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What should the empty CG be for the 601XL?

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David X

Joined: 10 Jan 2006
Posts: 154
Location: Princeton, NJ, USA

Posted: Wed Mar 01, 2006 5:26 pm Post subject: What should the empty CG be for the 601XL?

I'm interested in knowing what the empty CG is out there for existing 601XL owners - especially any who fly with a Rotax 912. 



I've seen a few kit-built with Rotax engines, and they all have relatively short noses and CGs between 14 and 15. Honestly, I think that's not good at all. In fact, I think it's horrible.



12 inches is tolerable, but you'd have to use the wing lockers instead of the rear baggage compartment if near max take-off weight (1320 lbs for US LSA category). 



I think that 10 inches empty CG would be closer to ideal. The SLSA built by AMD has a empty CG near 10 inches with a Continental O-200. By my calculations, they got it about as perfect as one could for the 601XL.



The attached spreadsheet can be used to play out a few CG scenarios for those who are interested. It's much better to know this info before you buy the firewall-forward kit.



Again, I'm interested in what sort of empty CG values are out there for the 601XL and what engine was used. My hope is that it will help the builders out there know what firewall-forward kits give the best results. Your help is appreciated.

WB-601XL.xls

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Spreadsheet to test CG scenarios.

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WB-601XL.xls

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Zodiac 601 XL - CZAW Built - Rotax 912S
DO NOT ARCHIVE

AndrewC

Joined: 28 Jan 2006
Posts: 8
Location: Scotland, UK

Posted: Thu Mar 02, 2006 2:34 am Post subject: Re: What should the empty CG be for the 601XL?

Hi David



I've got an XL with a Rotax 912ULS engine and my empty c of g is 339mm aft of the leading edge (my datum).  The airframe is the CZAW one though and I believe that the c of g limits are slightly better for their kits as they have a couple of design differences, the most obvious one being composite gear instead of metal.  My c of g range as approved by the UK authorities is between 350 and 500mm (although I think the most recent paperwork I saw allowed it to go back even further).



Hope that helps.



Andrew

David X

Joined: 10 Jan 2006
Posts: 154
Location: Princeton, NJ, USA

Posted: Thu Mar 02, 2006 9:21 am Post subject: Re: What should the empty CG be for the 601XL?

Losing 30 lbs by using the composite landing gear is most definately a plus for the CG. The aircraft have heavy-duty aluminum gear in the US.



The published CG range (for US) is between 300 and 450mm - i.e. between 20 and 30% MAC. The datum is the leading edge of the wing.



The UK standards for max weight are lower, I believe, which would lessen the CG issues considerably. 



What is the empty weight and the maximum gross weight of your ship?

_________________
Zodiac 601 XL - CZAW Built - Rotax 912S
DO NOT ARCHIVE

AndrewC

Joined: 28 Jan 2006
Posts: 8
Location: Scotland, UK

Posted: Sat Mar 04, 2006 3:46 am Post subject: Re: What should the empty CG be for the 601XL?

My empty weight is around 303kg and the UK authorities allow a 560kg max weight, though I've seen other countries allow up to 590kg, particularly with floats.  My permit to test extension came in this morning and it allows me c of g limits of 300mm to 520mm aft of datum, which isn't bad (though I don't particularly want to test that envelope too far...).  I've been working to limits of 350 to 500 and the aft-most limit on that range gives me two 86kg pilots, no fuel and 13kg in the rear baggage bay which is enough for me.



The composite gear also doesn't traverse the whole of the bottom of the fuselage in one piece, so it saves a bit more weight in comparison to the single piece metal gear that way too.

David X

Joined: 10 Jan 2006
Posts: 154
Location: Princeton, NJ, USA

Posted: Mon Mar 06, 2006 9:46 am Post subject: Re: What should the empty CG be for the 601XL?

I find it incredible that your aircraft is 45 lbs (20kg) lighter than the same aircraft built for the US market by the same company. Although with lighter landing gear, I don't see how the gear alone can make up the 45 lb difference.



The amazing empty weight (313kg/690lbs) makes your CG configuration acceptable even in the US market were the rear CG limit is 450mm ... so long as you don't have any rear baggage. As soon as you add rear baggage, the CG goes to hell. Even 9 KG (20 lbs) of baggage int he rear would practically limit the aircraft to one person (max of 110kg/240lbs).



The extended rearward CG of 500mm (or more) in the UK practically elliminates this problem. Of course, if the empty CG were closer to 250mm (10 inches), it wouldn't be a problem in the US market either.



BTW: How do you account for the amazing difference in empty weight?

_________________
Zodiac 601 XL - CZAW Built - Rotax 912S
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AndrewC

Joined: 28 Jan 2006
Posts: 8
Location: Scotland, UK

Posted: Mon Mar 06, 2006 10:15 am Post subject: Re: What should the empty CG be for the 601XL?

I've no idea as I'm not sure what differences there are between the US and European kits.  I don't have any gyro instruments though, seats are nothing special, no upholstery up the sides or in the baggage bays, simple fixed pitch composite prop, but it may be that the explanation - aside from the gear - lies in the fact that my plane is partly painted.  The wings and rudder are finished but the rest of the fuselage and the cowl need one more coat to be complete.  I suppose all the little bits add up.  Once her paint job is finished she'll be weighed again and I'll be able to see just how much fat she's picked up.  I'm expecting her to pick up another 5-10kg as a result of that and most of it aft of the c of g.

Martin Pohl

Joined: 30 Jan 2006
Posts: 118
Location: CH-8645 Jona SG, Switzerland

Posted: Sat Nov 11, 2006 4:46 am Post subject: Re: What should the empty CG be for the 601XL?

How comes that the XL can have an approved C.G. range of 300 - 450 mm in the U.S. (MTOW = 600 kg) and a much better range of 300 - 520 mm in the U.K. (MTOW = 560 kg)?



Link to PFA (UK) Zodiac XL type certificate: http://www.pfa.org.uk/Data%20Sheets/162B%20ZENAIR%20CH601-XL.pdf



The C.G. range is a matter of aerodynamics and not of the MTOW-limit. The MTOW mainly depends on structural strength (mainly wing bending which is the limiting factor for the XL maximum loading). Or am I not right?



Is there anybody who can explain? Is it perhaps because of the elongated elevator-trim-tab (over the whole left elevator width)?



Cheers from Switzerland

Martin

XL-builder, QBK Czech Aircraft Works[/url]

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Martin Pohl
Zodiac XL QBK
8645 Jona, Switzerland
http://www.pohltec.ch/ZodiacXL/Main.html

a.s.elliott(at)cox.net
Guest

Posted: Sun Nov 12, 2006 7:17 am Post subject: What should the empty CG be for the 601XL?

The allowable CG *envelope* is not normally a simple rectangular  box, that is you don't get the same weight limits throughout the entire CG  range.  Instead, it usually is a trapezoid, is skewed in one direction  (typically aft at the top) and may have some corners cut  off.  That means  that the allowable CG range usually both get  smaller and shifts aft as the weight increases.  The reasons for this are  not so hard to see.  

  

 At forward CG's, you need to have enough elevator and trim authority to fly  the plane through the flare without the nosewheel hitting first.  The  heavier the plane is at the same CG location, the more elevator force you will  need.  Since the elevator is fixed size, this gives a relation between the  reciprocal relation between the CG location and weight - more weight implies CG  forward limit shifts aft.

  

 Aft CG's tend to make the plane more and more sensitive in pitch.  Far  enough aft and the plane would actually be unstable.  This instability is  countered by the horizontal stabilizer.  Again, since the stab is a fixed  size, there has to be a balance to get an airplane that is flyable.  In  this case it depends again directly on the CG location and weight (mass  balance), and indirectly on the incidence angles of the flying surfaces  (aerodynamic balance).  But basically, it means that more weight implies  that the CG range must shrink.

  

 The relationship between gear capability and gross weight seems pretty  clear, but isn't.  That is rarely what puts the top limit on the CG  envelope, but much more often that is determined by the structural design limits  of the rest of the plane (wings, especially) for that particular  certification category (standard, utility, aerobatic) and the gear is added  later to ensure that it can properly support that design weight.  In fact,  many planes are certified for different CG envelopes for different  categories.  There may also be different limits for takeoff and  landing.  (The Bonanza was famous for being able to fly out of the CG rear  limit as you burned off fuel. Depends on where your fuel tanks are.)

  

 For taildraggers, the design process is a bit more involved, since there  are multiple landing types and can depend heavily on main wheel placement.

  

 Hope this made it a bit more clear.  Of course, this is a  simplification, since there are other aerodynamic and structural factors that  have to be considered, but you probably get the idea.  All basic aircraft  design texts have a chapter or two on this topic.

  

 Andy Elliott

N601GE (601XL/TD, Corvair, building)

   [quote][b]

David X

Joined: 10 Jan 2006
Posts: 154
Location: Princeton, NJ, USA

Posted: Sun Nov 12, 2006 7:23 am Post subject: Re: What should the empty CG be for the 601XL?

Martin,



The CG is about weight and not so much aerodynamics. It's the distribution of the weight that is the main issue. 



If you increase the MTOW of an aircraft, it depends on where you add the additional weight. If the extra weight is due to bigger Americans, then the weight will be added rear of the empty CG (on a 601XL) thus pushing the CG rearward.



As far as the CG range is concerned, it may be partly due to regulatory differences between USA and Europe, but I think that MTOW is also a factor in the difference in range. If you look at the POH for most small training aircraft, you will see two different sets of numbers for C.G.  One set is for "Normal" category and the other is for "Utility" category. The difference between the two is that MTOW for "Utility" is always lower and the CG range greater.



The large elevator trim tab is there so that you can trim the aircraft for level flight with full flaps; something required for the LSA category in the USA.



 	  Martin Pohl wrote: 	
		  How comes that the XL can have an approved C.G. range of 300 - 450 mm in the U.S. (MTOW = 600 kg) and a much better range of 300 - 520 mm in the U.K. (MTOW = 560 kg)?



The C.G. range is a matter of aerodynamics and not of the MTOW-limit. The MTOW mainly depends on structural strength (mainly wing bending which is the limiting factor for the XL maximum loading). Or am I not right?



Is there anybody who can explain? Is it perhaps because of the elongated elevator-trim-tab (over the whole left elevator width)?

_________________
Zodiac 601 XL - CZAW Built - Rotax 912S
DO NOT ARCHIVE

zodierocket(at)hsfx.ca
Guest

Posted: Sun Nov 12, 2006 8:38 am Post subject: What should the empty CG be for the 601XL?

Hi Folks, I have installed a W & B program for the 601XL on

www.ch601.org in the builder resources section. This is a great tool for

answering many of the questions I have seen lately on this thread. It

was developed by Gary Ray and is in Excel format. Excellent Job Gary.



Mark Townsend  Alma, Ontario

Zodiac 601XL C-GOXL, CH701 just started

www.ch601.org / www.ch701.com / www.Osprey2.com

--

Martin Pohl

Joined: 30 Jan 2006
Posts: 118
Location: CH-8645 Jona SG, Switzerland

Posted: Sun Nov 12, 2006 9:02 am Post subject: Re: What should the empty CG be for the 601XL?

Thank you for your answers.



(at)Andy: Good explanation. That indeed makes sense! I was just astonished that the U.K. limitations are broader than what Chris initially definded.



Rgds

Martin

_________________
Martin Pohl
Zodiac XL QBK
8645 Jona, Switzerland
http://www.pohltec.ch/ZodiacXL/Main.html

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