# Airfoil math nonexistent?

1. Oct 31, 2013

### pixelpuffin

I want a general idea of how much thrust an airfoil will produce at a set speed with a set shape
I would also like to know how much resistance it has in a set density gas
I know the math exists for liquids but wikipedia say that for gasses the math is to long and it thusly isn't shown there
I can understand why its a long bit of math but i just need an approximate answer even if the error puts it off by 30%

2. Oct 31, 2013

### Simon Bridge

That's easy - airfoils produce zero thrust.
Thrust is what engines are for ;)

You realize that "set shape" is very vague ... it covers the entire range of shapes of any kind.
I'm guessing you would like to be able to put in a crossection or something and figure out the lift for a particular speed and angle of attack?

I believe there are CAD packages for that.

3. Oct 31, 2013

### AlephZero

No, airfoils do produce thrust. The only problem is, it acts in the wrong direction to propel the plane. That's why it's usually called "drag" instead.

4. Oct 31, 2013

### Simon Bridge

... :)

5. Oct 31, 2013

### pixelpuffin

I need the calculations for a propeller which might just so happen to be the worlds smallest human carrying aircraft if i can ever build it
and before you mention it I REFUSE to use an angled blade to produce thrust that is not nearly as efficient

6. Oct 31, 2013

Well it still has to be angled even if it is an airfoil, and without more detail an answer is not really possible. There is no general equation for the "thrust" of a general airfoil in a propeller. Further, most propellers have a variable-section airfoil given that the tips move so much faster than the roots.

Last edited: Nov 1, 2013
7. Oct 31, 2013

### Simon Bridge

The mention of a blade suggests pixelpuffin is actually talking about a prop.
But I think the question has been answered: use a CAD program - the link in post #2 has a whole bunch of them.

8. Nov 1, 2013

### pixelpuffin

I don't have money to spend on this :/

9. Nov 1, 2013

### Simon Bridge

Some of those CAD systems are free-gratis - say the users.
May have changed a bit - but have a look around.

But you are basically saying you don't have the money to research your aircraft.
Solution = get the money.

Apart from that you will have to go into more detail for anyone to hep you.
Have you had a look through the various aerodynamics course materials - since you only want an approximation you should be able to come up with something you can use for back-of-envelope work.
http://www.dept.aoe.vt.edu/~lutze/AOE3104/airfoilwings.pdf

10. Nov 1, 2013

I don't see how a CAD package will solve his problem. That only provides him a means of designing a solid model of his airfoil. Determining aerodynamic forces would require a CFD package of some kind since there will be no analytical solution.

I would imagine the best way to go here would be to use something like XFOIL. That only applies to 2D wings though, and a propeller is a 3D wing, so it would only be a rough estimate to get started, with a full CFD package required to get accurate results.

11. Nov 1, 2013

### Simon Bridge

You are basically correct - though there exists software marketed as CAD which include CFD modules. Never used one myself but a quick search has people talking about various specialized aerodynamics CAD that do various levels of flow modelling etc.

Software is not always marketed to the labels we get used to. The point is: the complexity of the task requires software designed for the task or an extended period in an aerodynamics course. OR - information enough to settle things.

Remember the Mythbusters "concrete aeroplane" episode?

12. Nov 1, 2013

### pixelpuffin

xfoil seemed useful but it didn't work on my computer :/

13. Nov 1, 2013

That's probably user error then, as it is available for Linux, Mac OS and Windows. The only thing might be if its binaries are 16-bit, then it wouldn't work on newer versions of 64-bit Windows.

14. Nov 30, 2013

### truffaldino

Try this program (propeller calculator): it is free and error is surely less than 30%

http://www.mh-aerotools.de/airfoils/javaprop.htm

15. Nov 30, 2013

### rcgldr

See if you can find out the specs for the Gossamer Albatross, wings and propeller. Wiki article:

https://en.wikipedia.org/wiki/Gossamer_Albatross

16. Dec 1, 2013

### pantaz

17. Dec 17, 2013

### Baluncore

To model a propellor you must consider very many variables at many levels.

The airspeed and the propellor radius define a “transducer disc” model that lacks internal detail but will give accurate predictions of thrust to an aircraft. The “transducer” details can then be treated as a separate problem.

The propellor RPM and the number of blades are inversely related. One blade needs a counter balance, two blades are OK but possibly unstable, three blades are stable, while four blades are stable if the pairs are NOT within about 5° of 90°. The slower the engine, the more blades you will need.

The airfoil profile and it's angle of attack along the blades must be optimised for lift to drag ratio at all radii over variable airspeeds and engine RPM. It is the sum of all the profile lift that generates the thrust.

If you want to numerically compute propellor thrust you will need to get deeply into the complex transformation maths associated with the Joukowsky transformation of a circle into an airfoil.
http://en.wikipedia.org/wiki/Joukowsky_transform

If you go back to texts from 50 years ago, (when the slide rule was used by aeronautical engineers), you will find general guidelines and nomograms that will get you close to an optimum design.

I would recommend you get a copy of “Applied Aerodynamics” by Leonard Bairstow. The 1939, second Edn, chapter XII, pages 621 to 700 are all about airscrew theory. I am sure you can find a 2'nd hand copy through http://www.bookfinder.com . It is now available commercially for about US\$10 as an ebook. There may be a pdf out there on the web. If you are into the field it is well worth getting a copy.

18. Dec 17, 2013