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Ikarus
Topic Author
Posts: 3399
Joined: Mon Jan 08, 2001 10:18 pm

### Need Help With Lift/drag Coefficients +AoA

Hello!

We're finishing off our Aerospace Vehicle Design coursework - the conceptual design of a 88-seat regional jet. I am respponsible for wing & flap design and the aerodynamic analysis. In a few hours I'll have to hand in my report, but I have no idea whether the values I calculated are realistic or not. This is where I need your help. Could anyone give me some indication whether or not these results are acceptable?

1) Cruise
lift coefficient = 0.323
maximum lift coefficient of wing = 1.25
total drag coefficient = 0.0265
L/D = 12.18
angle of attack (wing mean aerodynamic chord) = 2 degrees

2) takeoff - before rotation
lift coefficient = 0.948
maximum lift coefficient with flaps = 1.47
zero lift drag coefficient (Cd0) = 0.0963
L/D = 6.09 (ignoring trim drag)

3) liftoff
lift coefficient = 1.215
angle of attack (wing MAC) = 4.38 degrees

4) initial climb (with takeoff flaps)
lift coefficient = 1.021
angle of attack (wing MAC) = 2.651 degrees

5) approach
lift coefficient = 1.23
max lift coefficient with flaps = 2.077
angle of attack (wing MAC) = -0.091 degrees
zero lift drag coefficient = 0.1115
L/D = 5.814 (ignoring trim drag)

6) above runway / just before touchdown
lift coefficient = 2.077
angle of attack (wing MAC) = 7.45 degrees

The wing is mounted at an angle of attack of 2 degrees towards the fuselage, so if you are a pilot, you'd experience 0 AoA during cruise, -2 degrees on approach, 5 degrees on touchdown etc. etc.

The reason why I'm asking is that some of the zero-lift drag values cause problems in performance analysis - my coworkers suggested they'd prefer the values to be 100 times less, because otherwise they get impossible results?

Anyway, I'd really appreciate if someone could point out whether any of these values are orders of magnitude away from what they should be, or roughly acceptable... so I can (if necessary) point out in the report that I believe the values to be right/wrong

Cheers

Regards

Ikarus

lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

### RE: Need Help With Lift/drag Coefficients +AoA

So lift coeff depends on altitude as well as AoA?
The meaning of life is curiosity; we were put on this planet to explore opportunities.

Minuteman
Posts: 260
Joined: Sat Aug 05, 2000 1:01 am

### RE: Need Help With Lift/drag Coefficients +AoA

Hi Ikarus,

Its been a while since I've done this kind of stuff and I left my notes from school at work, but I'll take a stab.

Initially, I thought your CD values were an order of magnitude *too small*. However, it looks like your cruise CL and L/D values are believable, so the CD must be correct as well.

My only other question is why the zero lift drag coefficient is on 5) Approach is 116% of the value for 2) Takeoff Before Rotation. Is this all due to increased profile drag?

Ikarus
Topic Author
Posts: 3399
Joined: Mon Jan 08, 2001 10:18 pm

### RE: Need Help With Lift/drag Coefficients +AoA

Thanks for the replies.

Lehpron: Lift coefficient = Lift / (dynamic pressure * wing area) - so it depends (among other things) on velocity (dynamic pressure = 0.5*density*velocity^2) , which in turn depends also on the Mach number, which depends on altitude. So here it varies with the assumed weights (MTOW at takeoff, 85% of MTOW at landing), the assumed velocities etc. etc.

Minuteman:
Much of this is guesstimation and assumptions. On approach, the plane uses more flaps, which in this case accounted for most of the extra drag. But how much flaps it uses for takeoff was pretty much guessed.

Cheers anyway. Thankfully I have handed in the report by now (probably one of my worst ever reports, as it was very rushed towards the end, and is structurally disappointing).

Regards

Ikarus

aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

### RE: Need Help With Lift/drag Coefficients +AoA

Ikarus, I'm not sure I understand your zero lift drag coefficient (Cd0) values. The two values you give for Cd0 are .0963 and .1115, correct? In Raymer's design book, he quotes typical Cd0s of .015 (jet aircraft), .02 (propeller aircraft), and .03 (dirty, fixed gear prop aircraft).

Also, I don't understand why you have different values of Cd0 for the same airplane. I guess Cd0 can change if the ratio of wetted area to wing reference area changes. I don't think it can change significantly between phases of a flight though. Perhaps it could with maybe some really big Fowler flaps or something that would make a big change in the wing area. But then again, the wetted area would increase as well, so maybe the ratio of wetted area to reference area wouldn't be affected too much afterall. Anyway, Cd may change at different times in the mission (due to its dependance on CL), but Cd0 should not change, correct?

Ikarus
Topic Author
Posts: 3399
Joined: Mon Jan 08, 2001 10:18 pm

### RE: Need Help With Lift/drag Coefficients +AoA

Aeroguy: Thanks for the advice. Unfortunately, I'm far away from my spreadsheets now, so I cannot check. I believe the formula I used was something similar to this:

Cd0 = sum of component drag coefficients (using form factors, wetted area ratios, and skin friction) + miscellaneous drag coefficients (flaps, undercarriage, leaks and proturbances).

The changes were not significant in the component drags (where wetted area plays a role), but the miscellaneous drag did have significant influence - at least in my use of the equations. Still, I see your point: The zero-lift drag while landing should probably not be 4-5 times bigger than the total drag in cruise. I might have made some mistakes, then. Just a question: The Raymer values you are quoting, are they for cruise or for landing?

Cheers

Regards

Ikarus

aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

### RE: Need Help With Lift/drag Coefficients +AoA

Those values I mentioned for Cd0 from Raymer were from a section where he was taking about cruise, so I'll make the bold assumption that those numbers are for cruise. I might be wrong, but I'd assume that Cd0 values for landing and takeoff would not be too much different. The reason I'm curious is that I'm wondering if it's really worth the time investment to calculate different Cd0 values throughout the mission. I suppose that's the more accurate way to do it, but if the values are very close, does it make much difference? You're probably in a better position to answer that question than I am since you were making spreadsheet calulations. The formula you mentioned looks about right, those miscellaneous drags aren't too much fun to calculate so maybe it's easy to make a mistake. Anyway, good luck with your design...

Ikarus
Topic Author
Posts: 3399
Joined: Mon Jan 08, 2001 10:18 pm

### RE: Need Help With Lift/drag Coefficients +AoA

Unfortunately, the zero lift drag during approach, takeoff (before, during and after rotation) and climbout were needed for some performance calculations (which someone else had to do), so I had no choice.

Anyway. I think I'll just let it rest (having handed in the report two weeks ago today). Better not to worry about all the mistakes at this stage...

Regards

Ikarus

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