Mission Segment Weight Fractions 2

[Ijfe39]

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
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]

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.

good luck with your project.

 

[IRstuff]

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

FAQ731-376

 

[rb1957]

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]

I left out the rest of the example for brevity. Here's the whole thing:

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).

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

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
7)Land W7/W6 = 0.995

Can be found as a worksheet from the following:

http://www.aircraftdesign.com/ASW.xls

Thanks.

 

[rb1957]

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]

"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.

 

[rb1957]

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]

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

FAQ731-376

 

[Ijfe39]

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]

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,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484