aircraft weight and C of G calculations
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aircraft weight and C of G calculations
Am currently in the process of studying for a Performance and Operations exam. One element of the subject has got me thinking...
Excuse the simplicity of the following question but...
Just how are `empty weight` details (as found in flight manuals) generated??? Is it really that simple - ie stick the heli on a giant set of scales?? If so I would have thought the scales would not be accurate enough...
For example the 206L-1 Longranger IIextract I will be using for my up and coming exam lists weight as follows...
as weighed empty weight 1020.6kg...
I am just haveing trouble getting my head around a set of scales (or hoist or whatever) that can measure to these tolerances - .6kg!!!
Or do they just run through the heli part by part, list associated weights and add up all the parts???
Also (unless I have overlooked it) the extract for this type (206L-1 Longranger II) does not specify a CofG position for sling loads etc along the longitudinal arm I realise that a load on a long line during flight will be dynamic and not remain in a fixed position but the weight will still be acting through one point on the body of the heli and would have some influence on overall Cof G positions when considering other weight, fuel burn etc......so what CofG `arm` length should be used for these calculations?? Logic suggests to me that the dimension given for the rotor mast (121.4inches, 3084mm) is the number of choice....information anyone
Excuse the simplicity of the following question but...
Just how are `empty weight` details (as found in flight manuals) generated??? Is it really that simple - ie stick the heli on a giant set of scales?? If so I would have thought the scales would not be accurate enough...
For example the 206L-1 Longranger IIextract I will be using for my up and coming exam lists weight as follows...
as weighed empty weight 1020.6kg...
I am just haveing trouble getting my head around a set of scales (or hoist or whatever) that can measure to these tolerances - .6kg!!!
Or do they just run through the heli part by part, list associated weights and add up all the parts???
Also (unless I have overlooked it) the extract for this type (206L-1 Longranger II) does not specify a CofG position for sling loads etc along the longitudinal arm
I realise that a load on a long line during flight will be dynamic and not remain in a fixed position but the weight will still be acting through one point on the body of the heli and would have some influence on overall Cof G positions when considering other weight, fuel burn etc......so what CofG `arm` length should be used for these calculations?? Logic suggests to me that the dimension given for the rotor mast (121.4inches, 3084mm) is the number of choice....information anyone
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GE.
As to the weighing with the good old Sea King they put scales under each wheel, take the figures & do some fancy maths relating to distance from the a/c datum point (subtract the number they first thought of and add their girlfriends weight) to get a weight to 10 decimal places. It will be close but I can guarantee it will not be totally correct. But so what!! It's close enough!!
Also load slings should be positioned relative to the centre of lift (main gear box) not c of g 'cos you'll get all sorts of bending moments in the frame. The SK has a 17 inch range for c of g could give some good stresses with a 6000lb load.
Have fun with them there exams!!
As to the weighing with the good old Sea King they put scales under each wheel, take the figures & do some fancy maths relating to distance from the a/c datum point (subtract the number they first thought of and add their girlfriends weight) to get a weight to 10 decimal places. It will be close but I can guarantee it will not be totally correct. But so what!! It's close enough!!
Also load slings should be positioned relative to the centre of lift (main gear box) not c of g 'cos you'll get all sorts of bending moments in the frame. The SK has a 17 inch range for c of g could give some good stresses with a 6000lb load.
Have fun with them there exams!!
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c of g longranger problems
Crazy problems to solve for the longranger P & O questions!!...has brought back bad memories of grade 7 maths classes....
`...two trains leave from different stations on travelling at 93 kpm the other...blah, blah, blah...`
you get the picture
Also Charlie great points about the hardpoint position....had only considered limitations along longitudinal and lateral axis. Normal axis did not enter my head!
Have there ever been `smart` ballast stations used onboard larger types (military, search/rescue for winching etc)- ie moveable ballast to counteract c of g movements.
Also during winching operations how does the crew know that while engaged in winching process the added weight of the crewman PLUS the victim will not exceed max lateral C of G limitations...or is the ship set up to deal with a victim who is `well above the average weight`
`...two trains leave from different stations on travelling at 93 kpm the other...blah, blah, blah...`
you get the picture
Also Charlie great points about the hardpoint position....had only considered limitations along longitudinal and lateral axis. Normal axis did not enter my head!
Have there ever been `smart` ballast stations used onboard larger types (military, search/rescue for winching etc)- ie moveable ballast to counteract c of g movements.
Also during winching operations how does the crew know that while engaged in winching process the added weight of the crewman PLUS the victim will not exceed max lateral C of G limitations...or is the ship set up to deal with a victim who is `well above the average weight`
Who in their right mind would put ballast (dead weight) on an aircraft just to compensate for a cg change? Bad enough to have to carry an extra battery in the nose of a 76B to make up for the heavier PT6 engines down the back, but at least a battery is useful.
The cg doesn't move all that much - moving a 90 kg pilot by 6 inches makes a poofteenth of an inch difference in the overall cg, so you would need a big ballast weight a long way from the cg to make a difference. Moving fuel around is how big planes do it, but helo fuel tanks are kept pretty much around the cg anyway. There is a system to move the battery along the tail boom in some Eurocopter birds.
For hoisting, there is a supplement in the manual to tell you how it goes. It is unlikely to ask that in the exam, just the simple stuff of the fuel moving in the LongBanger, and trying to catch you out at the most forward or most aft position.
The cg doesn't move all that much - moving a 90 kg pilot by 6 inches makes a poofteenth of an inch difference in the overall cg, so you would need a big ballast weight a long way from the cg to make a difference. Moving fuel around is how big planes do it, but helo fuel tanks are kept pretty much around the cg anyway. There is a system to move the battery along the tail boom in some Eurocopter birds.
For hoisting, there is a supplement in the manual to tell you how it goes. It is unlikely to ask that in the exam, just the simple stuff of the fuel moving in the LongBanger, and trying to catch you out at the most forward or most aft position.
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The scales used are not like a industrial version of the bathroom type but are infact load cells (which are normally half the size of a drink can) that are connected with wires to a box that reads each cell. You attach these to the tops of jacks and it measures the weight at each jacking point and when you swap them around three times and divide the sum of all readings by three you are left with the basic weight of the helicopter. The accesories are added onto that with a known weight from other sources eg STC's.
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.6kg isn't exactly close tolerance. The scales used are far more accurate, and the effect of any breeze in the hangar has to be accounted for. Yes, they put them on scales, usually 3 (one at each jacking point) and read the result. Now it's done with electronic scales, which are very accurate. To get the CG, you take the weight at each jacking point, which is a known station, and do some simple arithmetic. Our aircraft have to be weighed periodically, and are always weighed when they come out of refurb.
Senis Semper Fidelis
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Good morning G Eff,
Worry not about the weighing, I have a 50 tonnes overhead crane in one of my warehouses, it has an attachment that will allow anything to be weighed to a tolerance of 1 ounce( ie +/-half Oz) or whatever that is in metric, I use it to check weight coils of steel for transport and many weigh in the 25tonne range, so my customers are kept happy (and solvant)
Vfr
Worry not about the weighing, I have a 50 tonnes overhead crane in one of my warehouses, it has an attachment that will allow anything to be weighed to a tolerance of 1 ounce( ie +/-half Oz) or whatever that is in metric, I use it to check weight coils of steel for transport and many weigh in the 25tonne range, so my customers are kept happy (and solvant)
Vfr
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Please don't be fooled by the 0.1 lb on the weight statement (or the similar marking on the digital scale).
While these show a degree of precision (which denotes how fine the scale is), they should not be confused with accuracy, which denotes how close to the truth the measurement is. Most high quality scales are good to about 0.2 to 0.5% of the reading, so that a 4000 lb object is known to about 10 lbs.
While these show a degree of precision (which denotes how fine the scale is), they should not be confused with accuracy, which denotes how close to the truth the measurement is. Most high quality scales are good to about 0.2 to 0.5% of the reading, so that a 4000 lb object is known to about 10 lbs.
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Doncha love 'im?
When I was 18, my Physics teacher took a 40-minute lesson to describe the difference between precision and accuracy, and at the end I was none the wiser.
Now I'm 45, I read three lines by Nick, and it becomes clear.
Sorry to go off-topic a little, but this ability Nick has (in addition to all his others) is worthy of comment.
When I was 18, my Physics teacher took a 40-minute lesson to describe the difference between precision and accuracy, and at the end I was none the wiser.
Now I'm 45, I read three lines by Nick, and it becomes clear.
Sorry to go off-topic a little, but this ability Nick has (in addition to all his others) is worthy of comment.
Ascend Charlie
Not that I disagree with you, but for interests sake:
I have seen a well known Australian film pilot bolt specially designed lead weights around the tail rotor gearbox of a Jet Ranger to counteract a heavy nose mounted camera. To try to get the C of G within limits by using ballast in the baggage compartment required far too much weight.
Not the kind of thing you see every day, and the amount of lead weight astounded me, it was quite a few kilos!
Who in their right mind would put ballast (dead weight) on an aircraft just to compensate for a cg change?
I have seen a well known Australian film pilot bolt specially designed lead weights around the tail rotor gearbox of a Jet Ranger to counteract a heavy nose mounted camera. To try to get the C of G within limits by using ballast in the baggage compartment required far too much weight.
Not the kind of thing you see every day, and the amount of lead weight astounded me, it was quite a few kilos!
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Hilico,
Thanks for the compliment! My general belief is that when someone truly understands a subject, their explanations get easier to understand. Those who start off an explanation with "it is complicated" are usually telling you "I really don't know the answer, but here goes....."
Also, a good basic premise is that most intelligent people (about 2/3 of the human race, in my book, and maybe virtually all ppruners) can understand almost anything if the explainer understands the subject and wants them to know the answer. Those are two big if's!
Mr. Selfish,
I can't readily understand why the arm times weight does not equal the moment, because it really is that simple. The only possibility is that the as-weighed weight is corrected with standard items (each at a fixed location) to produce a corrected weight and moment, which is then used as the pilot's basis.
For example, if the weighing was done unfueled (often done for new manufacture and when coming out of major maintenance), the mechanic will adjust for "trapped fluids" to achieve the operating zero fuel weight. These trapped fluids include the fuel that is below zero on the gage (unusable fuel).
I wonder if this gets you closer to the answer.
Thanks for the compliment! My general belief is that when someone truly understands a subject, their explanations get easier to understand. Those who start off an explanation with "it is complicated" are usually telling you "I really don't know the answer, but here goes....."
Also, a good basic premise is that most intelligent people (about 2/3 of the human race, in my book, and maybe virtually all ppruners) can understand almost anything if the explainer understands the subject and wants them to know the answer. Those are two big if's!
Mr. Selfish,
I can't readily understand why the arm times weight does not equal the moment, because it really is that simple. The only possibility is that the as-weighed weight is corrected with standard items (each at a fixed location) to produce a corrected weight and moment, which is then used as the pilot's basis.
For example, if the weighing was done unfueled (often done for new manufacture and when coming out of major maintenance), the mechanic will adjust for "trapped fluids" to achieve the operating zero fuel weight. These trapped fluids include the fuel that is below zero on the gage (unusable fuel).
I wonder if this gets you closer to the answer.
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quote:
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Who in their right mind would put ballast (dead weight) on an aircraft just to compensate for a cg change?
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If flying an R22 solo and you weigh less than 170#, you are required to add ballast to remain within CG limits.
On all B-206 helicopters the cargo hook is located directly under the mast CG location. With any external load the a/c will always be within CG limiits (assuming the a/c was within limits with pilot, fuel, etc. before hooking up.) External loads do not affect CG on the 206 series.
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Who in their right mind would put ballast (dead weight) on an aircraft just to compensate for a cg change?
--------------------------------------------------------------------------------
If flying an R22 solo and you weigh less than 170#, you are required to add ballast to remain within CG limits.
On all B-206 helicopters the cargo hook is located directly under the mast CG location. With any external load the a/c will always be within CG limiits (assuming the a/c was within limits with pilot, fuel, etc. before hooking up.) External loads do not affect CG on the 206 series.
Mr Selfish
Yes, I have noticed the same. The difference is never enough to have any impact on the end result. However, I have never found the given moment agrees with my calculator and have never had a sensible explanation!
TeeS
Yes, I have noticed the same. The difference is never enough to have any impact on the end result. However, I have never found the given moment agrees with my calculator and have never had a sensible explanation!
TeeS
OK, what I REALLY meant about ballast for CG corrections was building the aircraft with a permanent adjustable ballast.
We have all had occasions where we had to carry a temporary ballast, whether it was for a hugely fat slug who insisted on being in the front seat (requiring 2 x 25-litre drums of water in the boot) to the afore-mentioned lead weights on the T/R gearbox because of the FLIR on the nose.
One of the dopiest things I saw was when our air force bought the first Iroquois helicopters from the US, they came equipped with Tacan. In Oz in the 60s, Tacan was only at the military bases, generally only one or two per state, and limited in range. Our military geniuses thought that because helos operated at low level out in the bush, Tacan would not be very useful, but instead of just turning it off, they paid to have the Tacan removed (after already having bought them) and replaced with an equivalent lead weight in the nose!!
We have all had occasions where we had to carry a temporary ballast, whether it was for a hugely fat slug who insisted on being in the front seat (requiring 2 x 25-litre drums of water in the boot) to the afore-mentioned lead weights on the T/R gearbox because of the FLIR on the nose.
One of the dopiest things I saw was when our air force bought the first Iroquois helicopters from the US, they came equipped with Tacan. In Oz in the 60s, Tacan was only at the military bases, generally only one or two per state, and limited in range. Our military geniuses thought that because helos operated at low level out in the bush, Tacan would not be very useful, but instead of just turning it off, they paid to have the Tacan removed (after already having bought them) and replaced with an equivalent lead weight in the nose!!
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In the early S76 days when completion shops vied for radio installation business, we delivered several S-76A's without a radio suite, usually just one comm and one nav to let them fly to the completion center. On those aircraft, we poured clever concrete weights to fit on the radio racks! Looked nice, some were even placarded by wise guy crews with nomenclature like a real radio.
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The arm for the cargo hook should be found in the appropriate supplement, at the back of the flight manual.
As for ballast, the 205's I last flew had ballast in the nose, as the owner was too cheap to fit a second battery, and glass windscreens. Mind you, the one that did have dual batteries, still needed a few plates of lead up front.
As for ballast, the 205's I last flew had ballast in the nose, as the owner was too cheap to fit a second battery, and glass windscreens. Mind you, the one that did have dual batteries, still needed a few plates of lead up front.
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Every 206 I ever flew had lead ballast in the nose. As for why it was there, the mechanics just said that it had to be there for CG purposes. I recall that one ship I flew regularly developed a metallic pinging sound, and I couldn't find the source for some time, but it began to worry me. During a stop I looked closely in the nose area, and found one of the bolts holding the ballast had backed out about a quarter of an inch, and the loose washer was hitting the bolt head when it vibrated in flight. A quick turn of the wrench by the mechanic fixed my noise. Funny what you can hear a hundred miles or so from land. 
