biplane stability question

[quimney]

I've built two biplanes so far and had an opportunity to observe a third. In each case they seemed very sensitive to down elevator. The slightest bit of down elevator causes them to dive hard. Response to up elevator seems more normal. I have been searching the web for hints but can't find any articles discussing pitch stability specific to biplanes.

The ft baby blender was my first. It seemed very pitch sensitive even with a very nose heavy setup. The second was an SE5 of my own design (can be found in the mad scratchbuilders section). It flew better than the baby blender but it is still seems pitch sensative compared to all the single wing models I have flown. My Se5 has zero incidence on the lower wing and 2 degrees up on the upper wing.

The motor is mounted 2 degrees down and the horizontal stabilizer is also mounted 2 degrees down. Throttle climb is very mild but there is still a little so I don't think I
I went too far there...

Any help would be much appreciated. Thanks

 

[Team_Monkey]

Those planes are "short coupled" meaning the fuselage is short. Those planes can be twitchy. If you make the fuse longer it will be more sedate. Imagine the long fuse, long tail of a pattern plane.

The other thing to investigate is if the pushrods are bending in one direction and in tension in the other direction. I had a plane with bad response in one direction and it turned out the pushrod flex only happened in flight.

 

[Bayboos]

+1 and +1 for Team_Monkey's response.

Because of biplane's relatively short wingspan (compared to monoplanes of the same effective wing area) it is often "overlooked" that the horizontal stabilizer geometry (size/distance from CoG) should be designed in relation to the effective wing area. That causes some "aesthetic problems": correctly flying biplanes have either relatively long tail (close or equal to wingspan), or relatively big horizontal surfaces (noticeably bigger than vertical ones); or both. Without any doubts, Baby Blender does not look like it would fly correctly - for me, it is surprising that it actually flies as good as it does. Your SE5 definitely looks much better, which confirms your experience; but apparently still non good enough (probably caused by small scale).

Pushrod flex is one thing to check; the other is that control arm (on the servo) and/or control horn (on the control surface) may not be "centered" correctly. The line between pushrod mounting hole and the servo/surface axis should be either exactly perpendicular to the pushrod, or moved in the same direction and to exactly the same angle on both ends. Otherwise you will always experience asymetric control response.

 

[quimney]

Thanks! The elevator pushrod is in tension on down elevator but it could bend going up making that direction more less effective. The pushrods I have are a little on the light side and they could be bending. As soon as it dries up a little outside I will try stiffer pushrods to see if that will make the response the same in both directions. Maybe cutting the rates after that will make it a little more docile.

I'll repost when I know more.... And then get to work on my plane stretcher, I'm pretty good at making them shorter already.

 

[Jbing]

Control rods

You can stiffen control rods by sheathing them with something like a coffee stirrer. The trick is finding one with a small enough diameter to be effective. I'm still learning to deal with all this super lightweight hardware. I'm used to building and flying gas models ranging from three to fifteen pounds.

 

[quimney]

Excellent point about the stabilizer vs wing area. I just built it to scale and never even checked. It seemed reasonable to my eye but I completely overlooked the wing area doubling.

The vertical stabilizer seemed really small so I increased its size some. It seems pretty neutral in the turn so I may have lucked out there with out affecting the appearance too much. I'm learning that scaling an existing plane involves compromise, its just a little tough to figure out what you can get away with until its in the air.

 

[AkimboGlueGuns]

It may also be an issue of incidence. The wings on a biplane must remain perfectly parallel with each other or disastrous results would ensue.

 

[quimney]

Great! I was hoping someone would bring up incidence. All I have been able to learn about it so far in relation to biplanes has been about its affect on stall characteristics. I can see that the real plane required a fair amount of up tilt ( I think that's positive ) on the top wing to fly properly but I'm not sure why other than for stall. I would love to learn more about this issue. What happens and why when the settings are varied.

 

[AkimboGlueGuns]

There are some theoretical advantages of having different incidences on the wings, however it would take a lot of experimentation. Most biplanes keep the incidence the same on both wings as it reduces airframe stress and it easier to make. I have a baby blender, and I have noticed the same issues you have had. It is pretty pitchy, but that is coming from the short fuse. It has similar downfalls to the pitts in that regard. It's very maneuverable, but very twitchy and sensitive.

 

[quimney]

The stiffer control rod made it symmetrical I dialed the aileron rates down a little and it flies pretty well. It sure is maneuverable it pulls so many g's that my normal method of velcroing the battery to the bottom of the battery box is not going to cut it. I keep flinging it out. And then I work the sticks like mad trying to bring it down easy. It's usually on the ground by the time I realize that no battery means no control time to build a battery box.

Thanks to all for the help!

 

[quimney]

One last thought... As I mentioned before my baby blender was very pitchy. One day I landed it a little hard and the top wing came off. Since I wasn't really enjoying the plane anyway I just cleaned off what was left of the wing strut, moved the battery, and tossed it in the air again. To my complete surprise the little plane was very flyable without the top wing. You had to go fast due to the very small remaining wing but it was not difficult to fly. I guess that's what started me wondering about what was so special about biplanes. Is it just the effect of doubling the wing area on pitch stability or something more.....

 

[AkimboGlueGuns]

It's not biplanes, it's the Baby Blender. Even comparing it to other short biplanes the fuselage is short. It has HORRIBLE coupling issues.

 

[Bayboos]

One last thought... As I mentioned before my baby blender was very pitchy. One day I landed it a little hard and the top wing came off. Since I wasn't really enjoying the plane anyway I just cleaned off what was left of the wing strut, moved the battery, and tossed it in the air again. To my complete surprise the little plane was very flyable without the top wing. You had to go fast due to the very small remaining wing but it was not difficult to fly. I guess that's what started me wondering about what was so special about biplanes. Is it just the effect of doubling the wing area on pitch stability or something more.....

The formula for horizontal stabilizer surface area is based on the wing area and distances from CoG. That means with the fixed distances, any change to the wing surface area corresponds to the change of required horizontal stabilizer surface area. Since you decreased the wing area by half (in fact significantly less than that, there is a word "effective" there somewhere; but still) without changing the tail surfaces at all, you ended up with effectively twice as big horizontal stab surface than the original Blender. No surprise that it started to behave well...