CG Off? Tail Heavy? - RC Groups
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Aug 29, 2017, 10:52 AM
Registered User
Question

CG Off? Tail Heavy?


So I had a maiden flight with my LX 70mm F-22 Raptor. Set up the dual rates as recommended by other threads/manual (75/25 for aileron and elevator with high rates at 100 which I only used during the take off process). The day was relatively calm, little breeze around 4-6mph. As I was attempting to trim the jet, I noticed that it tended to constantly exhibit signs of being tail heavy, even though I made sure the CG was where it was supposed to be according to the manual. The jet would constantly go nose up, even with trimming the elevator. I do know that my brushless motor was slightly tilted at an angle, which I'm assuming was attributing my nose to constantly want to go up, almost acting like thrust vectoring. I was able to bring the plane down, but I was curious if anyone had any input regarding what could've been happening. I fixed the angle of the motor and thought about adding a little weight to the front in hopes of correcting the nose up issue. I haven't flown the plane again, yet.

Anyone input would be greatly appreciated. Thanks everyone!
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Aug 29, 2017, 03:47 PM
O3D
O3D
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It's hard to give a definitive answer without more data.

But I'll assume that you are seeing the pitch up when the plane goes faster?

If so then moving the cg is forward will pronounce the effect.

What happens on most planes is that the center of lift is behind the cg and forms the fulcrum of a lever. The cg pulls down on one side of this "lever" and the (trimmed) elevator pulls down on the other side.

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When the plane goes faster, the cg down force stays constant but the elevator gets more airflow and pushes down harder, creating a pitch up.

Moving the cg forward INCREASES this effect because more elevator trim was needed for balance.

Moving cg back decreases the effect but also makes the elevator more responsive and the plane less stable overall. If the cg ends up behind the center of lift, then the plane will become wildly unstable.

Pitch up on speed is very common on high performance planes and there are three common approaches for addressing it:

1) On your radio, mix some elevator down trim into the throttle
2) Changing thrust angle
3) Move cg back (be careful)

Note that you can independently evaluate cg via a dive test (try Googling it)

Hope that helped!
Aug 29, 2017, 03:54 PM
Registered User
I don't have any EDF planes but I wouldn't think having the fan slightly angled would be a problem unless it was mounted right at the outlet. When you landed was the elevator trimmed down, i.e. actually angled down? Was the plane "pitchy" in flight, wanting to go up or down but not level, overly sensitive to elevator control? Were you running it at full throttle all the time? How did it behave at lower throttle settings?
Aug 29, 2017, 04:11 PM
yank and bank!!
Is there a thread for that plane here at RCG? What do they say about the plane?
Aug 29, 2017, 08:36 PM
B for Bruce
BMatthews's Avatar
It has been my "pleasantly brief" experience with ARF's that the CG locations given in the manuals are decidedly on the conservative side. So balancing where indicated in the manual might well still be highly "nose heavy" and the required elevator trim needed to fly well in the glide or low power settings is raising the nose when the speed rises by much.

If you can it's nice to isolate the CG position and elevator trim by using dive testing to set the CG and elevator without power for as much as is practical. Only THEN apply throttle and see how it works in connection with a proven CG and elevator trim and alter the thrust line to suit.

I realize that this isn't always easy. Heck, I'm fighting with a couple of ARF's right now on this very thing. It's tough to climb high enough to set the glide trim then do a dive test and then climb back up with a model that's fighting like a rodeo trained bucking bull. But persevere and don't lose sight of the goals.
Sep 01, 2017, 10:46 PM
Closed Account
Quote:
Originally Posted by O3D
What happens on most planes is that the center of lift is behind the cg and forms the fulcrum of a lever. The cg pulls down on one side of this "lever" and the (trimmed) elevator pulls down on the other side.

When the plane goes faster, the cg down force stays constant but the elevator gets more airflow and pushes down harder, creating a pitch up.
The CG for most airplanes is further aft than you have shown it. The old free flight model rule of thumb of "about 1/3 of the way back from the wing LE" is good for straight wings and typical horizontal tail sizes.

The center of lift for each surface (wing and horizontal tail) is about 25% of the chord of that surface.

The center of lift for the AIRPLANE is further back, it depends on the arm between the wing and tail and the size of the horizontal tail.

If you think about it, when the airplane is in trim, the center of lift must be at the CG. It can't be anywhere else; otherwise the arm between the lift vector and the CG would cause a pitch up or down. In which case the airplane would not be in trim. So "center of lift" doesn't really mean much when we are talking about the total airplane, vs. the wing or tail separately.

It is more useful to work with the "aerodynamic center" (AC) or "neutral point". This is the point where a CHANGE in lift appears when there is a CHANGE in Angle of Attack, the angle at which the wing meets the air. For a stable airplane, the CG always must be ahead of the AC. Why? Because an increase in AoA causes an increase in lift. So when the AoA increases, a lift component appears at the AC, forcing the nose down and reducing AoA. The opposite happens if AoA decreases. Thus a stable airplane tends to stay at its trimmed AoA, which is set by the horizontal tail incidence and elevator position.

So why does a stable airplane (flying at constant AoA) pitch up when speed increases? Remember that the lift and weight are both acting at the CG, but in opposite directions. Lift depends on the square of the airspeed, but weight is constant. So, when the speed increases, lift increases. Rather than causing the nose to pitch up, the whole flight path curves upward. It will continue to do so until the pilot applies down elevator (decreasing AoA) or until gravity increases drag, decreasing the speed.

The opposite happens with a decrease in airspeed. AoA remains constant, lift decreases; the flight path curves downward. The aircraft will continue to descend until up elevator is applied, or gravity overcomes drag and the airspeed is restored.

Move the CG back to the AC (neutral stability) and the airplane no longer maintains AoA. This will reduce or eliminate the "speed stability" just described. The airplane will be less susceptible to "phugoid oscillations" (porpoising), but the pilot will have to be very careful to maintain AoA manually.
Sep 01, 2017, 11:37 PM
O3D
O3D
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Quote:
Originally Posted by jruley
It is more useful to work with the "aerodynamic center" (AC) or "neutral point".
You're right. I should have said "neutral point" instead of "Center of Lift". Thank you for clarifying the terminology!
Last edited by O3D; Sep 02, 2017 at 12:15 AM.
Sep 02, 2017, 10:15 AM
Registered User
One step at a time !!! That way you know what the problem WAS . ENJOY !!! RED
Sep 06, 2017, 08:57 AM
Registered User
Seriously thank you all very much for your responses! That means a lot to me. It's amazing to be apart of such a helpful community of individuals who are all passionate about the hobby!!! You guys rock!
Sep 06, 2017, 09:00 AM
Registered User
Also I'm still figuring out this platform so I apologize for being lazy on responding to each individually post, which I will in due time once I get this platform figured out lol. Thanks again everyone!
Sep 06, 2017, 09:27 AM
Registered User
Quote:
Originally Posted by O3D
It's hard to give a definitive answer without more data.

But I'll assume that you are seeing the pitch up when the plane goes faster?

If so then moving the cg is forward will pronounce the effect.

What happens on most planes is that the center of lift is behind the cg and forms the fulcrum of a lever. The cg pulls down on one side of this "lever" and the (trimmed) elevator pulls down on the other side.

Attachment 10315606

When the plane goes faster, the cg down force stays constant but the elevator gets more airflow and pushes down harder, creating a pitch up.

Moving the cg forward INCREASES this effect because more elevator trim was needed for balance.

Moving cg back decreases the effect but also makes the elevator more responsive and the plane less stable overall. If the cg ends up behind the center of lift, then the plane will become wildly unstable.

Pitch up on speed is very common on high performance planes and there are three common approaches for addressing it:

1) On your radio, mix some elevator down trim into the throttle
2) Changing thrust angle
3) Move cg back (be careful)

Note that you can independently evaluate cg via a dive test (try Googling it)

Hope that helped!
Definitely going to try the Dive Test this weekend. I also ended up mixing just a little down trim into the throttle so I suppose it'll be just a matter of trial and error until I get the settings right with that. Thank you!
Sep 06, 2017, 09:48 AM
Registered User
Quote:
Originally Posted by 600Bob
I don't have any EDF planes but I wouldn't think having the fan slightly angled would be a problem unless it was mounted right at the outlet. When you landed was the elevator trimmed down, i.e. actually angled down? Was the plane "pitchy" in flight, wanting to go up or down but not level, overly sensitive to elevator control? Were you running it at full throttle all the time? How did it behave at lower throttle settings?
I never actually had the plan at full throttle during the flight, probably closer to 3/4 throttle, but even with fluctuating throttle position it still wanted to pitch up throughout the flight. So at times it was level, probably around 1/2 throttle, and then other times would pitch up like no other. I definitely think it was overly sensitive to elevator control which was surprising since I thought I set my dual rates appropriately, but I suppose it just takes some experimentation. This specific plane, stock version, uses two servos to control both elevator and ailerons, which I'm starting to realize doesn't seem to work well. I ended up installing a second set of servos to control ailerons independently from the elevator so I'm hoping making that switch will help with better overall control. Thank you for the input!
Oct 22, 2017, 08:19 PM
Registered User
Quote:
Originally Posted by jruley
The CG for most airplanes is further aft than you have shown it. ...

Move the CG back to the AC (neutral stability) and the airplane no longer maintains AoA. This will reduce or eliminate the "speed stability" just described. The airplane will be less susceptible to "phugoid oscillations" (porpoising), but the pilot will have to be very careful to maintain AoA manually.
Get steady hands or install a gyro.
Oct 23, 2017, 12:25 AM
B for Bruce
BMatthews's Avatar
It's actually not that bad unless it's taken to extremes. A neutral stability model is no worse than driving a car or truck down the highway. We make small changes as needed to stay between the lines almost without thinking about it. A neutral stability model is like that.

A SLIGHTLY negative stability model is like the same car but in a gusty crosswind where you need to make somewhat more frequent corrections and make then slightly larger. But still quite easily managed.

Folks seem to have this idea that ane amount of negative stability and the model will suddenly try to turn itself around and fly backwards at the slightest lapse from the pilot. But it simply isn't all that bad for small amounts.
Oct 23, 2017, 03:22 PM
Registered User
Quote:
Originally Posted by Marques2540
So I had a maiden flight with my LX 70mm F-22 Raptor.
You have only a few posts here, is this your first plane? I'm not being critical but knowing your experience helps a lot.

The stock CG recommendations are a starting point but never the final answer. Only flight reveals all.

Here's a chart that describes how to check balance/CG/etc in flight. Scroll down to How to Trim Your Aircraft. http://www.rcacf.com/RC_Advice.htm


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