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aerodynamic question, gaps or ridges?

Posted By:
Jay Smith
Homebuilder or Craftsman
#1 Posted: 1/9/2011 17:15:04

It's great to have access to a site like this where bonafide experts ring in.  How hard it must have been in the pre-internet era.

I'm skinning a BD-4 in foam/epoxy/glass.  My doors will open up (gull wing style).  I'm pondering whether to leave the doors flush with the skin with a gap of about 3/16" around or whether to extend the perimeter of the door skin with  thin  fiberglass that would sit over the fuselage skin, protruding  about .040.

It's about equal from a build standpoint.  Where is the aerodynamic edge?

Thanks in advance.

Thomas Stute
#2 Posted: 1/10/2011 13:15:05


in windtunnel tests on glider fuselages we have found that all kinds of canopy or door gaps produce a change in the boundary layer from laminar to turbulent flow. This of course produces drag. Since there have passed now more than 35 years since we made these trials, I really cannot say anymore (or quantify) if flush doors produce less drag, but I would imagine. However  my impression is that the envisaged gap width of 3/16" is a little to large. The experience we made with gliders points in the direction to minimize the gap to the utmost for drag reduction. Of course you need to have sufficient clearance for opening under all environmental conditions.(min and max operation temperatures)

Another major aspect for drag reduction at doors and canopies is the suppression of any airflow from inside to outside and vise versa.  Here gap sealings come in. There is a large variety of rubber profiles available which you can experiment with. In the past we've made the best results with inflatable tube-like sealings.

I hope my comments are of some help.

Good luck for your project.


John McGinnis
Young Eagles Pilot or VolunteerHomebuilder or CraftsmanAirVenture Volunteer
#3 Posted: 1/14/2011 00:44:49


The aerodynamic edge definitely goes to a closed gap.

Anytime airflow encounters a step-down, it is sucked in. Nature abhors a vacuum. If the flow over/inside a gap can be made relatively stable, a whirling little drag-reducing vortex may reside in it, but that is very unlikely. Most of the time it will just go on generating a turbulence that streams far behind the aircraft.

It's surprising for most of us, but laminar flow is nearly twice as tolerant of step-up conditions as of step-down and gap. Depending on other criteria, a rounded lip overlapping the forward edge of your door frame of up to .020" thickness could conceivably avoid adding a drag penalty altogether (see NLF presentation ). On the trailing frame, avoid the step-down condition.

If you can close the gap and stay close to flush... I have a job for you!

Eric Marsh
Homebuilder or Craftsman
#4 Posted: 1/14/2011 13:30:52

I'm just finishing drilling the nose skin to my wing and have a small area where there's a gap between the nose and lower skins. I'd say that it's .030"-.040" wide and the skin is .025" thick. The length is about 10" give or take a little. Clearly not substantial but I don't like it in either case. I'm thinking that when I paint the primer will probably fill it but have been wondering if it is likely to induce any drag. Thoughts anyone? This is my first project.



John Johnson
Homebuilder or Craftsman
#5 Posted: 1/14/2011 20:04:12

A small step up is much better than any gap.  Willie Messerschmidt used to lap his skins backward to standard practice.

He lapped the aftermost skin over the one in front of it, with a small bend down at the edge to be sure it sealed.  This allowed the flow to reattach and significantly reduced the drag penalty of the skin laps.  The step down tends to break the laminar flow and cause a transition to turbulent flow which increases the drag at the Reynolds numbers we experience in our aircraft.  Note that at very low Reynolds numbers turbulent flow may have lower drag than laminar flow.  That is the idea behind the "turbulator" strips they add to gear legs and propellors.  Even though these items are operating at Reynolds numbers well above where the turbulence would actually lower the drag. :-)


EAA 9135, Technical Counselor and Flight Advisor