# Airplane re-design stuff

karlmartin
Hello awesome science people.

I am a an aerospace engineering student and I am currently engaged in completing an assignment regarding a fictional re-design of the wing on a DeHavilland Twin Otter. I do have some difficulty though, since suddenly I have to independently manage to think about a lot of the details involved in the design of a wing.

The current uncertainty I am facing is that the original airfoil for the Otter had a design lift coefficient of 0.5. I have to see if I can find a better airfoil for the tasks of a maneuverable utility aircraft. Now, when I am trying to find a better airfoil, should I still aim to keep that design lift coefficient as the same number or can I change it? What are the consequences if I choose an airfoil with a different design lift coefficient? I know that if I choose one with a lower design coefficient, the airplane would cruise at a design coefficient that will correspond to a less than desirable lift coefficient. However, I am confused about what happens if my design lift coefficient is higher than the 0.5 of the original airfoil. Would a higher design lift coefficient be beneficial in some way? Could you guys please explain? Any help is greatly appreciated.

All the best,
Karl

## Answers and Replies

Mentor
Hello awesome science people.

I am a an aerospace engineering student and I am currently engaged in completing an assignment regarding a fictional re-design of the wing on a DeHavilland Twin Otter. I do have some difficulty though, since suddenly I have to independently manage to think about a lot of the details involved in the design of a wing.

The current uncertainty I am facing is that the original airfoil for the Otter had a design lift coefficient of 0.5. I have to see if I can find a better airfoil for the tasks of a maneuverable utility aircraft. Now, when I am trying to find a better airfoil, should I still aim to keep that design lift coefficient as the same number or can I change it? What are the consequences if I choose an airfoil with a different design lift coefficient? I know that if I choose one with a lower design coefficient, the airplane would cruise at a design coefficient that will correspond to a less than desirable lift coefficient. However, I am confused about what happens if my design lift coefficient is higher than the 0.5 of the original airfoil. Would a higher design lift coefficient be beneficial in some way? Could you guys please explain? Any help is greatly appreciated.

All the best,
Karl

Since this is a schoolwork assignment, it would normally need to be posted in the Homework Help, Engineering forum. But since it is fairly specialized in nature, I'll leave it here in the general Aero forum for now. Keep in mind, though, that you must do the bulk of the work on your assignment. We can help in answering specific questions, but you need to do most of the research and work on your own.

So for example, can you tell us a bit about the tradeoffs for different lift coefficients for different airplane tasks? And you should check your original post (OP) -- there appear to be several typos in it that make it a bit confusing...

Science Advisor
Homework Helper
Start with a basic question: what you you want your new wing to do, that the old one doesn't do already?

Until you answer that, you don't have any way to decide if a new wing design is "better" than the old one, or whether you want the lift coefficient (or any other parameter) to be the same or different.

Aero51
Before you decide to start changing things you need a very clear objective. In other words, what do you intend to improve by modifying the wing? If you do not have a clear goal your assignment is hopeless. Once you have established a goal, you can begin considering specific ways to go about achieving your objective. This is also known as conducting a trade study.

karlmartin
I'm sorry if my problem is confusing, the half-assedly put together nature of this assignment of mine is clouding my mind with questions and uncertainties. I'm sorry for posting this here as well, will do better next time.

Regarding my goals in the re-design of the airfoil - I've decided to try to improve the versatility of the of the aircraft by accomplishing these:

- making the linear part of the drag polar very steep so as to keep drag low at as many values of lift coefficients as possible. I reasoned that this should keep drag low at a wider range of speeds. However, it would mean more drag at the cruise phase of normal transport operations. Climb rate would also be negatively impacted, but since I've been given no guidelines as to what design goals to aim for, I'm inclined to not care.

- increasing the maximum lift coefficient value. I reasoned that this should allow operating at lower speeds without stalling or more payload. Also, it should allow for shorter take-off and landing(not considering the impact of high-lift devices)

- increasing the maximum Cl/Cd ratio that can be obtained. I reasoned that this should allow a longer endurance for aerial surveys or tourist flights. I thought it would also allow easier operation in mountainous or hard to risk areas: the pilot would have more time to make decisions regarding maneuvers since he can fly slower.

- making the moment coefficient curve less negative in most ranges of angles of attack. I reasoned that this should allow a size reduction for the tailplane area, thus lowering the weight of the aircraft. I know this impacts the dynamic stability but I chose to blatantly ignore that since I don't have the time to analyze everything. I wasted too much time already by being too indecisive and making problems very complicated.

In addition, I reasoned that if I increased the lifting capabilities of the airfoil I could perhaps decide to use less complicated high-lift devices and consequently reduce the weight of the wing.

I'd like your opinion on these requirements I set. Are they reasonable or do they make me seem simple in the head?

Moving on(assuming I'm not simple in the head):

Regarding the design lift coefficient, I calculated the weights at the beginning and end of the cruise part of the mission. From these, I calculated the necessary lift coefficients - 0.27 and 0.23, respectively. I have the design lift coefficients now. However, as I stated in my mission requirements, I am deciding to make the linear part of the drag polar steeper so as to be able to experience less drag at a wide range of lift coefficients. It seems to me that selecting an airfoil based on these cruise lift coefficients would be counterproductive. Am I correct to say that or would you say that it's still important?

Regarding my original post, I think understand now why the design lift coefficient was what it was in the case of the original airfoil. The cruise design lift coefficients were in the range of the inconspicuous laminar bucket in the drag polar of the original airfoil.

In conclusion, my new problems are:

- Are my mission requirements reasonable?
- Is my reasoning for the requirements correct or am I missing something a more experienced person would know?
- Is it wise to accept more drag at cruise settings to gain less drag at other lift coefficients?
- I can't really tell, because this hasn't been taught to us yet, but are my requirements impacting the maneuverability of the aircraft? If so, by what margin?

I also included as an attachment a screenshot from XFLR5, which I am using to analyze the airfoils, depicting the original airfoil and the airfoil that I think suits well for the requirements I set. The original one is a NACA 63A-516 and the other airfoil is a NACA 34015 on which I modified the aft part slightly to get a stable moment coefficient.

I'm sorry to have put you all through reading this. If there is a menial task you require me to do as a favor, please say so : D

Thank you very much for your time and help, I am eternally grateful.

Karl

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karlmartin
Sorry, "the linear part" of the lift-drag polar was the wrong thing to say. I meant that before it starts veering off to the right, I would like it to resemble a vertical line so there would be a similar low drag coefficient at most lift coefficients.

Science Advisor
Homework Helper
- Are my mission requirements reasonable?

I can't see any mission requirements in your post, just a list of things that you might change.

I would call a "missions requirements" things like
- better short-field takeoff/landing performance
- increased range and/or loiter time (I believe twin otters are used ifor SAR and surveillance applications)
- or whatever ....

But you need to put numbers to how much you want to improve such things, not just words like "better".

Then you can work out what your new wing needs to do to perform the missions. For example if you want to increase range, you probably need bigger wing fuel tanks (so maybe a thicker chord?) and you will also have a higher takeoff weight...

karlmartin
Well this is a nasty turn of events. Guess I'll have to figure something out in terms of the numbers...

Anyway, thanks a bunch, Aleph.

karlmartin
You know what guys, I'm not happy with how things have gone with respect to the mission requirements. I'm going to start over and get a new assignment. That basically means a re-sit, but I don't feel good about this frankly piss poor work that I've done. Better to do things again and well than to do them poorly. No need to reply to this thread anymore. When I run into more problems I'll turn to the homework section of the Engineering forum, as is proper.

Thanks for your valuable time and effort.
Karl

HowlerMonkey
The twin otter is a tough design to improve upon.

Mithrandir13
if in the assignment you were not given specific goals... you could have conjured'em up yourself....

but a wing is a big deal and in the real world.. a wing design has 10 man*years of trade studies!!!

example... a thicker wing.... structurally more efficient... drag penalty... where is the sweet spot?