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Author Topic: Determining the initial position for setting the CG  (Read 582 times)
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a23smith
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« on: December 11, 2020, 04:17:49 PM »

Hello everyone. I’m building a Miss Canada Senior/Senior Commercial ff rubber powered model airplane. The plan does not specify where the CG should be, and I have found several different recommendations for where it should be. They range from the rule of thumb (25-33%) to posts I’ve seen here (recommending around 50% for this model), to formulas I’ve found online [D=L*stab area/(wing area+stab area), where D is the distance behind the AC of the NP and then putting the CG 5-15% ahead of the NP; another based on the tail volume: CG=16+(36XTail Volume); another in an article by Steve Riley at FreeFlight.org], to 3 different online calculators. The recommendations using the formulas and calculators range from 60% to 92%, with 80% seeming to be about the mean. I would appreciate any help in understanding why there is such a large range in these recommended CGs and your advice on which method to pick for determining the best initial CG location.
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FLYACE1946
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« Reply #1 on: December 11, 2020, 05:04:04 PM »

When the horizontal stab is a lifting surface then a more aft position is possible. The variable locations you see are perhaps a personal favorite. The choice is based on many different things. Prop size, rubber size, motor length, etc. This is a very good model for flying but be careful to use a dethermalizer if you want to keep it. Good Luck.
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a23smith
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« Reply #2 on: December 11, 2020, 05:48:04 PM »

Thanks Flyace1946. I wouldn’t expect formulas and calculators to be based on personal favorites, but it helps to know some of the factors that influence the optimum CG position.
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BG
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« Reply #3 on: December 11, 2020, 06:19:25 PM »

Hi All,
so that 16+(37xTvo) formulae is an empirical formula that comes from plotting the relevant variables for a series of successful models. I believe it comes from the legendary McCombs book "making scale models fly". The issue for you is that this formula was developed for scale models to help designers figure out how much to enlarge the stab and where to locate the CG for the stab in question. Scale models have short moments and so their CGs are not relevant to duration ships which have longer moments, more dihedral and larger stabs. So, a revised formula was developed for old-time duration jobs like Gollywocks and Miss Canadas etc. (I believe by Don DeLoach or a FF colleague of his).  that formula is: 18+(40xTvo). The CG you get from this version of the formula will work well for your Miss Canada. As you might suspect my answer above also reveals why there is so much apparent disagreement regarding CG. The optimal CG depends very much on the design, competition rules restrictions, power in use, and the way the model is supposed to fly.

hope this helps.

BG

P.S. PM me for a spread sheet that will run the numbers for you if you don't already have one.


« Last Edit: December 11, 2020, 07:05:45 PM by BG » Logged

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a23smith
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« Reply #4 on: December 11, 2020, 06:28:19 PM »

BG,

Thanks, that’s very helpful information. I was thinking that the formulas and calculators were probably meant primarily for RC planes and gas motors, but I didn’t know. I will probably go with the result from the formula you provided, but I haven’t calculated it yet and I’m about to go get groceries now.

Alan
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a23smith
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« Reply #5 on: December 11, 2020, 07:58:13 PM »

BG,

Using the formula you provided [CG=18+(40XTVo)], the optimal CG would be 89.4%. In calculating the TVo I used the distance from the leading edge of the wing to the leading edge of the horizontal stabilizer. I have also seen the tail arm defined as the distance between the AC of the wing and the AC of the horizontal stabilizer, but that would change the result very little. Would you recommend I balance the completely loaded plane at 89% as a starting CG? If I use a folding propeller should the balancing be done with the propeller folded?

Alan
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a23smith
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« Reply #6 on: December 11, 2020, 08:13:08 PM »

BG,

On second thought, I believe it shouldn’t make any difference whether the propeller is folded or not when balancing the plane. Is that correct?

Alan
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billdennis747
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« Reply #7 on: December 12, 2020, 07:05:34 AM »

I would put the CG at 40 - 50% and then go and fly it. There are endless designs like this on Outerzone and of those that admit to needing a CG, they are 40 -50%
Prop folded
The only thing that might muddy the waters with this design is that it doesn't seem to have much wing incidence/decalage and if that is the case then it would need a rearward CG but then it might crash. I would trim it with tail incidence and the 40% CG
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BG
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« Reply #8 on: December 12, 2020, 10:43:38 AM »

Yes. Use the the 89% CG and balance with prop folded (you are trimming for the glide). I respectfully disagree with Bill (a rare departure from the norm). You could fly it at 40-50% but this CG position will probably require nose weight (more weight is bad) and it will limit the performance of the model which has a large stab and a longer tail moment. the 89% CG is exactly what I would expect for a Miss Canada. In my experience flying with the forward CG will make the climb sluggish and the glide uninspiring.

BG

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a23smith
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« Reply #9 on: December 12, 2020, 11:49:56 AM »

Bill and BG,

Thanks for the recommendations. There is only about 1.2 degrees of decalage, according to the plan, so I may use the 89% CG. If I get worried that it is at a high risk of crashing I will switch to a more forward CG.

ALAN
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billdennis747
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« Reply #10 on: December 12, 2020, 01:01:39 PM »

Let us know what happens!
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a23smith
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« Reply #11 on: December 12, 2020, 01:44:16 PM »

Bill,

Okay, but don’t hold your breath. I’m doing this mainly as a pass-time while I stay home to avoid the pandemic, and the building process (especially learning how to do a better job building the airplane) is the part I enjoy the most. Thanks again for the help.

Alan
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TimWescott
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« Reply #12 on: December 13, 2020, 06:02:48 PM »

Note that for any CG calculation, for any model (any airplane, really), for any purpose, the CG calculator is just going to get you into the ballpark -- after that, you need to fly, and trim the CG as necessary to fly better.

(Which leads me to ask - has anyone written a trimming guide for rubber models that goes into telling you when you need to change the CG based on its behavior?  Certainly if I need to use a lot of thrust adjustments to keep the nose down in a climb or if I see a large tendency to short-period fugoids in the glide I'm going to want to move the CG back and re-trim incidence.  But I don't consider myself to be an expert here).
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a23smith
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« Reply #13 on: December 13, 2020, 06:35:47 PM »

Tim,

I found a good article on that topic by Don Srull at free flight.org called “Getting That CG Right”, which was published in the September 2006 issue of MaxFax.

Alan
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lincoln
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« Reply #14 on: December 13, 2020, 10:45:46 PM »

I seem to recall that McCombs suggests getting the best trim at different c.g.'s and see which results in the longest times. However, this could involve 100 flights! I'd just get it as far back as is consistent with good behavior. I think the weight of the motor can shift a little as it unwinds, so a rubber model might need a slightly more forward c.g. than the same design as an electric. 

My inclination would be to balance the model for a good glide with the prop folded. However, the prop can be a bit destabilizing when in operation. On the other hand, when unfolded the prop's weight is further forward, which might make up for the destabilizing effect. Keep in mind that the only folding props I've used are on RC gliders where the effect is smaller and I haven't noticed it much, so this is theory.
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TimWescott
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« Reply #15 on: December 14, 2020, 12:22:32 AM »

... I'd just get it as far back as is consistent with good behavior. ...

Interestingly enough, this pretty much sums up my approach to CG adjustment for RC, CL, and FF too.
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calgoddard
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« Reply #16 on: December 14, 2020, 10:34:04 AM »

I am going to split my comments into two posts.

BG was spot on when he said in Reply #3 that “The optimal CG depends very much on the design, competition rules restrictions, power in use, and the way the model is supposed to fly.”

It is helpful to explain some of the fundamental concepts of model airplane aerodynamics in order to understand where to locate the CG of a given model.

A rubber powered free flight model airplane needs pitch stability so that it will automatically correct itself if the nose is too high (stall) or too low (dive). Pitch stability is achieved by locating the center of gravity (CG) of the model (with the rubber motor installed) so that it is ahead of the neutral point (NP) and by adjusting the difference between the angle of attack (AOA) of the wing and the stab (decalage). 

How far the CG is ahead of the NP determines the so-called “static margin of stability" of a model airplane. Static margin calculators are available on the Internet. The NP and the corresponding static margin of stability are generally based on: 1) the area of the wing; 2) the area of the stab; and 3) the distance between the wing and stab. The static margin of stability is usually expressed as a percentage of the mean aerodynamic chord (MAC).
 
The further the CG is ahead of the NP the higher the static margin will be and the more stable the flight will be, but at a cost. The decalage will have to be increased as the CG is moved forward and this makes the flight less efficient.  Excessive amounts of down thrust may be required to accommodate high launch torque and prevent the model from power stalling or looping if the CG is too far forward.  However, if the CG is too far rearward, the model will become difficult to trim and may not recover from stalls and dives.

If the CG is too far forward it will also be difficult to trim the model for a good slow glide. If the CG is too far aft it will be very sensitive in flight, especially powered flight, to small changes in down thrust, small changes in stab incidence, and warps, and will exhibit poorer duration in gusty conditions. 
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calgoddard
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« Reply #17 on: December 14, 2020, 10:36:14 AM »

This is a continuation of my immediately preceding post.

Set the CG at the location recommended on the plan with the prop and the rubber motor installed. Unfortunately, as has already been noted, the plan for the Miss Canada Sr. does not indicate the recommended location for the CG.

The optimum location for the CG must be established taking into account the weight of the rubber motor extending between the prop hook and the motor peg. Optimum rubber motor sizing for a particular model in terms of cross-section and weight is a subject which is beyond the scope of this post.  

If the model has a movable wing or pylon, move the wing or pylon rearward to move the CG forward. Move the wing or pylon forward to move the CG rearward. If the wing or pylon is not movable, add weight to one end of the fuselage/tail boom to get the CG right in the correct location.  Cabin models like the Miss Canada Sr., scale models, and other models that have a wing which is not movable longitudinally (functionally or aesthetically) often require some nose weight to get the CG in the correct location.
 
For scratch-built scale models with no plan and therefore no recommended location for the CG, locating the CG about 33% of the wing chord aft of the leading edge (LE) is a good starting point. Don DeLoach’s published formula for calculating the optimum location of the CG for a specific scale model seems to yield an optimum CG location that is aft of 33%, and sometimes aft of 40%.  

I seem to recall that Old Timer Rubber (OTR) models may have a CG somewhere between 50-60%. I checked the plan for the Gollywock II and it indicates a CG range which I calculated to be between 56% and 71%.

P-30’s and coupes typically have a CG somewhere in the 60-70% range.  

Keep in mind that if you change the weight of the rubber motor this will usually alter the actual location of the CG.  Some experts, like Wally Farrell, locate the rear motor peg so that an equal amount of the rubber motor extends fore and aft of the CG.  This way they can change the weight of the rubber motor with minimum adverse effect on the trim.

A model airplane also needs lateral (roll) stability – achieved via dihedral – so that it will automatically raise a low wing tip, and yaw (directional) stability – achieved via the vertical stabilizer - so that it will automatically return to a straight position (relative to the direction it is traveling).  How much dihedral and how much fin require more explanation, beyond the scope of this post. Use the dihedral and the fin size & location indicated on the plan, unless you are an expert flier.

Many tradeoffs are involved in trimming a rubber powered free flight model airplane.  Determining the optimum location for the CG of a particular model is one of them. I hope that this post and my previous post set forth immediately above are both helpful.
« Last Edit: December 14, 2020, 11:09:44 AM by calgoddard » Logged
Kevin M
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« Reply #18 on: December 14, 2020, 12:40:23 PM »

If it were mine, I would start with the C.G at 45-50%, by analogy with similar models I have. It may be you can go considerably aft of that as others have suggested, but I have acquired the habit of being conservative from flying heavier loaded RC models. With those, if the C.G. is too far forward, you land and move it back. If it is too far aft you don't always get to land, just pick up the pieces.
When you are ascertaining the best C.G,.in the trimming process, don't confuse trim for stability. Position the c.G. for acceptable stability and trim for glide angle with stab incidence. Then trim for the power phase with thrust-lines. That's my way anyway.
As regards C.G formulae, i find them useful on larger, higher-Re models (2-3m upwards), but not particularly so on these types, where what has been found to work empirically on similar models seems to me the best guide. Ideally for this find someone who has one that flies well and do the same.
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TimWescott
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« Reply #19 on: December 14, 2020, 12:42:57 PM »

... Unfortunately, as has already been noted, the plan for the Miss Canada Sr. does not indicate the recommended location for the CG. ...

Finding a model that has similar important dimensions and a CG marked would be another route to finding a "recommended" CG.  Important dimensions in this case are the wing and tail sizes, and the distance from the wing MAC to the tail MAC (this is, basically, what the CG calculators use).

I suspect there's a lot of models with similar dimensions to the Miss Canada Sr. from that same time period, which is good.  Unfortunately, I think you were just supposed to know where to balance things back then, so there may not be many with recommended CG locations.
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