SeamusG

Hi there - I'm tossing this thread into the Beginner's Formum because I believe that there's some good trimming basics that I'm missing that will be helpful for the newbies.

I have a Hog Bipe (HB)with a Saito 100 under the hood. I set its static CG using a Vanessa Rig. Being a biplane it's somewhat problematic knowing where the actual CGis. I thought things were somewhat nose heavy. The static lateral CGwas established using a wire-over-the-celing-joist rig. The control surface throws were set up to SIGspecs.

I've got 4 "good ending" flights on it. Definitely puts a smile on my face big time. But I noticed a couple of things. The HBis a bit twitchy in the back end. Rudder input moved the backend back and forth pretty aggressively. Most noticeable when I did a stall turn - the down line was really twitchy - kind of a tank slapper on the way down. My exponential is at 30%(DX7). Ditto observation on the elevator but less noticeable.

The 5th flight didn't have such a good ending to it. I was doing fly bys straight-n-level at 10 - 15 feet. Not trying to do a touch-n-go just checking out my visual reference points for getting the plane comfortably centered over the runway. Speed (I think)was about 1/3 throttle. The plane then started an aggressive glide path towards the concrete runway. I applied (what Ithought was)full throttle (smoothly but quickly) and applied back (up) elevator. The plane sped up but didn't climb. Seemed to do an aggressive flare at 5 feet (yuk). Nice 3-point landing with the throttle nearly full bore. The nylon landing gear bolts (31/4x20) sheared off and the plane veered a bit to the right into nice soft dirt and did 2 very nice cartwheels.

All things considered this will be a pretty simple repair. As a buddy used to say about the first scratch on a motorcycle - "ain't a virgin any more, just get back on and ride!".

Back to the thread. What are the basic steps in trimming a biplane. Please identify those things that are similar to a high wing/low wing trainer and those things that a biplane flying fool has got to tell you that come from real life experience (preferrably good experiences ).

TIAfor your input.

Villa

I have had three biplanes over a 30 year period. I always play a little with the CG location to see how performance changes. I have never found anything that is different about a biplane with regard to the CG or trim, as compared to any other RC configuration.

jetmech05

I currently fly a Pitts Python and have a Goldburg Ultimate that I don't fly much anymore.....trim on a bipe is just like any other airplane...I trim for thumbs off, straight and level flight at 1/2 throttle.......CG on any airplane is best checked in flight doing the 45 deg upline full throttle thumb off the elevator trick
What is different on a bipe is the drag....all the bracing for the top wing, and probably alot more leading edge than a mono wing....sounds like you got a little slow.....
If your having tail wag issues what servo are you running on the rudder?
Nice save by the way

SeamusG

I'm using a Hitec HS475 - very basic non-digital item.

I've learned a bit since setting this plane up using my DX7 18 months ago. This time I will look for 125% throw to achieve high rates then use the dual rate feature to knock the throws down to low rates. This will give me 125% servo rotation for both low and high rates. Stuff you learn just by asking ...

OkadaKeisuke

My Feedback: (1)

Just reading through this post and thought I'd ask if you could elaborate on this? I have always done the dive test...

~Noah

SeamusG

Apicture is worth 1000, em, uh, things? Noah - do ya need a detailed description?

I guess the good news is that I can recover the Hog Bipe. New scheme in the pic.

BTW- have you ever listened to Bill Cosby? Noah? Huh? Noah? Who is that? This is the Lord speaking! ...

OkadaKeisuke

My Feedback: (1)

SeamusG,
The picture says a lot. Very similar to the dive test. Nice scheme, and yes Bill Cosby is a funny guy!! I thoroughly enjoy the act he did about the cake for breakfast. Cake has eggs, milk and wheat, all things that make a well balanced breakfast...

~Noah

Lnewqban

Sorry to read about the mishap.
The plane surely had one of your fantastic cover jobs.

I would say that trimming is similar, except regarding the relative incidence angles of the upper and lower wings, which has been discussed in these forums before.

Some like more positive for the upper and some like same angles for both, which seems to have a pronounced effect in pitch stability and abruptness of wing's stalls.

The theory behind it is that the more advanced wing should be the first wing to stall, in order to help dropping the nose sooner.
Also, it has been discussed that both wing's upwash and downwash interfere with each other and the tail.

Another characteristic to consider is that the more compact configuration of bipes reduces the inertia in pitch, yaw and roll respect to a high wing/low wing trainer.

SeamusG

Ihad a 103 degree temp (thanks to the plague shared by my 6 mo. old son), miserable, when Ifirst saw that ("Himself"). Ihad tears in my eyes from laughing for two straight hours. There are times when mothers have absolutely NOsense of humor! Choc-late cake, choc-late cake, dad is great, choc-late cake ...

SeamusG

I spent many hours tweaking the incidence angles. While it's "apart" Ishould probably re-read those threads.

BTW- if you're interested in more details of this plane - this bipe a kit, not an ARF. If you're really interested Idid a build thread "Hog Bipe Build 2009" in the Kit Building forum.

OkadaKeisuke

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Thats the one!

dredhea

My Feedback: (13)

The 5th flight didn't have such a good ending to it. I was doing fly bys straight-n-level at 10 - 15 feet. Not trying to do a touch-n-go just checking out my visual reference points for getting the plane comfortably centered over the runway. Speed (I think) was about 1/3 throttle. The plane then started an aggressive glide path towards the concrete runway. I applied (what I thought was) full throttle (smoothly but quickly) and applied back (up) elevator. The plane sped up but didn't climb. Seemed to do an aggressive flare at 5 feet (yuk). Nice 3-point landing with the throttle nearly full bore. The nylon landing gear bolts (3 1/4x20) sheared off and the plane veered a bit to the right into nice soft dirt and did 2 very nice cartwheels.


Without video, it's nearly imossible to tell for sure, but it sounds to me like you were in a stall. If you were a little tail heavy, which you indicated as probably the case by excessive tail motion, and had insufficient airspeed, the tail would tend to drop somewhat. Once the AoA reaches a critical stage (aggressive flare), airflow will stay disrupted over the wings even if you gain airspeed. Continued up elevator input only makes matters worse as it will prolong the stall. If this is indeed what happened, a touch of down elevator to get the wings back to level flight would have corrected the situation. It's just very hard to push the stick forward when the plane is behaving badly at 5 ft. off the runway

Rodney

I have never had a Hog Bipe but after having 7 Aeromasters, 3 1/4 scale Moths, about 5 other large 1/4 scale bipes and many smaller ones, I have found that all flew best with the upper wing at about 1.5 degrees LESS angle of attack than the lower wing was set at. All also required about 3 to 5 degrees down thrust and about 3 degrees right thrust to track well. I did a lot of experimenting with the relative angles between the upper and lower wings and always wound up with the upper wing at a lower angle of attack than the lower wing was set at. When you build your model, I'd suggest you plan it so that you can do a bit of experimenting with this relationship and you will find that just minor changes can make a huge difference in how well the plane tracks and maneuvers. It can become quite a chore to change the relative angles for experimentation unless you plan ahead in the construction as the interplane struts and cabanes can become difficult to change.

Lnewqban

Very nice covering job!!

I have included that thread in my Favorites for future covering work references.

Thanks

Thatsakitbuilt

SeamusG,

I too have built the HB from a kit, and have about 10 flights on it. Must say, it was/is a learning experience. When building I reallly took my time in the wing build and mounting to make sure the incidence's the plans were calling out were right on. This is my first bi-plane and I still get the pucker factor when I fly it. However on my last flight, when I took off the plane was real sluggish in a climb, and very slow in response to get the plane higher and turn. I thought I maybe didn't let it get enough ground speed to lift off. It was so sluggish/waggly I was lucky I got it back on the ground in one piece. I checked the controls to make sure nothing was sticking. Everything looked good on the elevator/aileron/rudder inputs. So I took off again, but letting the plane get more ground speed before lifting off. Again the same symptoms. Got it back on the ground safely, and took the wings off and guesss what I found. My servo tray rails had come loose. Can't say if this could apply to your case, but I did have a few landings that ended up nosing the plane over on it's back a few times, which explained why my rails came loose. Didn't notice it during pre-flight, but it's part of my pre-flight now, to check the rails. I have a OS 75 on mine. Does your's have any issues on take off? Mine wants to track to the left. And I dont' think the plans showed any down thrust or right thrust numbers like most of the other SIG kits I've built.

SeamusG

Track to the left? If I'm not adding constant right rudder (or if the wind is in your face - 90 degrees from the plane's left) it will hang a hard left as throttle is advanced to take off speed - off the runway - necessitating wide open throttle just to survive.. I do not like the "whack it wide open and hit the elevator" style of take off. I like the slow increase in throttle, get the tail flying then a nice smooth lift off.

kerwin50

I agree with rodney

jester_s1

Rodney- I've heard that too about making the top wing lower incidence than the bottom. Do you have any explanation as to why it makes it track better? I know that many swear by doing it, but I can't understand why it would work better than the other way. Also, how are stall responses when the top wing is lower incidence than the bottom?

Rodney

I can not give you a technical reason, just that trial and error has proven it to me. Perhaps an aeronautical engineer might be able to. I think it is because the wings, being in close proximity (less than on chord length separation) makes the air flow different than if a single wing were being used. I have not found it detrimental in so far as stalling is concerned. I always want to be able to enter a true spin from a stalled attitude using only elevator and rudder (no aileron inputs) and all my bipes will do that. I also tinker with the CG location so that I need very little down elevator to maintain level inverted flight.

Thatsakitbuilt

SeamusG

I know what you mean. I've gotten better at takeoff, where I can get the tail to lift and then a smooth lift off. But sometimes those smooth liftoffs are headed left. May have to add some right thrust on the engine and see what that does. But I love the way it flies once it's in the air. I don't know if your's is like mine in cross winds, but mine is like sail. It swings the tail big time.

Cal

Gray Beard

My Feedback: (-1)

Jim, IF you really need some how to trim a bipe ideas i have a few items I have kept from old magazines. The IF is there because I would have to find them and put them into a file and email them to you. If you really need them I will mail them to you tonight.
Usually the reason for a sharper pull to the left on take off with a bipe is the P-Factor on a short coupled plane. Bipes like the Tiger Moth aren't effected like some short coupled plane like an AeroMaster or HOG. One old trick used instead of adding a bunch of engine right thrust angle is to have the left aileron set at neutral or centered and then add about 1/16 to 3/32 up in the right aileron to help the adversed Yaw.

Charlie P.

One thing I have noticed is, I suppose because they are so short-coupled, a biplane is more sensitive of horizontal stablilzer angle of attack.  Check to make sure your Hog is 0° or perhaps -1° (some negative helps avoid a "squatting" appearance in level flight).  If you applied full power from 1/3 throttle and couldn't gain altitude (only speed) it seems something was keeping the nose down. 

WWI bipes need about 1/2 throttle to land because of drag.  Do you have a lot of rigging on the Hog?

Augie11

I'm with Rodney on the negative incidence of the upper wing relative to the lower wing. I routinely use 1 degree on all my bipes (these include the HB, a Giant Aeromaster and two WW I's from Flair and BUSA). I also use 2 degrees left and 2 degrees downthrust. I've found this setup to work 'right off the board' for these bipes

As someone mentioned, WWI bipes with their generally lower wing loading and higher drag don't land like the HB. They require a bit of power and a nice nose level or slightly nose down attitude just prior to the flair. The HB pretty much lands like a low wing sport plane and that's really the beauty of it. Just get a good glide angle on final at a hair of power, cut the throttle as she crosses the threshold and grease her in.

Adjust your gal until you get her right and you'll have a friend you can fly under almost any conditions. Best of luck!

SeamusG

The Hog Bipe incidence as specified in the plans is simple: everything is at zero degrees incidence (stab, upper wing and lower wing).

Need to (re)check the thrust angles. I've keep nice notes just don't remember my frame of mind when I put them somewhere that I'd easily remember.

Iwas checking the control surfaces just to make sure that the hinges are in good shape as well as control rods and servos. While putting up/down tention on the elevator there was about 1/8" slop. Very tight at the horn/clevis. Very tight at the servo arm/clevis. However, the servo arm can be wiggled back-n-forth - internal servo slop. The elevator servo has hit the trash bin - nope, no teeth were missing or chipped. There was no damage to the elevator in the mishap (nicer way of saying pilot error resulting in a crash) so this condition could have been a contributor.

Thanks for your input all.

SeamusG

Missed your question - sorry Charlie

The cabanes are formed from 2 pieces of shaped aluminum (each a U connected by a wood sandwich across the tops). The 2 I-struts are 3/32" lite balsa. No other supporting structure. As both wings have ailerons there is a 4/40 control rod between the two ailerons.