrod (1/4 oz Carbon vs 1 oz  fiberglass). This should move the CG forward a bit which could help. The next thing is to check that the wings are all true (just finished the check all zero - zero). Next will check that the leadout position is at the correct height. I checked this by hanging the ship from the leadouts. Seems they could be a bit low since I changed out the heavy 4 oz wheels.

Last thing is to check out the engine thrust line. Since it has this tendency to turn down quick it seem there might be some down thrust causing the quick down turning. I'm thinking I might try some up thrust in the engine to smooth out the down turns.

Any thoughts??

Best,     Dennis

[Dennis Toth]

Did the check on the engine thrust line and it had some down thrust (the gauge indicated 2 deg, but I think it is less). I tried to repostion the engine and got about half way so I opened the mount holes one drill size. Also fitted the tongue muffler from RSM (this save 2 oz, which this ship needs). I then checked the vertical CG and it does need to have the leadouts raised about 1/4" to 3/8" to get them in line with the VCG. With 2 oz coming off the nose the balance CG shifted back about 1/8", I am making up a new carbon fiber pushrod with will save at least 1/2 oz off the back so the CG should come back to about where it is maybe a little farther forward which would be OK for this stage to trim. Ship is now 44 oz on 600 sq in.

Best,         DennisT

[Dennis Toth]

Been flying the Bipe for several weeks and feel the ship is very close. I tried an APC 11x5 and one of the older pop-in plastic spinners and removed the nose weight. The APC prop on the K&B 40 launching at 9300 rpm, this prop was recommended by Mike K and it really works on this motor. The ship is now smooth through the maneuvers and solid up top of the vertical eight and overheads (this ship is aimed at the Super Unlimited 52 event, flying the 1952 pattern). Level flight drifts a little, I have cleaned the lines and polished the solid leadouts to get them smooth. I might try adding back some nose weight and see how it impacts the turns.  

Best,       DennisT

[john e. holliday]

Now that is getting down to the fine tacks as the saying goes.   Glad she is working out for you.   It is fun when the planes do start flying the way you want them too. 

[Trostle]

I tried an APC 11x5 and one of the older pop-in plastic spinners and removed the nose weight. \

Best,       DennisT

So, where is the balance point now?

Keith

[Dennis Toth]

Keith,
Originally, with the heavy OS20 muffler and the rear engine mount it balanced 17% back of the LE of the total average cord (top wing plus bottom wing plus offset, pretty much what Sleepy suggested). With the new tongue muffler (RMS OS20 tongue) and the motor moved forward 3/8", APC 11x5 prop and standard snap together 2 1/4" spinner it is now at 18%. I think it is pretty close just a little drifting in level flight. I'm going to try adding a 1/4 oz prop washer to try and claim it down a little more. The ship is flying nice, able to get the OTS pattern solidly and lots of biplane fun.

Best,       DennisT

[Dennis Toth]

OK, I think the ship is dialed in. The last mod I made was replacing the solid leadouts with cable and eyelet guides. Flew it today and all the hunting is gone. I think the main factor was the leadout guides. The original setup was 1/16" brass tubing about 3/4" long that fit close on the solid leads. It seems that any yaw would cause a bit of friction around neutral and it had a slight hunt in level flight. I changed the to cable cause I figured I wanted to take a big swing and get this done. I have used solids in the past but with 3/32" tubing and never had trouble. Anyway, as has been pointed out early (I think Brett raised the pointed) it was a mechanical problem not and the airplane. If you are having a hunting problem look at all the linkages and make sure there is no binding or stiffness anyplace in the system (bend back the leadouts a little and make sure they are smooth).

Best,         DennisT

[Chris Wilson]

CG on bipe:  Use birds eye view.  It will give you total chord, le to te.  Balance percent as if this were the chord of one wing.  This has worked on all four of my original biplanes and on my triplane.  Even Howard seemed to agree.

How does that work if you have dissimilar wings?

[Dennis Toth]

Chris,
The "birds eye view" is a quick and dirty way to get you close. If you have a longer top wing then bottom wing (mine is this way) simply extent the dimensions of the bottom wing to the same length as the top and get the average cord with the offset, then balance as if it were a mono wing. If the top and bottom wings are really different (like the bottom wing is very small) use the CG Bipe spreadsheet that was posted back in this thread.

Best,     DennisT