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# Need Help in Take off weight estimation methods for a given mission profile

Question asked by blacrobous in #Coffee Room on Aug 23, 2009
blacrobous 路 Aug 23, 2009
Rank D2 - MASTER
Well hello all My name is Abhilash iam new to this forum.

Ive been facing a very hard problem for like 2 months or so for just estimating the take off weight of an aircraft.

Ive Used 3 books so far

1.) Design of aircraft by thomas corke
2.) Jon Roskam's 8 volumes on design of aircraft
3.) AIAA series on aircraft design by daniel P raymer

We know that for every aircraft may be a civil or a combat it has its own mission profile,Range,etc... parameters.

The design values i took just for testing were from thomas corke' textbook.
When i did continous iterations using the formulaes mentioned i found out that i was getting around 7000lbs of difference from the mentioned spreadsheet values.I did it with advanced calculators,Used every piece of mathematical software to solve the problem. Roskam and raymers methods were out of the stage since they were too difficult to understand since i am still a novice.

Could Anyone specifically a aeronautical or an aerospace design engineer suggest me or find me a better easier accurate method of estimating the take off weight of particulary a civil aircraft ( forget about combat).

Thank you😁 Posted in: #Coffee Room
imjiant 路 Aug 28, 2009
Rank E1 - BEGINNER
Hello Abhilash,

The take off weight of an aircraft can have several names and factors. For instance, it is common to see a take off weight associated with a ramp weight (usually on on larger AC) although they are quite different. Is the take of weight you're seeking a Wto or MTOW or the like? If so and you're looking for a number based soley on the peformance aspects of the craft (i.e. not what is structurally possible), then the value you seek is most often dictated by the stall speed (Vs, Vs0, Vs1, typically) as this is the lowest value for given design parameters that the craft can achieve flight. The equation for this that I use is Vs=((W/S)/(rho Clmax))^1/2. Where Vs is, of course, the stall speed, W/S is the wing loading--the ratio of weight to planform wing area, rho is the air desity for the altitude and conditions in question and Clmax is the maximum achievable lift coefficient for the wing (i.e. using any lift altering apparatus such as flaps or slats). If you are sizing to another parameter or performance characteristic, let me know and I'll help as I can. Also, be sure to check your units. I know this is a simple point and I don't mean to offend in this suggestion, but it is all too easy to find yourself orders of magnitude off due to such an oversight.

J
blacrobous 路 Aug 30, 2009
Rank D2 - MASTER
well these values iam taking are structurally possible values or to estimating fuel consumption on a whole for a already working aircraft

for example i take some civil aircraft like an A320 i get its specifications and i would write down a mission profile for it. Then i want to calculate for example fuel consumed by this aircraft for a journey of lets say 1000 nautical miles. I need to calculate fuel consumption at each stage like take off , cruise , loitering etc.. The total fuel consumed + my estimated weight would give me the total weight of the aircraft before take off

the main parameters i used till now are machnumber, aspect ratio , structural factor, l/d ratio, coeff of lift , drag ,engine TSFC values ( min and max),altitudes of flying,payloads,etc..
imjiant 路 Aug 31, 2009
Rank E1 - BEGINNER
I see. So you already have zero-fuel weight for the ac in question and via the profile you're trying to find the weight of fuel consumed. Using the mission profile for this can be very tedious and will require iterations since, for example, the rate of climb (ROC) is: ROC=excess power/weight; and the altitude you choose for the cruise and the time to climb will affect the fuel consumed, therefore initial weight, therefore ROC and so on. However, if for a 170000lb ac you're only off by less than 5% using this method, that's not bad. Burning somewhere in the range of 40lbs/min (I think that's pretty close to an A320 burn) if you add on one hour of reserves, that drops your error by 2400lbs.
blacrobous 路 Aug 31, 2009
Rank D2 - MASTER
that iterations are giving me problems that is why i want any new method that is in existence that could solve it 馃槢

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