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Ijfe39 (Industrial)
20 Jul 09 7:05
Hello,
I'm currently reading Aircraft Design:A Conceptual Approach by Raymer. On page 31 there is an example of sizing calculations performed on an anti-submarine aircraft:
Mission Segment Weight Fractions:
1) Warmup and takeoff W1/W0 = 0.97
2) Climb W2/W1 = 0.985
3) Cruise R = 9,114,000 ft
C = 0.0001389 1/s
V = 0.6M x (994.8 ft/s) = 596.9 ft/s 字串7
L/D = 16 X 0.866 = 13.9
W3/W2 = e{-RC/VL/D} = e-0.153 = 0.858
Where would you perform iteration here until your only off by 1% of the desired weight? The aircraft is assumed to weight 10,000 lbs. Thanks.
rb1957 (Aerospace)
20 Jul 09 8:29
looking at fig 6.1 in Raymer, it does say "iterate".
taking nothing from Dan Ramyer, and assuming your W3/W2 saying that travelling 9E6ft, = 2E3 miles, consumes "only" 15% of the weight of the plane; i'd say that doesn't make sense.
look at the fuel fraction of any large long range transport, fuel makes up something like 40-60% of the AUWt. 字串4
good luck with your project.
IRstuff (Aerospace)
20 Jul 09 10:41
The distance is only 1726 mi, which seems awfully short for a full search patrol. I think the calculations ignore on-station time, which might be double or triple the specified range. Also, not clear whether the range specified is one-way or two-way.
TTFN
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rb1957 (Aerospace)
20 Jul 09 11:00
it is a small a/c, at 10k AUWt; and yes the calc does ignore loiter time (but loiter means to me, stooging 'round investigating a suspect, as opposed to the surveilence cruise; i guess you could build 15min reserve into the mission weights.
Ijfe39 (Industrial)
20 Jul 09 13:05
I left out the rest of the example for brevity. Here's the whole thing: 字串5
ASW Requirements
The key requirements for this mission for our hypothectical antisubmarine would be: the ability to loiter for 3 hrs at a distance of 1500n miles from the takeoff point. While loitering on-station, this type of aircraft uses sophisticated electronic equipement to detect and track submarines. For the sizing example, this equipment is assumed to weigh 10,000 lb. Also, a four-man crew is required, totalling 800 lbs. The aircraft must cruise at 0.6 Mach number.
L/D Estimation
For initial sizing, a wing aspect ratio of about 10 was selected. With the area of the wing and canard both included, this is equivalent to a combined aspect ratio of about 7. The author then gathered the wetted area ratio (Swet/Sref) from a chart in this chapter from historical trend(actually comparing this design to a Beech Starship)This yielded a wetted aspect ratio of 1.27(i.e., 7/5.5). Finally a maximum lift-to-drag ratio of about 16 was given(again from historical trend).
字串6
ASW sizing calculations
Mission Segment Weight Fractions(Britsh Units)
1) Warmup and takeoff W1/W0 = 0.97
2) Climb W2/W1 = 0.985
3) Cruise R(Range)= 1500 n.mi=9,114,000ft
C(fuel consumption)=0.0001389 1/s
V(velocity)=0.6Mx(994.8 ft/s)=596.9 ft/s
L/D(lift/drag)=16x0.866 =13.9
W3/W2=e{-RC/VL/D}=e-0.153 = 0.858 字串6
4)Loiter E=3 hrs = 10.800 s
C=0.4 1/hr = 0.0001111 1/s
L/D = 16
W4/W3 = e{-EC/L/D} = e-0.075 = 0.9277
5) Cruise(same as 3) W5/W4 = 0.858
6) Loiter E= 1/3 hr = 1200 s
L/D = 16
W6/W5 = e-0.0083 = 0.9917 字串4
7)Land W7/W6 = 0.995
Can be found as a worksheet from the following:
http://www.aircraftdesign.com/ASW.xls
Thanks.
rb1957 (Aerospace)
20 Jul 09 13:25
1) "equipment" is not the same as "aircraft" ... more like payload (originally you said "the a/c weighs 10,000 lbs")
2) the problem answers your question ... when your guess (B21) equals the calculated result (B35) you're finished.
3) "they" frown on student posts
Ijfe39 (Industrial)
20 Jul 09 13:35
"3) "they" frown on student posts" - where else can I get assistance. This wasn't the first site I actually visited. There were 4 other sites I went to including a blog site from Raymer himself:http://02028f5.netsolhost.com/aircraftdesignblog/?p=1#comment-116 whom hasn't answered as of yet. This is the only site that has provided such helpful assistance. And anyways I'm not a student.
字串3
rb1957 (Aerospace)
20 Jul 09 14:04
you may not be a student, but you are working on an assignment (as opposed to real live work).
and thx for the star !
IRstuff (Aerospace)
20 Jul 09 14:17
In any case, when you do get to the end of the chain of fractions, the plane is at 66.4% of takeoffweight. But, in such design, 1% accuracy on this number is NOT the endpoint. The analysis conditions described essentially are IDEAL conditions. Throw in head winds, storm detours, etc., and you'll find that you need to put in a fairly substantial reserve, to ensure the survival of the plane and crew, assuming you don't want them to be ditching the plane in the middle of the ocean. TTFN
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Ijfe39 (Industrial)
21 Jul 09 12:35
字串2
Thanks IRstuff. I know what you mean but this is just the initial introduction to sizing an aircraft by the author before actually getting into more of the specifics later on. Appreciate everyone's replies.
KENAT (Mechanical)
29 Jul 09 17:20
Ijfe39 been a while since I used the book.
I'm not sure about some of your assumptions, take a look at what the S3 did, or even the Tracker, even allowing for newer materials & smaller electronics, I'm not sure you mass makes sense, depending what mass it is.
http://en.wikipedia.org/wiki/S-3_Viking
KENAT,
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