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Old Sep 07, 2011, 10:05 PM
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How will moving my motor forward for a better cg affect my plane?

I've installed a lighter motor in my Parkzone Extra 300 and have been using a about an ounce of washers to get the CG where I wanted. I spaced it out a little further, about 3/8", to try to get it to balance without washers. How will or might making the plane longer by spacing the motor forward affect it's flying? I fly general aerobatics and try some 3d moves with it although it doesn't cooperate very well usually. The plane is already pretty pitch sensitve. Thanks
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Old Sep 07, 2011, 10:16 PM
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Generally, flying nose heavy is safer in initial flights because you can keep control of the plane. However, it will need a higher speed for flight, will be unstable and hard to control in slower flight and will need a higher speed/good landing technique.

Planes like the PZ Extra 300 are designed for pattern or acrobatic flight and moving the CoG back (or forward) can have a dramatic effect. More expereinced pilots will generally have the CoG back because the plane is better at lower speeds and will tip into acrobatics faster. You can do a good mix of fast and slow acrobatic manouveres. The plane might be little easier to land becasue you can get it into a higher alpha position and the conrol surfaces will remain effective with prop wash at lower speeds.

Having said this, start with the CoG at the recommended point (eg battery pushed as far forward as it will go (straps over the lead wires forthe stock battery)). I doubt that a new motor would need any weight addition. As you probably know, the manual instruction on placement of the battery is wrong and pushes the CoG too far back - very bad if you are not used to this style of plane. You don't needmore than about 1cm deflection either way for the elevator, this will help reduce snap rolls.
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Old Sep 07, 2011, 11:00 PM
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Moving the motor won't have any noticeable effect whatsoever, apart from the change it makes to your balance point. The change to the balance point is extremely significant.

Moving the motor forward to make up for a lighter-than-normal motor is a pretty standard practice. Just make sure you check the balance before you fly.
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Old Sep 08, 2011, 12:46 AM
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Theoretically having longer moment arm on the nose will mean that the plane has increased moment of inertia in pitch and yaw so becomes a little more sluggish to pitch and yaw input and also carries on rotating a little longer after the controls are centralised. This is because the moment of inertia is proportional to moment arm squared, so increasing moment arm increases moment of inertia even though the weight of the motor/washers is decreased and the CG is the same.

Having said all of this, relatively small changes are unlikely to be noticeable.

Steve
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Old Sep 08, 2011, 06:40 AM
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Originally Posted by JetPlaneFlyer View Post
Theoretically having longer moment arm on the nose will mean that the plane has increased moment of inertia in pitch and yaw so becomes a little more sluggish to pitch and yaw input and also carries on rotating a little longer after the controls are centralised. This is because the moment of inertia is proportional to moment arm squared, so increasing moment arm increases moment of inertia even though the weight of the motor/washers is decreased and the CG is the same.

Having said all of this, relatively small changes are unlikely to be noticeable.

Steve
+1

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Old Sep 08, 2011, 08:14 AM
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+1 again on the inertial moment thing. As far as CG is concerned, you have to deal with that however you can, and moving the motor out is certainly a better option than adding dead weight. But keeping the plane's mass as near as possible to the center of mass is also a good goal.

I learned this racing small sailboats. You want to keep the crew weight centered, in order to minimize rocking and rolling of the boat from wave effects. It's also the principle of mid-engine sports cars -- less likely to skid or slip in tight turns.

I love a model plane where the battery sits right on top of the CG.
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Old Sep 10, 2011, 04:05 AM
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If the plane is "tail heavy", the plane may be unflyable without power. (stalls)
(A lot of people died over the years because of a tail heavy condition combined with high gross weight in full size aircraft.)

If "nose heavy", the plane will dive without power and or control deflection. This assumes that the trim is not changed. In a full size light plane, with power at flight idle or landing, the trim is usually changed to set up a reasonable rate of descent. Take off trim and cruise trim are also different settings. Model planes "should" be trimmed so that they will glide with a reasonable rate of descent with no power.

A model's CG (within the allowable limits) should usually be set so that the elevator has a slight to no deflection in a glide. Some prefer the elevator deflection to be slightly up, rather than down or none.
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Old Sep 10, 2011, 08:26 AM
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Originally Posted by chuck75 View Post
If the plane is "tail heavy", the plane may be unflyable without power. (stalls)
A tail heavy plane is not unflyable. It is just difficult to fly because it has negative pitch stability. Even if the airplane is tail heavy the elevator still works. With no power the plane won't stall if the pilot applies down elevator to keep the airspeed up.

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Originally Posted by chuck75 View Post
In a full size light plane, with power at flight idle or landing, the trim is usually changed to set up a reasonable rate of descent.
The pilot sets the trim for the desired airspeed, usually about 1.5 times stall speed. The pilot contros rate of descent with the throttle.

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Originally Posted by chuck75 View Post
Model planes "should" be trimmed so that they will glide with a reasonable rate of descent with no power.
No, they should be trimmed for straight and level flight at cruise power. If you trim them for a reasonable rate of descent (whatever that is) with no power then you will constantly be holding up elevator during the normal flight routine.

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Originally Posted by chuck75 View Post
A model's CG (within the allowable limits) should usually be set so that the elevator has a slight to no deflection in a glide. Some prefer the elevator deflection to be slightly up, rather than down or none.
A model's CG position has nothing to do with the glide. The fore and aft CG position determines the model's pitch stability and and the model's sensitivity to pitch commands via the elevator. Those are the parameters used to set up the CG. Elevator deflection for the glide is just like the full size aircraft, it is set for an airspeed just above stall speed. Its actual physical relationship to the stab is inconsequential, it's the aerodynamic results that count.

Larry
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Old Sep 10, 2011, 11:22 AM
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Originally Posted by Lnagel View Post
A tail heavy plane is not unflyable. It is just difficult to fly because it has negative pitch stability. Even if the airplane is tail heavy the elevator still works. With no power the plane won't stall if the pilot applies down elevator to keep the airspeed up.
A tail heavy plane is not necessarily unflyable. It becomes unflyable if at takeoff speed, the elevator lacks the aerodynamic force to keep the nose level. The the plane is only crashable.

If this plane is obscenely overpowered, our preferred option, it may be possible to climb vertically after the unflyable plane takes off, accelerating to a speed to which the elevator now has enough force to keep the nose level. But until you can get to that airspeed the plane is not flying, so we will continue to call a plane in that tailheavy condition unflyable.

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Originally Posted by Lnagel View Post
The pilot sets the trim for the desired airspeed, usually about 1.5 times stall speed. The pilot contros rate of descent with the throttle.
I'll buy that for a dollar! Right on the money there.

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Originally Posted by Lnagel View Post
No, they should be trimmed for straight and level flight at cruise power. If you trim them for a reasonable rate of descent (whatever that is) with no power then you will constantly be holding up elevator during the normal flight routine.
Not quite. You are correct that a plane should be trimmed for a certain speed at level flight. You can do this for any flyable throttle setting.

Now strange things happen after you've set your trim. Let off on the throttle and your plane assumes a more nose down attitude and settles into a stable attitude with the same airspeed as your "cruise" setting!

Give the plane throttle and the nose comes up, the plane settles down into a climb at that same "cruise" airspeed. You would have to input down elevator to gain speed and maintain level flight, not up as you state above. I'm sure you meant down and understand this.

Cut the motor entirely and the nose drops further than the first example, but the plane stabilizes at the same airspeed as if you were at level cruise. In other words, no matter what your trim speed for straight and level flight you will pitch down in a power off glide.

Elevator determines speed. Throttle determines altitude. The interaction of the two determines pitch.


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Originally Posted by Lnagel View Post
A model's CG position has nothing to do with the glide. The fore and aft CG position determines the model's pitch stability and and the model's sensitivity to pitch commands via the elevator. Those are the parameters used to set up the CG. Elevator deflection for the glide is just like the full size aircraft, it is set for an airspeed just above stall speed. Its actual physical relationship to the stab is inconsequential, it's the aerodynamic results that count.

Larry
You're talking to a sailplane pilot here. CG position has EVERYTHING to do with the glide. You completely understand why because you explain it perfectly above.

Most people set up their sailplanes with the CG at the factory recommended location, on a Parkzone Radian it's 63mm aft of the leading edge of the wing. Most of the time this is too far forward for an efficient glide. But it makes for a very stable plane, and that's what the manufacturer is worried about.

Setting up for most efficient glide means knowing what speed your plane needs to fly for its highest lift to drag ratio. The faster you fly, the higher the drag. The further from zero degrees incidence your elevator is, the higher the drag. Each airfoil has its own preferred speed. But in general, flat bottomed and cambered airfoils like to fly just above stall speed for best l/d.

Now, if your CG is too far forward you will have to use up trim to force the nose up and slow the plane down to reduce drag. You're making drag to reduce it! And after you have the plane slowed down, the elevator may lose authority to hold the nose at the right attitude, the nose then drops because no force is keeping it up, the plane accelerates until the elevator regains authority forcing the nose back up to where the elevator again loses power and the plane is oscillating: porpoising. Note that in the glide porpoising is caused by the CG being too far forward!

Safety issue: when your plane porpoises because of bad CG position and you stall it close to the ground, depending on how nose heavy your plane is there is a tradeoff between the time required to resume control and the time to impact with the ground. Sometimes the ground wins this one. Nose heavy planes are not necessarily safer to fly.

The important thing is the with CG too far forward there are multiple reasons related to necessary elevator trim angle that increase drag and destroy your minimum sink rate.

Now with the CG in a better place, your plane no longer stalls in the way you are used to. In fact it may not give you much warning at all because the CG is much closer to the center of lift. Instead of precipitously dropping the nose, the plane will just mush through the air at stall speed if it is a spirally stable aircraft. Otherwise it will drop a wing and become a demon you can't control if your CG is too far back.

What you are looking for is a place where you are still comfortable to control it and at the same time does not cause the nose to drop when the speed falls. On the Radian, FOR ME, 70mm back from the leading edge of the wing was magical. The plane suddenly slowed magnificently, the sink rate fell in half and my Radian was just plastered to the clouds. The difference in the glide from moving the CG 7 mm was not just significant, it was astounding!

It left starting elevator trim in a very slightly down position. That can be because of decalage, the angle between the angles of incidence of wing and stabilizer. And it is why the famous "dive test" for setting CG has very little validity in an aircraft which does not have a full flying stabilizer.
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Old Sep 10, 2011, 12:26 PM
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Originally Posted by Rockin Robbins View Post
A tail heavy plane is not necessarily unflyable. It becomes unflyable if at takeoff speed, the elevator lacks the aerodynamic force to keep the nose level. The the plane is only crashable.
A rearward CG makes the elevator more sensitive, not less. The problem with a too far back CG is nothing to do with the elevator lacking aerodynamic force. What you get if you move the CG too far back is an unstable or 'divergent' condition. This means that the plane will constantly try to deviate from any heading it's on, that's what makes it hard/impossible to fly. This would be true even if you flew vertically up.. the plane would still be constantly trying to deviate from it's heading.

PS.. and the 'dive test' works just as well for a plane with normal elevators as it does for a plane with full flying stab... it's a test of static pitch stability and static pitch stability has nothing to do with how the elevator works.
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Old Sep 10, 2011, 02:54 PM
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Duplicate post redacted.....
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Old Sep 10, 2011, 02:55 PM
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Originally Posted by JetPlaneFlyer View Post
A rearward CG makes the elevator more sensitive, not less. The problem with a too far back CG is nothing to do with the elevator lacking aerodynamic force. What you get if you move the CG too far back is an unstable or 'divergent' condition. This means that the plane will constantly try to deviate from any heading it's on, that's what makes it hard/impossible to fly. This would be true even if you flew vertically up.. the plane would still be constantly trying to deviate from it's heading.

PS.. and the 'dive test' works just as well for a plane with normal elevators as it does for a plane with full flying stab... it's a test of static pitch stability and static pitch stability has nothing to do with how the elevator works.
You're not thinking straight here. It is the distance between the center of lift and the center of gravity that alters the mechanical advantage the elevator has, and hence its ability to develop enough force on the airframe to offset the imbalance caused by that distance.

When the CG is too far forward that seems easy to think about. The nose is too heavy. Unless you can generate enough downforce on the tail to overcome the downforce on the nose, the plane can't fly level.

The same is true when the CG is too far back and the CG is actually behind the CL. Then it is the tail that is too heavy and the elevator has to lift the tail in order to produce level flight. Just as I said, if the elevator is unable to lift the tail to a level condition, that plane cannot fly. There just isn't enough elevator authority. It is exactly the same situation as CG too far forward but somehow you understand one and not the other condition.

Yes, from the standpoint of trying to fly the plane it has negative stability and probably cannot be flown by humans without aid. However, sticking a good gyro on there will do wonders. The F-117 is negatively stable and can't be flown except by computer. The stick does not control the control surfaces, it tells the computer where you want the plane to go. The computer then performs whatever hocus-pocus it deems necessary to make the plane do that. But the negatively stable plane is flyable, just not by humans!

You're confusing stability with flyability. They are not synonymous.

Better talk with sailplane fliers about the dive test. It can lie big time. There are too many variables at play for a simple test like that to be 100% reliable. It assumes too much.
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Old Sep 10, 2011, 03:08 PM
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Originally Posted by Rockin Robbins View Post
You're not thinking straight here. It is the distance between the center of lift and the center of gravity that alters the mechanical advantage the elevator has, and hence its ability to develop enough force on the airframe to offset the imbalance caused by that distance.
Nope, my thinking is straight as an arrow on this. There is a much larger effect at play and that is stability. A stable plane always tries to fly at it's trimmed angle of attack. In order for the elevator to alter the angle of attack is has to overcome the planes natural stability. The more stable (nose heavy) the plane is the more force the elevator has to generate to deviate from the trimmed condition. This is why a nose heavy (very stable) plane is sluggish on elevator despite the longer moment arm to the CG. Surely you have experienced this if you have flown a nose heavy plane?

The reverse is also true. An unstable plane is always trying to diverge from straight and level flight. The elevator then hardly has to do any 'work' to nudge the plane in one direction or another. Once the elevator applied a small 'nudge, the instability take it from there. This is why tail heavy planes are very twitchy on the elevator and also why some jet fighters like the F-16 are designed to be tail heavy (unstable), that way they have much faster reaction to elevator control and can manoeuvre quicker.

There are plenty of aerodynamic text books that will confirm what I've said here. But probably no need to open a book, just try flying your plane in a nose heavy condition and you will see that the elevator is less effective. I would not recommend trying the tail heavy (unstable) test though.

Steve

PS. I agree that the dive test works best on gliders and aerodynamically 'clean' models. That's nothing to do with elevators and all moving tails, it's because it's a test of the sensitivity of the plane to changing airspeed and planes with a lot of drag don't accelerate much when you dive so don't respond as well to the test. The test still works, just not quite as well.
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Old Sep 10, 2011, 04:00 PM
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I flew the plane with the motor spaced out 3/8" as I mentioned in the original post and it flies the same. Still needed weight to get it to balance where I want but not quite as much. It looks like it would be impractical to space the motor out far enough to get it balance without the washers, unfortunately.
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Old Sep 10, 2011, 07:16 PM
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Originally Posted by JetPlaneFlyer View Post
Nope, my thinking is straight as an arrow on this. There is a much larger effect at play and that is stability.
You are still confusing stability with flyability. Although you can not fly such an unstable plane because it does what you say it does, with a gyro system you can fly it by inducing artificial stability as I outlined above. There are a lot of RC planes now flying that people cannot fly without use of the automated systems.

A plane with the CG back of the CL is still flyable but not by unaided humans. It suffers from the same elevator dampening effect as a nose heavy plane. The elevator must generate enough force to overcome the CG/CL lever moment. When the plane is moving too slow for that to happen the plane cannot be controlled.

The elevator is most effective, that is, needs the smallest control movement to produce a maximum pitch effect on the plane, when the CG and CL are coincident. At that point it takes any non-zero force to induce a pitch change and the plane is neutrally stable. This is still a humanly unflyable condition, although it is not negatively stable yet.

Maximum elevator effect happens when the CG and CL are the same point. Moving the CG either direction progressively and proportionally makes that elevator less effective at all speeds and not effective at all below a threshold speed which increases proportionally with increased distance between the CG and CL.

A reference backing up my opinion of the dive test. It changed my Radian in an amazing way and proved that the dive test only shows the relationship between CG and stab decalage/elevator position. If the decalage is wrong, changing the CG will compensate and make the dive test look okay. Your plane will have the wrong CG. It will not perform optimally. Because of all the nasty interactions between CG, CL, airfoil, decalage and elevator position, Mr Agnew's rather grueling test procedure is what we are left with if we want our plane to perform its best. On the other hand if you have a power plane you really don't care that much and would rather not play with the limits of controllability for the purpose of getting the best glide ratio. You have a motor and stability is more important to you. Comparing needed control trim at normal and inverted flight is also a useful trimming tool that I know one Radian flier uses. That would be great to use in a power plane. Fun too!

@krexken good show! Looks like you either leave it the way it is or build an extension for your nose with new firewall if you're gung ho about saving every gram of extra weight on the plane. Depending on your much weight you have now and how much static thrust you have, you might get better 3D performance by doing the extension, or you might do quite a bit of work and not be able to detect any difference. It's trial and terror time!
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Last edited by Rockin Robbins; Sep 10, 2011 at 07:34 PM.
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