Flat Airfoil Aft Section and Sealed Flaps

[Ken Culbertson]

After months of deliberation, I was convinced to go with a wing that had the Thundergazer (geo bolt) wing instead of one with a thick rounded aft section and recessed flaps.  After much reading here I cannot find any discussion of why the flat aft section of the geo bolt airfoil is superior to the more curved aft part of say the MaxBee.  In fact, I seem to find the exact opposite.   Since my decision is more practical than theoretical (a lost foam jig is available), I am wondering if I am making a mistake.  No argument that the Thunderbolt is a superior class airfoil judging from the number of top fliers using it.   But are they using it because it is better in their eyes, or do they just know it so well, plus they have the lost foam jig too.

My second question challenges the established task of sealing hinge lines.  Is there a possibility that sealing can actually worsen performance of a given design?   The initial flights on my Trifacta were a disaster.  It cornered great but tracking was impossible and hunting even worse.  I thought it was the unsealed flaps (removable so I didn't seal them right away) so I sealed them and it got even worse.  That is when I discovered the warp.  Fixed the warp and almost all of the issues went away.  Tracking well and no hunting.  So, I resealed the flaps and the hunting returned, and the corners were harder to lock.
I removed the seals and it went back to being well behaved.  What am I missing?  My plan is to get the plane completely trimmed (as if that ever happens) and the changes I need to make in flying to adapt to logarithmic behind me then reseal them again and test.

Ken

[Tim Wescott]

Dunno about the flat aft airfoil section (AKA "ice cream cone") because I haven't flown one personally.  But I do know that there are some folks that don't like it.  So -- build both, and report back!

As to hinge line sealing -- In my experience, it has always helped.  But (A) I don't build fast, and thus haven't tried it a whole bunch, and (B) people who I know fly well say that sometimes a plane just likes unsealed hinge lines.  To me that says you should try both, and accept what works.

Or you should try the sealed hinges and make absolutely sure you're not binding up the control system -- I'm sure that sticky controls have a way less subtle effect than sealed hinge lines, so if you stick up the controls with your hinge tape, that effect will dominate.

[Ken Culbertson]

AKA "ice cream cone"

Tim it is not an Ice cream cone.  They are flat from the highpoint.  This is only flat for the last 30% going into the flap.

Ken

[Dave_Trible]

Is this electric?  Using ball-link controls?  My thought is some stick-tion in the controls, maybe flaps and elevator not perfectly aligned and the sealing tape making the stick-tion a little worse......the seals will help some airplanes but I can't imagine how they can aerodynamically hurt anything-only mechanically.

Dave

[Dennis Toth]

Could be you built a very good light ship and the flap/elevator ratio needs may need adjusting with the seals.

Best,   DennisT

[Brett Buck]

After months of deliberation, I was convinced to go with a wing that had the Thundergazer (geo bolt) wing instead of one with a thick rounded aft section and recessed flaps.  After much reading here I cannot find any discussion of why the flat aft section of the geo bolt airfoil is superior to the more curved aft part of say the MaxBee.  In fact, I seem to find the exact opposite.   Since my decision is more practical than theoretical (a lost foam jig is available), I am wondering if I am making a mistake.  No argument that the Thunderbolt is a superior class airfoil judging from the number of top fliers using it.   But are they using it because it is better in their eyes, or do they just know it so well, plus they have the lost foam jig too.

My second question challenges the established task of sealing hinge lines.  Is there a possibility that sealing can actually worsen performance of a given design?   The initial flights on my Trifacta were a disaster.  It cornered great but tracking was impossible and hunting even worse.  I thought it was the unsealed flaps (removable so I didn't seal them right away) so I sealed them and it got even worse.  That is when I discovered the warp.  Fixed the warp and almost all of the issues went away.  Tracking well and no hunting.  So, I resealed the flaps and the hunting returned, and the corners were harder to lock.
I removed the seals and it went back to being well behaved.  What am I missing?  My plan is to get the plane completely trimmed (as if that ever happens) and the changes I need to make in flying to adapt to logarithmic behind me then reseal them again and test.

Ken

   The most likely interpretation is that the lack of a seal is covering up some other problem, or, the seal is adding stiction. That's why it is so difficult trying to sort out stunt planes, there are potentially dozens of little things that all interact. Of necessity, you can only build a few airplanes so you are tempted to jump to conclusions about design details when in fact it is something else.

     Brett

[Ken Culbertson]

Hi Ken:

The Geo-Bolt (Thunder Gazer...) wing is not a flat back airfoil. True, it doesn't have a ton of curve aft of the high point, but it does have some. I know, I cut the Lost-Foam fixtures in which Bill Werwage's original Thunderbolt wing was built. I also cut the fixtures in which Dave Fitzgerald's wings have been built.  I probably cut the Lost-Foam fixture that you have access to as well.

If you are concerned about the airfoil shape in relation to airplane performance, remember that both Billy and Dave have won World Championship titles using that wing design...

Later - Bob Hunt

I have done more research and you are correct.  What prompted that comment was a tracing I was given that did indeed have a straight back from 70% to the TE.  I was given the impression the tracing was accurate and I asked a friend who has a geo bolt in a Bear to check his.  I was told it was flat.  I would never question an airfoil used by so many champions.  When the plane flys better than I can fly it, my time goes into reversing that equation! 

Thanks for responding!

Ken

[Ken Culbertson]

   The most likely interpretation is that the lack of a seal is covering up some other problem, or, the seal is adding stiction. That's why it is so difficult trying to sort out stunt planes, there are potentially dozens of little things that all interact. Of necessity, you can only build a few airplanes so you are tempted to jump to conclusions about design details when in fact it is something else.

     Brett

I am pretty convinced you are right.  I have several trim sessions to go before I get the balance between my timer and the logarithmic flaps in line.  After that I am going to reseal them.  It is very possible that I am confusing the difference in how you approach a corner with trim.

Ken