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Move the wing for better CG?

cubdriver2 said:
My Pa11 would land and takeoff shorter and slower from the back seat

Glenn
Click to expand...

The further you go the better.. until its too far however we normally fly in front of a Cub, with a big engine it's not balanced
even before flaps add to the equation.

Anyone know the math or what the result of moving the wing will bring? If we are moving the whole fuselage
back relative the forward moving center of lift it doesn't seem like it would take much to get to the sweet spot again.
Yes the wings movement will add to the forward CG but would think its a limited effect with a short arm.

What if the center of lift approaches the CG point? Normally the center of lift remains behind the CG.. what is that result
as it gets closer?

I know there are some engineers on board :lol:

Please PM if you don't like to broadcast.
fknapp_at_alarmspro (dot) com

Thank you!
 
What if the center of lift approaches the CG point? Normally the center of lift remains behind the CG.. what is that result
as it gets closer?
Click to expand...
The tail doesn't have to push down as much in order to keep the nose up. Therefore wing lift is not wasted counteracting the down-push of the tail. Not as much drag from either wing or tail. And of course, pitch stability is reduced. Again, I'm not an aero engr, so I realize this is way-basic. But it's a start?
 
Instead of moving the wing, Why not just move the seats and controls aft? After all we are just talking about a CG adjustment.
 
Little_Cub said:
The further you go the better.. until its too far however we normally fly in front of a Cub, with a big engine it's not balanced
even before flaps add to the equation.

Anyone know the math or what the result of moving the wing will bring?

Thank you!
Click to expand...

The way to go about the math.
Since you have handled as well as built all the components of your craft.
With the wings off, do a weight & balance of your fuselage. Do this with you seated in the plane. You may need tail weight without the wings, maybe not but get the numbers.

You know what your wings weigh but you need to know where their balance point is fore-aft.
Your wings also provide where your center of lift is.

I expect for the math, use the LE of the wing as your datum.
Now run your W&B but on paper, move the full mass of your fuselage fore-aft till you feel it is where in the lift range you want the mass.

I expect the wing will want to be further forward, the numbers will tell you.

Also run numbers with the engine moved forward from where it currently is should you want to regain room up front. Might consider just moving the engine on it's mount for part of this allowing you to fine tune after the build such that if the plane is not comfortable it can be tuned with different engine mounts rather than adding tail weight.

Not sure if you have done this yet, I am sure you are not running any washout but you might consider having adjustment room in your struts to put a degree or so of positive twist in the wing. :wink:
 
Little_Cub said:
The further you go the better.. until its too far however we normally fly in front of a Cub, with a big engine it's not balanced
even before flaps add to the equation...
Click to expand...

Now that you brought up the big engine...
I thought your early performances at Valdez were awesome--
a lightweight Cub with a stroked 85 spanking the hotrods.
The Jerry Burr lightweight thing carried out to the nth degree.
I was kinda surprised (and I will admit, kinda disappointed) to see you show up later with a 320 bolted on.

Now you're talking about what sounds like a real major mod (moving the wing attach) to alter the CG,
due to the heavier Lycoming engine.
Have you considered just going back to the Continental?
Lighter & simpler is often (if not usually) better-
and that why originally I found your Little Cub so intriguing.

This kinda makes me think of the Cessna 180--
over the years it got heavier & heavier as they "improved" it,
but some of us still prefer the lighter early models.
 
hotrod180 said:
Now that you brought up the big engine...
I thought your early performances at Valdez were awesome--
a lightweight Cub with a stroked 85 spanking the hotrods.
The Jerry Burr lightweight thing carried out to the nth degree.
I was kinda surprised (and I will admit, kinda disappointed) to see you show up later with a 320 bolted on.

Now you're talking about what sounds like a real major mod (moving the wing attach) to alter the CG,
due to the heavier Lycoming engine.
Have you considered just going back to the Continental?
Lighter & simpler is often (if not usually) better-
and that why originally I found your Little Cub so intriguing.

This kinda makes me think of the Cessna 180--
over the years it got heavier & heavier as they "improved" it,
but some of us still prefer the lighter early models.
Click to expand...

Agree with the smaller-lighter C85 but it just didn't pull hard enough for the eventual goal.. F_U_N factor was over the top!
We don't want to change Lil Cub we are talking about a new build.. HP is good and the nimble feel is still available..
just gotta get to the sweet spot without messing too much else up.. like leg room or visibility. With Lil Cub we cut the firewall back 5"
with a little weight in the tail it's in the fun zone again.

We would like to build that perfect machine.. plenty HP, leg room and comfortable, lotsa visibility, fun flyer.. too many times we sacrifice something.

Speaking of Jerry.. he was at Oshkosh this year and (flying his certified J-3) beat all the Carbons, HP Super Cubs and all except Steve with the hot rod Just Aircraft in Experimental class! A lot to be said for consistency.. one good stick.
 
Isn't part of the reason for extending the SQ-12 and Rev 2 to re-gain elevator and trim authority to take advantage of the high lift/high AOA wing mods? Why is extending the tail aft a bad thing? It provides a good benefit-weight ratio. Judging by the SQ-12 videos and the pirep gross loads it carried it was well balanced. The Rev 3 wing may escalate things to another level, too. Interesting stuff.
 
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what about, if talking of moving wing forward, instead add another layer of slats out front of existing ones??? do they effect the wing CL???

since they came here all nicely stacked together in the crate....
 
I have a simpler solution. Just sell the Cub and the house and get a helicopter. And then get a second job to cover the maintenance costs.
 
stewartb said:
Isn't part of the reason for extending the SQ-12 and Rev 2 to re-gain elevator and trim authority to take advantage of the high lift/high AOA wing mods? Why is extending the tail aft a bad thing? It provides a good benefit-weight ratio. Judging by the SQ-12 videos and the pirep gross loads it carried it was well balanced. The Rev 3 wing may escalate things to another level, too. Interesting stuff.
Click to expand...

I believe the new SQ is trimmed however adding to the arm (tail) or adding weight makes the control forces heavier.. haven’t spoken with Wayne.. still dreaming! In a perfect world it seems best (for everyday fun machine) to simply balance what works and limit the mass. Your right though.. wing candy adds another dimention to chase.

I know how these dreams usually end.. ‘pop’!

Enjoying the process however [emoji12]




Sent from my iPhone using Tapatalk
 
Little_Cub said:
The further you go the better.. until its too far however we normally fly in front of a Cub, with a big engine it's not balanced
even before flaps add to the equation.

Anyone know the math or what the result of moving the wing will bring? If we are moving the whole fuselage
back relative the forward moving center of lift it doesn't seem like it would take much to get to the sweet spot again.
Yes the wings movement will add to the forward CG but would think its a limited effect with a short arm.

What if the center of lift approaches the CG point? Normally the center of lift remains behind the CG.. what is that result
as it gets closer?

I know there are some engineers on board :lol:

Please PM if you don't like to broadcast.
fknapp_at_alarmspro (dot) com

Thank you!
Click to expand...

Frank,

Here's my spin on it.

With a conventional plane the centre of gravity (C of G) is in front of the centre of lift (C of L) and a downforce is maintained with the tailplane to keep the plane in equilibrium. See attached pic.

You need to maintain a margin between C of L and C of G. Once the two coincide you no longer need down force on the tail and the aircraft becomes unstable. Even if the margin is too small the aircraft may become difficult to control. The RV-6 is a good example of the latter, they are easy to get into an aft C of G situation and a friend of mine told me that when he was doing the flight testing for his, he was pushing forward on the stick and the nose was still coming up. This is exasperated with low wing aeroplanes due to the geometric relationship between the wing and the C of G as the angle of attack increases. The opposite extreme that we can all relate to is flying an empty C206 where they fly like dogs and you can need power in the flare.

Putting the flaps down actually helps. Cubs pitch nose up with flap which is the same as having a more aft C of G isn't it? This is to do with the centre of lift moving forward once the flaps go down. (Yeah, I know, lots of new drag above the drag line too.) The C of L also moves forward as the angle of attack increases, so as you slow up they become less stable and the lower stick force isn't just to do with less airflow over the surfaces.

There are a couple of other factors in play here too. One is pitching moment and the other is downwash. I have a compilation of articles written by Barnaby Wainfan (sp?) that might've been in kit planes magazine and he explains pitching moment at least. His stuff is really easy to read, if you want to do something like this, I recommend you find a copy.

I did a test flying programme for a Murphy Rebel and with the flaps down, there was not sufficient elevator authority available to flare it. Stalling it one time it even bunted, indicating the tailplane and elevators gave up flying. The flaps cover about half the span and are quite large chord and go right up to the fuselage and I put the elevator authority issue down to the probably significant downwash coming off the wing. As I understand it from reading, the Bearhawk has a gap between the wing and the flap to maintain some proper airflow over the tailplane when the flaps are down. Cub guys might not be doing the right thing closing up the gap between the flap and the fuse………...

The thing that would put me off the most in moving the wing forward on a Cub type build, is that you will be no longer seated in line with the leading edge of the wing, significantly affecting visibility in the turn. It will be like a 152. Try and find some air to air pictures of the Rans S-7 and you will see the Captain is seated even further forward than a Cub with fantastic visibility.

Just this week I had a disagreement with a colleague who claimed the USA35B airfoil was just a Clark Y in drag (pun intended) and I disagreed saying the USA35B coefficient of lift (Cl) is much better than a Clark Y. At home he laid the data for various airfoils over each other and had to eat crow about the USA35B. But one of the airfoils he put on the graph was a Liebeck airfoil that he is keen on. It has a much better Cl than the USA35B and more importantly has a much lesser pitching moment which means your elevators would be more effective, which is really partly what you are after. More than one way of skinning the cat!

I am an amateur aerodynamicist, so I offer my apologies in advance if I have not got some things quite right or my explanations are difficult to follow.

Cheers,
Andrew.
 

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Frank,
So, now that the event has been run, what did you build?
I do not even know if you had the new plane there. I see 33 combined this year, very nice.
Presuming you did have the new plane, how far did you move the wing to achieve the balance in the new plane?
 
CharlieN said:
Frank,
So, now that the event has been run, what did you build?
I do not even know if you had the new plane there. I see 33 combined this year, very nice.
Presuming you did have the new plane, how far did you move the wing to achieve the balance in the new plane?
Click to expand...

We were flying Lil Cub.. same as last year. We are still teasing all the possible changes.. that seems to be
almost more fun than flyin' it ;-) you get it done then you always consider 'the next'!

With the new flaps Lil Cub is doing about everything we can ask.. it balances very well with almost no stick pressure.
For the next one we want to attain 100+ and have low end, it will be more of a daily traveler.
 
Nice, I was suspecting there was not enough time to engineer and build the new plane in time.
 
fobjob said:
Reduce the wing angle of incidence, and you reduce the pitching moment, resulting in a nose-up condition. At full forward CG, the stab should be slightly lower in it's slot, and the elevator deflected slightly up. When my wing was mounted at zero degrees incidence, (and washout correct) the pitch trim was halfway to the full nose-down position, and the elevator down at a noticeable angle. Other than the vicious spin (seven turns minimum recovery) that resulted from a stall, it still flew pretty good!

*only tried to kill me once.....
Click to expand...


Fobjob The zero incidence was a contributor to the spin problems? Because of........? Thank you, for your post. Jonny O
 
To have positive pitch axis stability, the tail has to maintain a down force, so that when your speed reduces, the negative lift on the tail reduces, and the nose drops....with the tail producing positive lift even at forward cg, and worse as useful load increases, stalling the bird results in the tail dropping, which produces an enhanced stall, which instantly becomes a spin. The problem was frustrated by numerous people telling me that the root wing incidence should be zero degrees.(wrong!) So, I bought a set of micro vg’s to keep me alive until I could believe my lying eyes....they reduced the seven turn spin recovery into a one-third turn recovery. A few years later, it got squeezed through the jig in Belgrade, Mt. , and the proper angle returned....who would have thought that someone would weld the dang wing onto the fuselage at the wrong angle? No wonder they jumped at my low-ball offer! This bird flew a lot of power line patrol like that, too! Nothing in the log books, either..... ( the power company people that supervised the line patrol operations would have had a hemmorage , had they known that.....)


*good thing for them I’m not vindictive.....
 
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Well, just to enlarge on this a bit, when the stall arrived, it did so with zero warning, and the spin developed instantly in the direction that the airplane was most comfortable with at the moment....when I tried to “walk” it back to neutral, it would overshoot each time and diverge, more and more violently. About as unstable a situation as you could imagine, yet there was enough tail area and length of fuse between cg and tail, that the plane behaved pretty normally, and did not try to porpose, even at aft cg.
A good reason to immediately stall/spin test any new airplane you acquire.....
 
Mauleguy said:
I would say before you acquire!
Click to expand...
They only let me “fly” it after I gave them a deposit, and then only with one of their guys in the back seat. I wonder if their pilots knew they would die if they stalled it at low altitude. By their 65mph wheel landing, I wondered how much skill they actually tried to acquire on that aircraft. I learned many years after that they wrecked it doing stall turns at just above the ground between the runways on a Sunday, (jackassing around) when the tower was not manned, then hauled it into a hanger and “repaired” it free of any FAA supervision.....In spite of all that, I was still glad that I had acquired a Cub, so all I had to do was stay alive until I figured out what was wrong and how to fix it......
 
Ummm. By the way of his thinking even putting on the first notch of flaps would turn a plane into a spinning death trap. That was not the problem(wing incidence angle). The real problem was addressed in reassembly after rebuild. Most likely simplily getting the ball to agree with what the plane was actually doing.


Sent from my iPhone using SuperCub.Org
 
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