SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Sep 05, 2008, 05:25 PM
MAAC 6251
Ottawa, Ontario, Canada
Joined Jan 2002
699 Posts
Question
What effect on the characteristics of an airfoil does the position of the maximum thi

What effect on the characteristics of an airfoil does the position of the maximum thickness have?
For example how are two airfoils different if one has the max thickness at 40% chord and another at 33% chord, or even 30% chord?
Rob.
RobZ is offline Find More Posts by RobZ
Reply With Quote
Sign up now
to remove ads between posts
Old Sep 05, 2008, 11:29 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,316 Posts
In the old model books the rule of thumb was that the further ahead the high point the later and softer the stall would be. This assumed some fairly routine airfoil shapes.

But really you're not all that extreme with a 40% high point. I though you were thinking of the 60% and more back max thickness points such as found on the NACA laminar flow series of airfoils.

Back to the more model style stuff though....

For example an Eppler 474 is quite hard to stall. Some buddies used this airfoil for control line combat on my recomendation and found that it worked far better than the previous "standard" which had been the Phil Granderson Tadpole Special. The Eppler was thinner but had a shape that the air preffered so it stalled at a higher angle of attack then the TS. They did fairly well with it for a couple of years.

These days you'll find the max thickness points or the highest point on an airfoil are all over the place within reason but where the high point is located is a function of both the thickness profile and the camber. What really allows the airfoil designers to break some of the old "rules" is the ability to study the pressure distributions that are able to be calculated with such programs as Xfoil and the Eppler program. By avoiding sharp changes in the pressure curves and controlling the slope of the pressure curves they can give us airfoils that work with shapes that would normally be considered unwise to try. Such as the Selig 1233 heavy lift airfoil.

However along with this comes the need for extreme accuracy in duplicating the shapes in many cases. Variations of as little as a couple of percent on some of the more extreme shapes will totally ruin the performance promise. And on airfoils of this sort do not even THINK about using open structures covered with flexible films. The sag between the ribs and spars will completely ruin the performace again.
BMatthews is offline Find More Posts by BMatthews
Reply With Quote
Old Sep 23, 2008, 09:07 PM
Registered User
Congress, AZ
Joined Sep 2001
4,874 Posts
Get one of the airfoil programs, Tracfoil or Profili and do enough experimenting to answer your questions.
GaryO is offline Find More Posts by GaryO
Reply With Quote
Old Sep 25, 2008, 11:25 AM
Registered User
Joined Nov 2002
86 Posts
Quote:
Originally Posted by RobZ
What effect on the characteristics of an airfoil does the position of the maximum thickness have?
For example how are two airfoils different if one has the max thickness at 40% chord and another at 33% chord, or even 30% chord?
Rob.
Hi RobZ,

There are probably several effects. One pretty important characteristic of the point of maximum thickness the point of minimum pressure (on the top surface) will be close to the maximum thickness. This is important because from the leading edge to the minimum pressure, the pressure is constantly decreasing, along the surface. Thus, from the leading edge to this point, the air on the surface is flowing toward lower pressure, which is desireable. After the minimum pressure point, the air encounters an 'adverse pressure gradient'. The adverse pressure gadient tends to cause separation and/or transition to turbulence. One component of a wing's drag can be reduced by delaying the transition to turbulence, which can be achieved by delaying the onset of the adverse pressure gradient. If you look at some of the various laminar flow airfoil sections, you will probably notice the the point of maximum thickness is relatively far back. This doesn't mean that every airfoil section with a rearward point of maximum thickness will achieve laminar flow or low drag, unfortunately.

banktoturn
banktoturn is offline Find More Posts by banktoturn
Reply With Quote
Old Sep 25, 2008, 05:48 PM
MAAC 6251
Ottawa, Ontario, Canada
Joined Jan 2002
699 Posts
Thanks Bruce and banktoturn.
Rob.
RobZ is offline Find More Posts by RobZ
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question Reflex effect on hovering characteristics Chatenever 3D Flying 2 Aug 11, 2008 10:49 AM
what will the effect on the TM be flyfishr Parkflyers 5 May 29, 2002 12:02 AM
Finding the 'incidence line' on an airfoil tempest411 Electric Sailplanes 8 May 25, 2002 01:58 PM
what effect does chord length have on wing characteristics? Ben74 Power Systems 6 Jul 24, 2001 02:31 AM
Stock Up on Equipment or Get an LMR Glider, What do you Guys Think? AndrewK Electric Plane Talk 6 Jun 17, 2001 07:43 PM