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Cozy weight and balance

CG Weight and balance Dimensi

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#1 Mike Arndt IE

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Posted 23 July 2016 - 12:01 AM

Hey everyone : )

 

I've been learning about the Cozy Mark IV for years now, and I'm trying to make a CG/ weight and balance calculation but having a hard time. I've found various spread sheets that compute it for a complete aircraft, but I'm trying to find out the distances (X axis only) between the tip of the nose, the front wheel, the rear wheel, the CG and the fire wall.

 

I've read up on the details of engine weight vs position to attain the original cg, (Link) but I'm trying to calculate a ratio, specifically, "Shifting the engine X inches back will add Y lbs weight capacity to the front seat".

 

Understandably, shifting the engine back will necessitate more nose ballast when flying solo, but as long as the total CG/weight and balance remains in the stable range everything should work. Yes, shifting really far back will make takeoffs/landings more difficult by decreasing the distance between prop and runway on rotation, but to me its worth the trade off. My wife and I together are about 450 to 475 lbs, and we do everything together, so no way is she taking the back seat on this.

 

Can anyone here provide these dimensions?

Nose tip to front wheel = 

Front wheel to rear wheel = 

Rear wheel to CG = ( not sure if target CG is before or aft of rear wheel)

Rear wheel to firewall =

 

Any help would be appreciated... 



#2 Kent Ashton

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Posted 23 July 2016 - 10:21 AM

Pages 39-40 of the POH should give you what you need.  Try this link or just google cozy mk-IV POH

www.cozy1200.com/geeklog/filemgmt/visit.php?lid=2

 

I have flown with 470 front seat wt and as a couple, my wife and I fly at 410.  However, I would make a couple observations:

 
The first is that jiggering the CG by shifting the engine is not a good idea.  It will require huge ballast when flying solo.  Also, I have to be very careful ground-handling my airplane when empty with no ballast in the nose to avoid a tip-back.  When my wife and I fly dual with no ballast, I have to be very careful to hold the nose down until she get in the airplane.  If the canard inadvertently rises more than 12-20 inches, I am not going to be able to stop it.  This happened to me once when I was helping an elderly lady off the strake.  I forgot I did not have ballast and broke the tip of my prop.  If you shifted the engine aft, you'd have to be even more careful with ground handling to avoid a tip-back
 
With 50 lbs of ballast and a 25 lb lead weight hanging in the nose gear, mine will sit on three wheels but you would have to use substantial weight + ballast to roll your airplane around.  It gets to be a bit of a hassle.  Too, that weightlifting is hard on the back.  You could use a nose lift but the electric lifts put a lot of stress on the strut.
 
Now, you and your wife could fly it as designed at 475 lbs.  You would put more stress on the nose gear which is not that robust, and you would notice a higher rotation speed and longer takeoff roll.  On a hot day with a heavy load, you might find the takeoff roll very interesting.   :-)   But the airplane would fly OK.  You wouldn't notice much difference there.    In an off-airport landing though, you would carry more speed and the crash would be less survivable.  
 
Short answer: I don't think the Cozy is the right airplane for you.  One of the Vans 2-seat models might work.  An RV-10 would be quite satisfactory, I'd guess.  The Bearhawk could easily accommodate you and it has a fairly fast cruise speed.

Edited by Kent Ashton, 23 July 2016 - 10:21 AM.

-Kent
Cozy IV N13AM-650 hrs, Long-EZ-55 hrs


#3 Mike Arndt IE

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Posted 23 July 2016 - 11:41 AM

Agreed, I was thinking the same thing about the ballast when empty, but I had a work around for that...

 Aym3G.jpg

Admittedly its a little different, but the moment is (weight in lbs X lever arm in inches), so if the ballast was in the form of a long iron rod that locked into the nose and stuck out like this, that would give a lot of extra inches without so much unwanted weight. Parking it would be fun, but if I manage to build a aircraft, I'll bet I can figure out some way to push it around on the ground without tipping over. 

 

Thank you for the link to the POH, but I couldn't get the measurements I needed from that (at least, I didn't see them when I downloaded it and looked around pages 30 - 35). 

 

Ultimately this idea needs to be settled by calculation - if the engine needs to move 3 ft back to make this work then the idea is impractical and no good. If it needs to go back 6 to 8 inches then maybe it could be done. I just need those aircraft dimensions to run the calculation and see where reality stands vs my dreams of grandeur : ).

 

Nose tip to front wheel = 

Front wheel to rear wheel = 

Rear wheel to CG = ( not sure if target CG is before or aft of rear wheel)

Rear wheel to firewall =

 

Thanks for your reply!  :D



#4 Kent Ashton

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Posted 23 July 2016 - 01:48 PM

Try pp. 39-40 but some of what you ask will have to be estimated from the diagram, p. 39.  You probably want to download Marc's wt & bal spreadsheet and play with it.

http://cozybuilders.org/docs/

 

To revise and extend my remarks:  :-)

For you, flying the unmodified airplane solo would be fine.  You would use less ballast than I do.

 

Flying the unmodified airplane with your wife would also be satisfactory.  The published front seat limit (425) is a rather arbitrary choice IMO.  You are only 25-50 lbs over that.  It puts a little more stress on the nose gear but the canard can easily handle it.  You wouldn't see much beyond a slightly higher rotation speed and takeoff speed and a small landing speed increase.  Maybe slightly more trim drag because the canard is loaded a bit more.  I used to fly a Cozy-III occasionally way forward of the published forward CG.   I was nervous about the III's nose gear and a bit with the speeds but mostly, it flew just fine.

 

Maybe I was too quick to say it's not the right airplane for you but flying it as designed seems to be a lot less problematic than shifting the engine.  I would bet there are owners already flying at the weight you mentioned.   I imagine Marc will comment or you could get more opinions at the cozybuilder group list.


-Kent
Cozy IV N13AM-650 hrs, Long-EZ-55 hrs


#5 Mike Arndt IE

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Posted 23 July 2016 - 02:31 PM

Wow, thanks for the info : )



#6 petroelb

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Posted 23 July 2016 - 04:56 PM

One other thing you'll definitely want to keep in mind is that if you shift the engine back, you decrease the prop clearance at takeoff and landing.  So, if you want to shift the engine back you may also have to use a smaller diameter prop to maintain the required clearance. 



#7 Mike Arndt IE

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Posted 24 July 2016 - 09:06 PM

Made some headway on the calculation: Moving the engine doesn't help (or hurt) as much as you might think. Compared to the ideal CG of 100 (where smaller numbers are inches forward of 100 and larger numbers are inches aft) the front seats and engine are equal distances from the CG, so one more lb of engine balances one lb of front seat pilot or passenger. 

 

If anyone else is out there looking at the front seat weight question, here's what I used:

 

All values in inches

Nose Ballast = -3

Front seat/pilot = 59 

Forward FG limit = 97.5 

Ideal CG = 100 

Back seat/baggage = 101 inches

Aft CG limit = 102

Fuel tank = 103

Firewall = 123.3

Engine = 142

 

So the engine at 42 aft is just about the same as the front seat at 41 inches forward of ideal CG.

 

So moving the engine around is a major tech mess, and more to the point, its not very productive.  :wacko:

 

At this point, the only solution that still makes sense would be to use the same 'fiberglass pipe with lead ended counterweight' and mount them out on the wings, outboard of the ailerons. If I glassed in just the mount on each side, and made to pipes about 3 ft long weighing 15lbs each with a cg about a foot aft of the wing tip, then 30 lbs ballast counters 60 lbs of front seat weight plus the ballast probes could be removed to return the CG to normal when not flying with a heavy front seat. The wings already produce the vast majority of the lifting force (duh), so in flight 15 lbs of weight/twist per wing probably wouldn't be enough to matter.

 

What do you think? Mad science :P , or better to just chance it and fly past the forward CG as some are doing today?  



#8 Marc Zeitlin

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Posted 24 July 2016 - 11:38 PM

Ideal CG = 100

What makes you think that the "ideal" CG (if such a thing exists) would be at FS-100? Personally, I always like to fly at the aft limit of 102", if possible
 

At this point, the only solution that still makes sense would be to use the same 'fiberglass pipe with lead ended counterweight' and mount them out on the wings...

I was hoping to stay out of this discussion - 99.8% of the time, discussing MAJOR aircraft changes with folks that don't even own the plans yet is less than optimally productive. I don't even want to start discussing why this is a really bad idea, but it ranges from flutter, to structural issues, to complexity, etc., etc.
 

The wings already produce the vast majority of the lifting force (duh), so in flight 15 lbs of weight/twist per wing probably wouldn't be enough to matter.

a) The canard produces from 25% to 35% of the total lift, depending upon CG and airspeed. Majority, certainly, but "vast"? But that's immaterial to this question.
b) Is "probably" a reasonable level of safety? You want to hang 15 lb. off the TE of a wing that in total, might weigh 80 lb. and was not designed for the loads that such a weight would impart, especially in high "G" situations or high roll/yaw/pitch rates. I would strongly suggest a different path.
 

What do you think? Mad science :P...

Yeah, pretty much.
 

or better to just chance it and fly past the forward CG as some are doing today?

Why do you think that those are the only two possibilities? Who tells you that they regularly fly forward of the FWD CG limit?

I see one relatively obvious choice (aside from the "lose weight" admonition, which is a) none of anyone's business, and b) unlikely) - leave the canard at the original span (3" longer each side than the current span) and move the CG RANGE forward by approximately 2". The aerodynamics of the plane thus stays about the same with the same Neutral Point margin and same Deep Stall resistance, with a higher front seat weight limit. As long as the new CG range is CLEARLY noted in the POH, plans, and on the IP, you're good to go.



#9 Kent Ashton

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Posted 25 July 2016 - 08:13 AM

I see that the Bearhawk has made airfoil improvements that give an extra 5-8 MPH.  This is a much better option for you.

http://aero-news.net...18-3112730bf952


-Kent
Cozy IV N13AM-650 hrs, Long-EZ-55 hrs


#10 Mike Arndt IE

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Posted 25 July 2016 - 07:35 PM

Mark,

 

Thank you for the insight, I hadn't considered that weight might induce a flutter issue, and I have no desire to go the way of Zubair Khan

The wing area is 88.7 sq ft, and the canard is 14.7 sq ft, with a span of 12.6ft. At a guess, say 2 of those 12.6 ft are in the nose, so if I read this right, the lifting surface of the canard is 14.7 sq ft - 10.6 ft long and 1.38 ft wide. The max weight is 2050 lbs, and if the canard is lifting 25%, then the wing loading is about 34.7 lbs per square ft.

 

If we lengthen the canard by 3 inches per side (keeping the original size : ), that adds 0.6 ft X1.38 ft = 0.828 sq ft or 28.7 lbs more lift on the canard.

 

The canard CG or center of lift is about station 20, so a delta of 60 to ideal CG. 28.7 lbs x 60 inches = 1724 inch lbs.

 

The front seats are at station 59 or 41 inches delta to ideal CG. Y new lbs in the front seat X 41inches = 1724 inch lbs so Y = 42 lbs additional front seat weight.

 

All with no mad science  :). Thanks for the tip!

 

 

Kent,

 

While the Bearhawk is certainly built for the weight, I don't normally use off airport runways, and the shorter range, slower top speed, and high kit purchase price don't fit our family goals of fast long distance flight.  There are faster planes, but a stone will fly if you strap on a big enough engine (I wouldn't want to pay the fuel bill though). I just don't know of any other four place experimental that can match the cozy for range and speed in the 180 to 200 hp range.



#11 Marc Zeitlin

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Posted 25 July 2016 - 09:14 PM

Thank you for the insight, I hadn't considered that weight might induce
a flutter issue, and I have no desire to go the way of Zubair Khan[/url].

Uggghhh. Zubair Khan did not crash due to flutter. After reading the article you reference and discussing it with the main author, it was clear that they didn't understand airplanes or how this crash occurred. They hypothesized, guessed, and did not use the resources available to them to determine the real
cause, which was a deep stall, not flutter. The author admitted that they were just thinking out loud. No flutter. Just junk science.
 

The canard CG or center of lift is about station 20, so a delta of 60 to ideal CG. 28.7 lbs x 60 inches = 1724 inch lbs.

I still don't know what you mean by "ideal CG".

My plane has a front seat weight limit of about 450 lb. with the shorter canard, for reference. For heavy folks, think about using an O-540 (which a few people are doing). With 100 lb. extra aft of the firewall, you can then create a CG range and CG wherever you want within the approved range, even with a short canard. But you end up with a heavier aircraft, and need to take that into account when choosing components and operating limits for the POH.

Edited by Marc Zeitlin, 25 July 2016 - 09:15 PM.


#12 B52

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Posted 02 October 2017 - 02:38 AM

I would think something as simple as placing a bathroom scale under a deployed nose wheel and noting the weight readings. Has anybody don this and published the the safe weights according to different loads. It is noted that each individually built cozy would have slightly different CG. it can be determine by placing trestles under the wing root then moving the plane so that the whole plane is balanced. This will indicate the empty weight CG. The relevant loads are added to get the desired CG.

 

There's a patent for commercial airliners of a devise that reads the nose wheel suspension. A data processor will generate expect weight and balance index.



#13 TMann

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Posted 02 October 2017 - 12:55 PM

I would think something as simple as placing a bathroom scale under a deployed nose wheel and noting the weight readings. 

 

Placing all the wheels on scales is pretty standard for determining your empty W&B.

After that you just do it as you would any other GA aircraft (Arm & Moment).

 

Most of the Tablet Software for Aviation (ForeFlight, WingX etc) has it built in. You set the Arm for each station then plug in the weights. Pretty simple.

 

You'll learn all about this in Ground School.


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