Original Posted By: "walt evans"
This is in response to Chuck Gantzer's post of yesterday, suggesting the best wayto compare Pietenpols with regard to empty weight center of gravity (EWCG)is to express EWCG in inches aft of the wing leading edge. I'd like to offerthe opinion that a better way is to express EWCG is as a percentage of mean aerodynamicchord (MAC), particularly when discussing homebuilts that typicallyhave dimensional variances among individual aircraft. The datum is an imaginary vertical plane from which all fore and aft measurementsare taken, to determine the arm of a particular item of weight. On a factory-builtaircraft, the datum is specified in the type certificate data sheet.In a homebuilt (i.e., non-type certificated) it is wherever the builder wantsit to be. The important thing is that it be a location easy to measure from,preferably one from which you can drop a plumb bob straight to the floor, allowingyou to mark the datum on the floor, again for ease of measurement. Youdon't want to select as a datum a point whose location is changeable, like (ona Pietenpol) the wing leading edge, or the front face of the propeller. On aPiet, the firewall is probably as good a location as any.If you assume the firewall face as your datum, your EWCG will be some number ofinches aft of the datum. Your wing leading edge (LE MAC) will also be some numberof inches aft of the datum. Take the EWCG location in inches, and subtractthe LE MAC location in inches. Divide the result by the MAC length in inches.The result is your EWCG expressed as a percentage of MAC. For example,if an aircraft has an EWCG 120 inches aft of the datum, and a wing leading edge100 inches aft of the datum, and a MAC of 60 inches, your EWCG is at 33 1/3%MAC. ((120 - 100)/60)Ted Tuckerman ________________________________________________________________________________
Pietenpol-List: Center of Gravity Computations
Re: Pietenpol-List: Center of Gravity Computations
Original Posted By: Ted Tuckerman
Ted,The final "go/no go" for CG is if the balance fits into the "window" on the wing,no matter where it is. You can go through all the loop de loops and stretchthis and that, but the final numbers are on the wing from the leading edge.Think this is the common denominator that Chuck was needing. :)walt evansNX140DL ----- Original Message -----
Ted,The final "go/no go" for CG is if the balance fits into the "window" on the wing,no matter where it is. You can go through all the loop de loops and stretchthis and that, but the final numbers are on the wing from the leading edge.Think this is the common denominator that Chuck was needing. :)walt evansNX140DL ----- Original Message -----
Re: Pietenpol-List: Center of Gravity Computations
Original Posted By: "Bert Conoly"
Subject: Re: Pietenpol-List: Center of Gravity ComputationsIn a message dated 2/15/04 5:01:41 PM Central Standard Time, ws133b341(at)cox.net writes:>Ted, Determining the E.W.C.G. could certainly be done as a percentage of Mean Aerodynamic Chord, but it would require an additional calculation. Keeping everything in inches, is just easier and simpler to compare. My intention is toshow a common baseline to compare. The datum called out in the plans is the Firewall. For comparison, everyone should keep it there, and deal with the math of negative numbers. Theproblem with this, is the negative arm, ahead of the firewall, which makes it more complicated when doing weight and balance. Positive times Positive equalsPositive. Negative times Negative equals Positive. Negative times Positive equals Negative. The contemporary location of the datum is ahead of the prop,that way all numbers are a positive number. One thing to keep in mind, that I forgot to mention yesterday, is that B.H.P. called out the LOADED Center of Gravity Range (envelope) is between 1/4to 1/3 of the chord. This is between 25% and 33 1/3% of the chord, or between 15" and 20", aft of the Leading Edge. Also keep in mind this is for the'Pietenpol FC10' airfoil. And remember, this is the LOADED Center of Gravity range, not an Empty Weight Center of Gravity Range. There is no E.W.C.G range.Fly safe, and stay in your envelope !!Chuck GantzerNX770CGin serious need of warmer weather________________________________________________________________________________
Subject: Re: Pietenpol-List: Center of Gravity ComputationsIn a message dated 2/15/04 5:01:41 PM Central Standard Time, ws133b341(at)cox.net writes:>Ted, Determining the E.W.C.G. could certainly be done as a percentage of Mean Aerodynamic Chord, but it would require an additional calculation. Keeping everything in inches, is just easier and simpler to compare. My intention is toshow a common baseline to compare. The datum called out in the plans is the Firewall. For comparison, everyone should keep it there, and deal with the math of negative numbers. Theproblem with this, is the negative arm, ahead of the firewall, which makes it more complicated when doing weight and balance. Positive times Positive equalsPositive. Negative times Negative equals Positive. Negative times Positive equals Negative. The contemporary location of the datum is ahead of the prop,that way all numbers are a positive number. One thing to keep in mind, that I forgot to mention yesterday, is that B.H.P. called out the LOADED Center of Gravity Range (envelope) is between 1/4to 1/3 of the chord. This is between 25% and 33 1/3% of the chord, or between 15" and 20", aft of the Leading Edge. Also keep in mind this is for the'Pietenpol FC10' airfoil. And remember, this is the LOADED Center of Gravity range, not an Empty Weight Center of Gravity Range. There is no E.W.C.G range.Fly safe, and stay in your envelope !!Chuck GantzerNX770CGin serious need of warmer weather________________________________________________________________________________
Re: Pietenpol-List: Center of Gravity Computations
Original Posted By:
Chuck- The interesting thing about using a EWCG is that it's really easy touse it to determine how far out to extend an engine mount to accomodate adifferent engine, prop, cowling, or Heaven Forbid, adding ballast to get theloaded CG right. As long as you use a CG datum between the aircraft CG andthe engine CG, it's a simple exercise to build up a spreadsheet and playwith engine weights, prop weights, motor mount dimensions etc. I found thefirewall a great datum to use. But you're right that the EWCG has to bereferenced back to the LOADED CG at some point.I recently went through this exercise in doing the W&B for a BoredomeFighter. Had to move the engine WAY out front for a VW engine.It's all about the math.Bert----- Original Message -----
Chuck- The interesting thing about using a EWCG is that it's really easy touse it to determine how far out to extend an engine mount to accomodate adifferent engine, prop, cowling, or Heaven Forbid, adding ballast to get theloaded CG right. As long as you use a CG datum between the aircraft CG andthe engine CG, it's a simple exercise to build up a spreadsheet and playwith engine weights, prop weights, motor mount dimensions etc. I found thefirewall a great datum to use. But you're right that the EWCG has to bereferenced back to the LOADED CG at some point.I recently went through this exercise in doing the W&B for a BoredomeFighter. Had to move the engine WAY out front for a VW engine.It's all about the math.Bert----- Original Message -----
Pietenpol-List: Re: Center of Gravity Computations
Original Posted By: "Robert Haines"
Regarding the location of Datum, although I would normally argue for thefirewall, this situation would be better suited to a Datum of the leadingedge.The firewall is a great location for an individual aircraft to work outweight and balance. You could change engines or relocate the wing and theDatum does not change. Also during the building process, the firewall Datumis physically available once the fuselage takes shape, which is well beforethe engine or wings get mounted. It's hard to hang a tape measure on aspinner tip when the engine is not mounted.Although, for this database which is being compiled, the most helpfulinformation would be a compairison of the CG and it's location on the cordbetween different aircraft. Specifically, a trend could be seen in thatmost well flying aircraft all have their CG at the same cord location. Thiscould be used as a indicator to identify that an new aircraft may have aproblem if it's CG with respect to the cord is outside of the norm.Also, I use "cord" here as opposed to mean aerodynamic cord (MAC) simplybecause there is no taper to this wing and why confuse the issue. Also, itis my understanding that no one deviates from a wing with a 60" cord, sousing the leading edge as a Datum provides a stable location for stating CGlocation and their relationship (i.e. if I mention that CG is 15" from LE,it's safe to say that it's at 25% since the assumption is that cord is 60").Also, empty weight CG is not desirable since pilot weight is designed intothe aircraft. If a pilot were somewhat heavy and compensated for with aforward relocation of a battery or a weight, the EWCG would be forward whencompaired to other aircraft.Robert HainesDu Quoin, Illinois________________________________________________________________________________
Regarding the location of Datum, although I would normally argue for thefirewall, this situation would be better suited to a Datum of the leadingedge.The firewall is a great location for an individual aircraft to work outweight and balance. You could change engines or relocate the wing and theDatum does not change. Also during the building process, the firewall Datumis physically available once the fuselage takes shape, which is well beforethe engine or wings get mounted. It's hard to hang a tape measure on aspinner tip when the engine is not mounted.Although, for this database which is being compiled, the most helpfulinformation would be a compairison of the CG and it's location on the cordbetween different aircraft. Specifically, a trend could be seen in thatmost well flying aircraft all have their CG at the same cord location. Thiscould be used as a indicator to identify that an new aircraft may have aproblem if it's CG with respect to the cord is outside of the norm.Also, I use "cord" here as opposed to mean aerodynamic cord (MAC) simplybecause there is no taper to this wing and why confuse the issue. Also, itis my understanding that no one deviates from a wing with a 60" cord, sousing the leading edge as a Datum provides a stable location for stating CGlocation and their relationship (i.e. if I mention that CG is 15" from LE,it's safe to say that it's at 25% since the assumption is that cord is 60").Also, empty weight CG is not desirable since pilot weight is designed intothe aircraft. If a pilot were somewhat heavy and compensated for with aforward relocation of a battery or a weight, the EWCG would be forward whencompaired to other aircraft.Robert HainesDu Quoin, Illinois________________________________________________________________________________
Pietenpol-List: Re: Center of Gravity Computations
Original Posted By: "Robert Haines"
One additional thing about why I suggest listing CG from LE in inches (thisis in response to Ted's email), it's easier for the individuals anddecreases the likelihood of error. It is safer to personally have the rawdata and perform the calculations oneself than to have multiple individualsall performing a single calculation. The individuals would only then haveto operate a tape measure and a plumb bob. Not trying to say that the groupcan't do math, it's simply a scientifically smarter thing to do.Also, it should be stated at what position the aircraft is in when themeasurement is taken (it is assumed that the top longeron should be level,but again, this should be stated for record).Robert HainesDu Quoin, Illinois----- Original Message -----
One additional thing about why I suggest listing CG from LE in inches (thisis in response to Ted's email), it's easier for the individuals anddecreases the likelihood of error. It is safer to personally have the rawdata and perform the calculations oneself than to have multiple individualsall performing a single calculation. The individuals would only then haveto operate a tape measure and a plumb bob. Not trying to say that the groupcan't do math, it's simply a scientifically smarter thing to do.Also, it should be stated at what position the aircraft is in when themeasurement is taken (it is assumed that the top longeron should be level,but again, this should be stated for record).Robert HainesDu Quoin, Illinois----- Original Message -----
Pietenpol-List: Re: Center of Gravity Computations
Original Posted By: "Robert Haines"
Coffee just kicked in....What the heck was I thinking?! You can't measure CG with a tape measure!Gesh,Robert----- Original Message -----
Coffee just kicked in....What the heck was I thinking?! You can't measure CG with a tape measure!Gesh,Robert----- Original Message -----