If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Overweight takeoff / flight
Howdy again,
After reading NTSB reports that attribute the cause of the accident to exceeding the airplane's maximum takeoff weight, I began wondering about the effects of an overweight takeoff within C.G. limits. Specifically, what would I have to do differently when flying an airplane that's heavier than what the POH specifies. I am not supporting the practice, of course, so let it be purely educational. Corrections, additions, and comments welcome. I would start by considering the increase in weight as comparable to an increase in load factor. Hence, all your aoa-related speeds would increase by the square root of the load factor. Vs, Vx, Vy, Vglide, etc. would all increase. Va would also go up. Now, by virtue of rotation speed being a function of stall speed, I conjecture you'd have to liftoff at a faster airspeed which would equate to a longer takeoff roll. Then, after pitching for your faster Vy airspeed, you'd notice a decrease in climb rate at full power due to the increased power requirement. During cruise, you'd notice a reduced cruise speed and an increase in stall speed. At approach to landing, should you bump up your approach speed, you'll find yourself sinking faster when chopping off the power even though your glideslope will remain the same. Since your stall speed is invariably higher, you'll eat up more runway when landing. So to sum up: Takeoff: higher takeoff distance, higher rotation speed. Climbout: lower climb rate at higher Vy speed, same angle of climb for obstacle clearance at higher Vx speed. Should Vx not be flown faster, a poorer angle of climb would result, making obstable clearance doubtful. *I may be wrong here* I am not sure if the max. angle of climb is constant regardless of weight...my calculations don't show so...could someone clarify? Cruise/Maneuvering: lower cruise speed, higher maneuvering speed, higher clean stall speed. Approach to maintain glideslope & descent profile: higher approach speed, higher sink rate for a given power setting. Higher dirty stall speed. Landing: higher landing distance Question (1 of 2): Seems to me that flying "overweight" is possible if you're aware of the performance reductions. So why do you read so many NTSB reports with probable causes listed as "overweight takeoff, exceeded performance limitations"? As you slowly pull the yoke to rotate, wouldn't a pilot *realize* through control forces, feel, gut feeling that something is wrong? Question (2 of 2): When considering accidents due to exceeding maximum takeoff weight, do the majority occur during takeoff? If so, is it typically due to not reaching proper liftoff airspeed for that increased weight, stalling, and spinning to the ground? Would this scenario be consistent with failure to set the flaps/slats to their takeoff value? Alex |
#2
|
|||
|
|||
|
#3
|
|||
|
|||
In article , Koopas Ly
wrote: Approach to maintain glideslope & descent profile: higher approach speed, higher sink rate for a given power setting. Higher dirty stall speed. Landing: higher landing distance Once you have burned sufficient fuel to bring the weight back down into the max gross/useful load range, why should landing parameters be any different from published? Published numbers are for max gross weight. |
#4
|
|||
|
|||
Koopas Ly wrote: Howdy again, After reading NTSB reports that attribute the cause of the accident to exceeding the airplane's maximum takeoff weight, I began wondering about the effects of an overweight takeoff within C.G. limits. Seems to me that you have listed most of the effects correctly. One thing you should consider, however, is the fact that the balance envelope for most (if not all) planes gets narrower at the top. In other words, the more weight you put in an aircraft, the closer to the center of lift that weight has to be. At some point, all of the weight will have to be in the front seat. I have read of cross-Atlantic ferry flights in which the aircraft was loaded to weigh about 1.6 times the normal MGW. In one account, a Bonanza loaded that way took over 6,000' to get airborne. George Patterson A man who carries a cat by the tail learns something that can be learned no other way. |
#5
|
|||
|
|||
An old pilot once told me, when I was a young pilot, "...sumbitch flies a
hell of a lot better overweight than it does outta gas..." JG |
#6
|
|||
|
|||
"Koopas Ly" wrote in message
om... I would start by considering the increase in weight as comparable to an increase in load factor. Hence, all your aoa-related speeds would increase by the square root of the load factor. Vs, Vx, Vy, Vglide, etc. would all increase. Va would also go up. I take issue with Va. At first thought, it should go up as sqrt( m/m0) with m the new weight and m0 the maximum gross at which Va is quoted. This since at a higher Va, we can maintain the same AOA as we did at m0, so the G forces at stalling AOA never exceed the design limitations. BUT, there are 2 things (at least) which contribute to the setting of Va in the first place. One is the limitation of 'heavy things in the plane', such as a bag of sand in the baggage compartment. If this is the limiting factor, then Va should indeed scale as sqrt(m/m0). However, there is also the 'torque on the wings' (low wings) or 'force on the wings' (struts on Cessna). If you are pulling 3.5G with a higher gross weight, you'll be exerting more force than was designed for at certified gross. So to be safe (hah!, we're talking about overloading dammit), then unless you know exactly which type of failure limits Va in the first place, you'd be best off using Va for certified gross and not scaling it up. -- Dr. Tony Cox Citrus Controls Inc. e-mail: http://CitrusControls.com/ |
#7
|
|||
|
|||
BUT, there are 2 things (at least) which contribute to the setting
of Va in the first place. Neither one of the things you mentioned is given in Part 23 as a requirement for Va. Part 23 uses the speed solely to provide the design requirements of the elevators, ailerons, and rudder. Since the forces on these control surfaces will not vary with weight, you certainly can't scale it up. |
#8
|
|||
|
|||
When considering takeoff parameters, don't forget the increased rolling
resistance of overloaded tires...this will affect acceleration and takeoff distance. Bob Gardner "Koopas Ly" wrote in message om... Howdy again, After reading NTSB reports that attribute the cause of the accident to exceeding the airplane's maximum takeoff weight, I began wondering about the effects of an overweight takeoff within C.G. limits. Specifically, what would I have to do differently when flying an airplane that's heavier than what the POH specifies. I am not supporting the practice, of course, so let it be purely educational. Corrections, additions, and comments welcome. I would start by considering the increase in weight as comparable to an increase in load factor. Hence, all your aoa-related speeds would increase by the square root of the load factor. Vs, Vx, Vy, Vglide, etc. would all increase. Va would also go up. Now, by virtue of rotation speed being a function of stall speed, I conjecture you'd have to liftoff at a faster airspeed which would equate to a longer takeoff roll. Then, after pitching for your faster Vy airspeed, you'd notice a decrease in climb rate at full power due to the increased power requirement. During cruise, you'd notice a reduced cruise speed and an increase in stall speed. At approach to landing, should you bump up your approach speed, you'll find yourself sinking faster when chopping off the power even though your glideslope will remain the same. Since your stall speed is invariably higher, you'll eat up more runway when landing. So to sum up: Takeoff: higher takeoff distance, higher rotation speed. Climbout: lower climb rate at higher Vy speed, same angle of climb for obstacle clearance at higher Vx speed. Should Vx not be flown faster, a poorer angle of climb would result, making obstable clearance doubtful. *I may be wrong here* I am not sure if the max. angle of climb is constant regardless of weight...my calculations don't show so...could someone clarify? Cruise/Maneuvering: lower cruise speed, higher maneuvering speed, higher clean stall speed. Approach to maintain glideslope & descent profile: higher approach speed, higher sink rate for a given power setting. Higher dirty stall speed. Landing: higher landing distance Question (1 of 2): Seems to me that flying "overweight" is possible if you're aware of the performance reductions. So why do you read so many NTSB reports with probable causes listed as "overweight takeoff, exceeded performance limitations"? As you slowly pull the yoke to rotate, wouldn't a pilot *realize* through control forces, feel, gut feeling that something is wrong? Question (2 of 2): When considering accidents due to exceeding maximum takeoff weight, do the majority occur during takeoff? If so, is it typically due to not reaching proper liftoff airspeed for that increased weight, stalling, and spinning to the ground? Would this scenario be consistent with failure to set the flaps/slats to their takeoff value? Alex |
#9
|
|||
|
|||
| | Question (1 of 2): Seems to me that flying "overweight" is possible if | you're aware of the performance reductions. So why do you read so | many NTSB reports with probable causes listed as "overweight takeoff, | exceeded performance limitations"? As you slowly pull the yoke to | rotate, wouldn't a pilot *realize* through control forces, feel, gut | feeling that something is wrong? You would not necessarily feel heavier control forces if the airplane was trimmed properly. Heavier control forces as you rotate would indicate a forward cg, not over weight. You could be grossly over weight and have very light control forces if the weight was mostly in the back. Most noticeable is that the airplane does not accelerate as quickly as usual. If you are in the habit of flying overvweight, you might not notice anything wrong at all. Add in a hot day, short runway, and high altitude and suddenly you are going to find yourself bitten by bad habits. | | Question (2 of 2): When considering accidents due to exceeding maximum | takeoff weight, do the majority occur during takeoff? If so, is it | typically due to not reaching proper liftoff airspeed for that | increased weight, stalling, and spinning to the ground? Would this | scenario be consistent with failure to set the flaps/slats to their | takeoff value? Many airplanes take off from normal runways without flaps. A pilot can easily forget to set flaps for short or soft field takeoffs. A lot of pilots are also taught just 'plane' wrong. Consider the Cessna 172M, for example. Most pilots are taught to set the flaps at 10 degrees for a short field takeoff. Most aftermarket checklists tell you to do this, even the ones designed for older Cessnas. Surecheck sells checklists that are supposedly designed specifically for the 172M but they contain this error. But read the manual. It tells you that if you set the flaps at 10 degrees you will lift off the runway more quickly, but that you will climb more slowly and you might not clear an obstacle at the end of the runway. The manual says to use 10 degrees of flaps only when the runway is soft or is short but there are no obstacles on climbout. But the idea that you use 10 degrees of flaps to do a short field takeoff is so pervasive that I have had train my students in how to educate examiners on this issue. Newer Cessna 172s use 10 degrees of flaps for all short field takeoffs, so when transitioning from one model of Cessna 172 to another, be sure to read the manual thoroughly. |
#10
|
|||
|
|||
In article , Greg Esres
wrote: BUT, there are 2 things (at least) which contribute to the setting of Va in the first place. Neither one of the things you mentioned is given in Part 23 as a requirement for Va. Part 23 uses the speed solely to provide the design requirements of the elevators, ailerons, and rudder. Since the forces on these control surfaces will not vary with weight, you certainly can't scale it up. G-forces are directly related to weight. Since the size of the control surface is directly related to the forces exerted on it, control surfaces are dependent on weight. |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
RAF Blind/Beam Approach Training flights | Geoffrey Sinclair | Military Aviation | 3 | September 4th 09 06:31 PM |
AOPA Stall/Spin Study -- Stowell's Review (8,000 words) | Rich Stowell | Aerobatics | 28 | January 2nd 09 02:26 PM |
us air force us air force academy us air force bases air force museum us us air force rank us air force reserve adfunk | Jehad Internet | Military Aviation | 0 | February 7th 04 04:24 AM |
"I Want To FLY!"-(Youth) My store to raise funds for flying lessons | Curtl33 | General Aviation | 7 | January 9th 04 11:35 PM |
AOPA Stall/Spin Study -- Stowell's Review (8,000 words) | Rich Stowell | Piloting | 25 | September 11th 03 01:27 PM |