rt3232

My Feedback: (6)

OK all

I need some help, I am in the process of finishing an ARF bipplane that I purchased about 3 years back. When I recived it I did not open it for about 3 months as my bench was full at the time. any way after opening I found it was NOT what I thought I bought so it wasd not returnable and it went in the corner, I have now decided to see if I can make it fly and after looking thru the box and all of the parts there is NO instructions and it was missing the fuse cabaines no big deal so I can make my own. The only making on this thing was a vitercal and horizantal C/L on the fire wall and after measurig the box frame I established a thurst line, Using that as my referance, fitted the lower wing at 0 incidance, it turns out the horzintal stab has 2 deg's of positive, which I think is ok, fitted the upper wing at 11/2 negitive incidance. this matches a 1/3 weeks special that I have. but everything else is different. So with all of the guts in stalled except the batteries. Now I am looking for a way to figure out the correct C/G so any help on how to figure this would be greatly appericated. A singal wing bird is easy but I have never had to do a bipe with out instructions.

cheers Bob T
AMA13377

Charlie P.

Biplanes are a bit different than monoplanes as the wings are often staggerred and the 23%to 25%of chord rule-of-thumb doesn't apply as easily. What model is it?

Gray Beard

My Feedback: (-1)

If it's constant cord then the 25% of the wing cord works very well, if not then I would use one of the on line CG calculators. Hard to really know without knowing what plane it is and what type of wings.

rt3232

My Feedback: (6)

Sorry guys I did not mean to confuse the issue, but there actualy is no name, and you guesst it I bought it off of (flea bay) as best I can discribe it, it is vary similae to a pitts but it AINT will try to post a pic tonite as I am off to work

Cheers Bob T

JohnBuckner

My Feedback: (1)

If it is straight cord wings then it is still easy to estimate a good starting point for the CG which for most airplanes I prefer 25% MAC of the collective cord. Collective is a a key word and an important technique for doing the estimate for biplanes.

Folks are bringing me airplanes at the field all the time for a quick estimates and its not that hard to estimate a biplane/triplane or more. This is not some computer program but a simple method that works well for me and you can make your own decision, I,ve even received unpleasant Pm's in the past for suggesting something that was not a computer program so if your interested I will elaborate further after I return from the field this morning.

What I believe Gray Beard was getting at is the name of the airplane is not really important but what is will be the configuration. So here are the questions:

Is one or both of the wings swept?

Are the wings staggered, if so how much and is it positive or negative stagger.?

Is the wing straight cord?

Or are one or both wings tapered?


A shorter lower wing is usually not a problem when using the 'collective or projected Planform' estimate. Actually biplanes are simple but what I have had the most trouble with are canards and in one case a straight cord flying wing "flying plank".

John

rt3232

My Feedback: (6)

Ok guys

The lower wing is a stright wing 11" cord, the upper wing ia also 11 " cord, but it is tapered anth aft edge of the upprt wing is 3 3/4 aft of the bolt as shown in thw rear cabain

I think a pic uploaded

Cheers Bob T

JohnBuckner

My Feedback: (1)

OK RT the technique I spoke of would work well on your lovely Pitts S-1. What you need to do is assemble the airplane with both wings and with the larger airplanes such as this one its easiest to work with someone else. The idea is to stand the airplane on its nose with you helper holding it near vertical.

Now you step back around fifteen feet or so. The idea is to get your eyeline perpendicular to the airplane and looking as if you were above the airplane and the wing outline you will now see is the projected planform as well as the collective outline of both wings. from this perspective it will become easy to visualize just where 25% or the quarter cord is by a visual estimate. With just a bit of practice you will be amazed how easy it is to imagine where that point is and what I recommend is to practice this technique on any of your other airplanes the easy ones with straight cord and the taper wings as well as swept.

Is this precise aero engineering - Certainly not but is a method you can practice and actually amaze yourself with a little effort.
Now visualizing that one quarter point (25% MAC) is of course not the perfect CG perhaps for every airplane but for the majority of types we fly it is a very practicle and safe starting point for much of what we fly. Likely you pitts will ultimately be best at around 28% but always start a little forward than aft and you can adjust CG to how the airplane flys from your safe starting point,

Just yesterday one of the fellows presented me with I guess what would be considered a sixty size and it was an unknown bipane he had refurbished it only took minutes to step back and pick out that quarter cord. After taping on an eight inch cresent wrench to the top of the cowl to adjust to that point the airplane flew quite solid and graceful with no suprises.

I know this is no fancy computer project but it has worked very well over the years as a quick and satisfactory method for all those 'mystery airplanes' and its well worth learning as one of those useful tools.

John

rt3232

My Feedback: (6)

Hi John

One correction both wings are constant core the upper wing is swept back at about 10 deg's. with both wings on and using a square on the thrust /center line fwd to aft the come bind cord is right at 18 inches at the fuse, But when looking at it as you have discribed the 25 % point seams to be farther back.

Will try to post more pic,s this afternoon but today is honneydue day I am sure you know what I meanbut will post as sone as I can.
well it it off to work for now.

Cheers Bob T
AMA13377

JohnBuckner

My Feedback: (1)

Bob I believe the reason the quarter cord seems more to the rear when you look at it as I described is because you are looking at the collective outline of both wings and not just one wing that we are so used to and for these purposes the collective outline is what we need to base our estimates on.

Now with your straight bottom wing and with that swept top wing of 10 degrees that means the trailing edge of your collective view is going to be straight and the leading edge is going to have just a bit of a bump in the middle. If that leading edge were you to imagine was straight from the tips its very easy to estimate the quarter cord using that view from afar as I described (practice, practice) and then just allow the point just a smidge further forward because that area forward in that 'bump'.

Agine practice, on you own airplanes even better your buddies airplanes That are know good flying airplanes. Stand 'em on their noses get way back and note your estimate of what you think 25% is. Then just check the airplanes in your fingers and see where they are actually flying them at. Yes some will be just a bit further aft than the quarter cord and that's fine but what we are looking for is a known safe place to start with. Of course fine tuning later can be move aft a bit but on that pitts you really want that 25%.

John

rt3232

My Feedback: (6)

Well quess what, I have been moving along with my head in you know ware. Gota have the muffler befor I can balance this bird.

We have a bit of a sound problem at our field (cranky old geeezer 800 ft from us) So I am having a custom muffler built from a mockup with some extra sound suppression, ie inlarged can and stainless steel pot scrubber material stuffed in side (not packed) from some research I was doing last winter T/T a fellow that make special dirt bike and go cart mufflers and that is what he is duing any way he tells me it takes the sharp ness out. so I will set this one on a rack till the muffler get hear..

Time to be flying any way

Cheers Bob T

JohnBuckner

My Feedback: (1)

Yup, post up a picture when its done.

John