Wings-RCU

My Feedback: (2)

Jerry, I'm with v-snap on this one. A more forward CG does not help stop snapping tendencies, In fact, it will make it worse! Move the CG aft if it's not ballooning on landing or climbing inverted. Does it always snap to one side (to the right)? If so, it may be an engine thrust or Yaw (Rudder) problem.

Red B.

If your aircraft doesn't suffer from any unwanted wing twist, i.e. wash-in, the only thing you can do to reduce the problem is to remove weight.
A Fuji 32 (is that the engine?) weighs in at 4 lbs incl. mount and muffler. That is a lot considering the aircraft is 1.20 size.

/Red B.

Jerry_Hailey

As good as this thing was flying the past two days, I don't think I want to change a thing. At least for a while. I think most of the problem was the uneven elevator. The left elevator had more travel that the right, plus the torque of the Fuji. It was a hand full to fly. Now the thing can turn sharp, loop, tumble, Hover ( plane can, Pilot needs practice). I had put this engine on this plane just to break the engine in for a GP 81' Super Chipmunk. But now I am re-thinking the plans, This Cap is FUN.

Stickbuilder

This may sound like a dumb observation, but I always thought that aerobatic airplanes were supposed to be, "
snappy" ... could be a dumb observation though
[8D]

Dr1Driver

Aerobatic planes ae supposed to be highly maneuverable. They are not supposed to have bad habits which may crash the plane is normal flying situations. This plane has a reputation for snapping on "normal" approaches, and more power than usual is needed. The nose must be kept down and the airspeed must be kept up.

Dr.1

BusterBust

My Feedback: (3)

I have a cap 232 as well. when i first got it (brand new)...it was SUPER tail heavy, i had ST 1.4 to put on it, which by specs should have been reasonable. I had to use metal plates attached to the side of the firewall box to get it to balance right. It was big time twitchy. I had to dumb down the throws on the elevator allot. Then Problem 2, the 1.4 engine was barely hauling the plane around, in the turns the plane looked like it wanted to slide right out of the sky like a dinner plate sliding off a tipping over table. After tinkering with the engine and the plane and watching it being about as graceful in the sky as a hippo doing ballet.....i went out and got a gas powered 52cc engine for the plane, now the cap has no problems in the sky and it only seems to be a little bit over powered, it still does snap when i pull back to hard on the elevator...but thats where discipline comes into plane on the power and elevator issue.

Al Lewis

I have the World Models CAP 232 46R and flew it slightly nose heavy. Really had to try hard to make it snap. I balanced it perfectly (added weight to the tail) and it snapped on every maneuver. Took off the weight and fly it slightly nose heavy with no snapping.

Wings-RCU

My Feedback: (2)

Lou, that is very unusual. Something else might have been going on. Usually snapping is reduced by moving the CG aft. It will make the elevators seem a little more sensitive forcing you to reduce the elevator throw, but true snapping should have been reduced. A more nose heavy plane will need to fly with a higher angle of attack - hence, it will fly slower, will seem more stable (because the elevator trim is fighting harder to maintain level flight) and will snap sooner at the same rate of elevation change. At least in the full scale world that is the way it works.

RTK

My Feedback: (1)

Wings-RCU---That is what they taught me eons ago, all about AOA, or something else I have forgotten.

sillyness

My Feedback: (25)

Get rid of that Fuji... even if you solve the snap problem. An OS 1.60FX would be a good motor. You will have an entirely new spritely and fun airplane on your hands. That Fuji is just like strapping a cinder block to the nose.

rmh

IF-- two models weigh exactly same weight and are identical -except for balance -which will snap the most easily?
The one with the most aft cg -not the otherway around.
The stabilizer/elevator can more easily produce more AOA on the wing - the further aft the CG is located .
This is where the trouble begins
Irrespective of the CG OR the weight - the AOA of the wing determines the snap ( wing is stalled).
also - if two are identical in size and shape and cg but are are different in weight --- they will both stall at same AOA
different speeds - but same AOA
Most scale 232 CAPS work well at 25-30% CG- best 3D crap at 30% and still very stable
looky here
The bottom line - in about 99.9% of snappin Caps - the problem is the same - toooooo friggen heavy and usually underpowered
The heavier it is - the faster it must fly - you cant fool mother nature .
many kit CAPS I have seen are simply bricks - a few new ARFS are quite good and will fly very easily but under 1.20 size I have not seen light ones

Wings-RCU

My Feedback: (2)

Dick, I think we are talking about different things. What I am trying to say is that you will be able to pull a tighter loop with less elevator input with a further aft CG before snapping. If you think it is snapping easier, it's because you have effectively increased your elevator authority.

You are right; the AOA determines when the wing stalls irrespective of CG or weight.
The difference is; the nose heavy plane, like an overweight plane, must fly at a higher AOA just to maintain level flight. (Reducing AOA is the reason we move the weight back. It saves fuel and makes a plane fly faster) Pull a 30 foot loop and the nose heavy plane -like the over weight plane- will snap before the further aft - or lighter- plane. (My point) A further aft CG makes it easer to get a higher AOA with the same elevator input causing a snap. I think that is what you are talking about.

So:
If you want to pull tighter loops before snapping, move your weight back.
If you want more elevator travel before snapping, move your weight forward.

sillyness

My Feedback: (25)

The elevator does not fly the plane through the loop... the wing does. All the elevator does in change to AOA of the wing. With an aft CG it will take less elevator because it's easier for it to change the AOA of the wing.


True. Due to up elevator trim (downforce on the tail) required to compensate for the weight in the nose.

rmh

consider this:
the weight and speed determine the wing AOA for level flight on conventional model such as this

not the cg

IF the plane weighs the same - the angle of attack on the wing required for any given speed is the same- irrespective of cg.

example : take a given plane and add a movable weight to shift cg at will
fly it and move the weight around -if it weighs 100 lbs the wing still must produce exactly 100 lbs lift -to hold a steady state level attitude .
However --the trim drag increases -the plane slows
This is the change that occurs from shifting CG forward
the power applied must increase as the DRAG increases -to hold same airspeed giving the 100 lbs lift but trim drag required to hold wing at necessary AOA-- adds total drag --the wing AOA remains the same IF the speed remains the same
restated
more elevator deflection (more trim AND drag) is required as cg moves forward
this makes for better stability AND recovery is easier as adding elevator makes far less pitch change for drag produced .


will you buy that?

sillyness

My Feedback: (25)

For models, mostly... it would take more research... there are definitely some effects, but I don't know if they are noticable enough with the limited CG ranges we play with.

For full scale, no. I see CGs that can vary between 15%-30% of MAC. There are very noticeable difference in the way the aircraft handles and the fuel economy realized... we always try to load the aircraft with a middle to aft CG to get better fuel economy (higher speed for same throttle setting). The reason we get the better fuel economy is simple... a nose heavy airplane want to tip forward (nose down moment). The tail must compensate for this by pushing down in the back (nose up moment). To the wing, both of these canceling moments feel like extra weight (you can figure how much by figuring the nose down tipping moment caused by weight in front of the aerodynamic center, set it all equal to X times the distance from the tail to the aerodynamic center, and solve for X) So, because of the extra "induced weight" on the wing, the AOA must increase, increasing induced drag (total drag = induced drag (or the drag created by the production of lift) + parasitic (or profile) drag). The shorter coupled the plane, the greater the penalty. The longer coupled the plane, the further forward the CG can be.

The stab drag changes due to trim changes do exist, but they only work in conjunction with AOA changes... after all... all an elevator is is an AOA machine!!!! It does nothing else but control AOA!!! Trim drag is greatest at lower speeds, which also happens to be where induced drag is greatest... like I said... AOA machine. Also... a properly designed airplane will have nil trim at cruise speeds with a CG in the ideal area of the CG envelope (stab incidence adjusted for this)... so trim parasitic drag should increase with any CG changes aft OR forward of the ideal CG. Even with zero trim, a down force is still be generated.

So... to sum up, this is the statement I disagree with:

For any conventional airplane to be longitudinally stable the wing's lift must actually be slightly more than the weight of the airplane. The further forward the CG, the more lift it must produce. Planes with T-Tails and adjustable stabs are designed as such to allow very nose heavy situations (wide CG envelopes) to allow a variety of load plans and odd sized cargo to be carried. When they do carry these loads, they pay a penalty in the form of induced drag (fuel economy) due to the increased wing lift and stab lift (a stab is an inverted wing... actually pushing down) required.

Wings-RCU

My Feedback: (2)

Sillyness, very well put! Thanks.

sillyness

My Feedback: (25)

Actually you may want to re-quote me... I made some corrections (bad math) and tried to clear it up a little.

Cheers

rmh

Nope guys -I won't -can't buy it
the why is right in your answers .
why?
If weight of the craft remains the same
the lift required remains the same
However
the power required changes
so
the nose heavy aircraft must use more power to get wing to the necessary AOA-which is the same -at the same speed and weight to produce the same lift .
The efficiency goes all to hell - but the AOA of the wing is not changed .
Power requirements really jump -
that is where the need for proper CG for best perfomance comes in
. I understand the "effective loading thing "
but that is just drag at work - if you spend the power to get back to the same speed at the same weight - the AOA will be back to the same place .

sillyness

My Feedback: (25)

These statements make are inaccurate:

Wrong for the reasons quoted above. It's simple newton stuff... if something pushes one way, something else must push the other way to stay in equilibrium. The tail pushes down, so something else must push up. That something is lift. Here's another good example... it's not exactly the same, but it's something you can see. Watch an RC heli hover from the rear... notice how it's leaning slightly to the right? That's cause the tail is pushing to the left in a hover to counter rotor torque. The problem is that a linear force (tail rotor push) is being used to provide the counter torque, so something must also counter this linear force to stay in equilibrium (keep it from drifting left). This something is provided by tipping the rotor disk to the right. The lift vector is now tipped slightly right... a sort of lateral thrust. This lateral thrust now counters the TR thrust.

Now view the same heli from above, nose to the left. Draw this on a piece of paper. The rotor torque is the same as the torque created by the CG being in front of the aerodynamic center. The stabs pushing down is the same as the TR pushing down (as you are viewing it). The extra lift required by the wing is the same as the heli's disk tipping up (really right, but in your current perspective it looks up) and providing a small thrust vector lateral to the disk's lift.

Wow... look at that... you not only got a lesson in aero, but a free lesson in relativity also!!!

It takes more power because drag is higher (only possible explaination). This drag comes from where? A higher AOA.

Just wrong. Sorry.

This would be a great place for George Hicks to jump in. Dick, you and George both write for the same magazine. I can only assume you have a way to contact him. I'd be glad to eat my words after seeing a valid moment diagram that proves me wrong.

sillyness

My Feedback: (25)

Just so people know where I am coming from when I say a higher AOA increases induced drag.


A wing generates lift perpendicular to the chord line. Since planes fly at a positive angle of attack, the lift vector is tipped aft slightly. You can spilt this lift vector into two components... one that opposes gravity, and one that is 90 degrees, effectively opposing the direction of flight. The component that opposes gravity is felt as usable lift. The component that opposed the direction of flight is felt as drag. So... immediately the wing is generating more lift than weight... it is generating the vector sum of vertical lift and induced drag!

Now, move the CG forward, and the tail must push down harder. The wing must oppose this. At the same speed, the only way it can do it is to increase AOA (or use flaps, slats, etc...). When the AOA increases to get the extra lift, the wing's lift vector tips further aft also. When you split it into the vertical and horizontal vectors we discussed in the previous paragraph, you'll find the horizontal (drag)vector has increased disproportionately to the increase in lift required to oppose the increased elevator down force (because the vector wing's lift vector tipped aft). This is why we really notice the need for extra power with a forward CG.

rmh

OK
to reduce your comments to one point:
increased tailplane downforce needed to produce required AOA on the wing --is ADDED to the wing lift and thrust requirement.
granted
but increasing thrust (and holding original speed) solves this.
As a silly example :
a plane is flying along at 100 mph
and total lift is 1000 lbs exerted -holding the 1000lb plane at an even level 100 mph
buy that?
now we sliiide a weight forward - and at the same time applying power to keep lift at 1000 lbs exerted.
the thrust needed has increased -but the speed and AOA remain constant .
Y/N?

Wings-RCU

My Feedback: (2)

Yes, if you move the CG back to the Center of lift, the total amount of lift the wing needs to produce is less, so the AOA is also reduced. So.. if the AOA is higher in a nose heavy condition, it is also closer to the critical angle of attack (Stall). This is the reason I think moving the CG forward is not going to help and may be detrimental to a snapping plane.

rmh

Why do you not include the fact that the plane will speed up if the AOA is left alone?
reducing thrust will maintain the speed and we are back to same speed and AOA for same lift.- just less need for thrust.
I see three things as interactive - the speed - the AOA and the thrust required to hold the speed .
change the trim drag required ( by changing CG) and correct with thrust change - the AOA and speed can stay the same


what if we deployed a chute - greatly increasing drag

Would a higher AOA be needed?
not if the thrust could be increased to hold airspeed .
Lousy example?

sillyness

My Feedback: (25)

Still not correct.

All of this assumes constant speed, constant weight, changing CG (and power as required... or we can forget power... make it a glider. A nose heavy glider falls faster).

Sum all the force vectors... the total must = zero to be in a steady non-accelerating state (i.e. non changing climb or descent profile).

What is pushing down? Aicraft weight (W) + stab downforce (SD)... correct?

What is pushing up? Lift (L), correct?

So, L - (W + SD) = 0... correct?

So, if CG moves forward, SD must increase, correct?

So, looking at the equation... if SD goes up, what else must change? The only thing that can change is lift.

L = 1/2*Cl*Roe*A*V^2 (Cl = coefficient of lift, Roe = air density, A = Wing Area, V = velocity)

So, if V stays the same, A stays the same, Roe stays the same, and 1/2 is an equation constant, the only thing that can change is Cl.

Cl depends solely on airfoil shape and AOA. Airfoil shape isn't changing, so AOA must.

The AOA increase, in turn, increases induced frag as explained in post #45. Thus, the increased required thrust.

There is also a SLIGHT amount of extra drag from the tail itself, but in a well designed airplane, not much.

I don't know how to be clearer about this. The math and Newton can't be argued with. That was a linear explaination... in reality it requires a vector explaination, but the angles are fairly at cruise and the linear explaination is fairly accurate.

This is all fairly complicated and difficult to explain in this forum. I really wish Mr Hicks was around. After reading his articles I'm sure he'd be better at explaining this.

Here's one for you all to mull over... see who can be the first one to a draw a picture explaining this... a wing is actually producing LESS lift in a steady state climb than it does in level flight!!! . You'll have fun with that one. I'm out

sillyness

My Feedback: (25)

Jerry... I'm sorry this thread has taken such a turn, but it's an interesting discussion.

In your situation, first I'd get rid of weight. Second, I'd experiment with CG forward AND aft... in reality there is a CG sweet spot... no right answer as to which direction you need to go from where you are. I had a WH 28% Extra some time ago (before the really big bug bit me)... too nose heavy OR too tail heavy and it would get snappy. In the middle... just right Just play.