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Smaller Wing Tips for Speed

B

Bob Axsom

Well Known Member
I have experimentally modified many things on our RV-6A and participated in over two years in SARL sanctioned cross country air races. My experience and observation of others is the easiest way to increase the speed of an RV is to replace the wing tips with homebuilt shorter ones on the order of 3" span each.

Bob Axsom
 
So the bat wing tips don't make a difference in speed as far as you know Bob??

Frank
 
can you post some pix ? Please?



you obtain additional speed, at the expense of lift. Is that the correct assumption?
 
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Bob Axsom said:
I have experimentally modified many things on our RV-6A and participated in over two years in SARL sanctioned cross country air races. My experience and observation of others is the easiest way to increase the speed of an RV is to replace the wing tips with homebuilt shorter ones on the order of 3" span each.

Bob Axsom
Click to expand...

What kind of speed increases expected ? What kind of stall increases?
I was thinking of modifying my ones , ie cutting them down some.
 
How 'bout shape?

Bob,

Seems that many racers at Reno have clipped wings...some radically so (take a look at Rare Bare's wings...very short span!):
rarbear.jpg


My wings are clipped 3.5 inches, much the way I'm told many Rocket wings are. I'm considering making a pair of racing wingtips, but am not sure what might be the best shape. Wondering what your thoughts are on best (fastest) shape...symmetrical, more curved on top/flat on bottom, flat plate, etc.

I took a few pics at the Pagosa 100, and will post them here:

Here are my current tips (slightly curved on top, slightly increasing span towards the trailing edge:
wtmills.jpg


Wayne Hadath's F1 tips look somewhat similar to mine:
wthadath.jpg


John Huft's appear to be a little more symmetrical (top to bottom shape) with the tip edge more towards chord center, versus a bit lower on mine and Wayne's (I think):
wthuft.jpg


Mike Thompson has some flat racing tips, seen here:
wtthompson.jpg


And Tom Martin's very fast F1 EVO has flat tips (and I think that may be standard on the EVO wing...not positive though, but it's a very different wing altogether):
wtmartin.jpg


So what does your research and experience say with respect to tip shape? With an already shorter wing, I don't want to overdo this, but I'm very interested in your thoughts on size and shape. Thanks much!

Cheers,
Bob
 
A few answers

The plane is faster in top speed by 3 kts and the plane is different in other ways - slower climb, higher stall speed, faster roll rate, etc. - that you would expect from a smaller wing. The word I get is Van's found no measurable speed increase by going to the bat wing tip. Mike Thompson cut off a set of tips and added plexiglass wing tips (eyeball data) and claims a similar speed increase. The shape of mine in plan view are an exact copy of the lower surface of the wing airfoil with a maximum span of 3".

Bob Axsom
 
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Photo and question

IMG_3511.jpg


Here is a photo of mine on the ramp at Taylor. Question for Bob Mills - Has Mike removed the plexiglass tip plates from his wing tips? Mike and I both race in the RV Blue class and we are most competitive.

Bob Axsom
 
Bob,

First thing how is the wife doing.
Mike did take the plexi off. Don't know if his speed changed
or not.
Can't take my tips off so I guess I will be behind you when I get
racing, Anteneas in the wingtips.
Where is John Huft getting all the speed in that 8. Looked at his web
site and he put on the speed.
Incedence in the Horizontal Stab ????????????:rolleyes:

Hope it works and hope to be with you guys next year.

Missed ya at Grace flight.
 
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Hi Bruce

Jeanine after 3 surgeries and 5 hospital stays I think has turned the corner to improvement. She starts radiation therapy Monday and Chemotherapy sometime a little later due to waiting for the broken leg to heal.

John Huft has done a magnificent job with his RV-8 modifications with a lot of focus on the firewall forward. He is a teriffic guy and engineer but I still have him in my sights. It is a carrot on the stick situation but I'm dumb enough to keep pursuing it and happy with the challenge.

Bob Axsom
 
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Bob,

Glad to hear that Jeanine is showing steady improvement. I'm certain that your all-out, being-there-for-her support has been a key factor. She's in my thoughts and prayers as she continues treatment.

On Mike's airplane, I saw no plexi on the tips in Pagosa, just the small, flat tips.

I looked at John Huft's website, and he's sure done a lot of tweaking...it certainly has paid off! I'm trying to develop a plan for incremental mods, starting withe basic clean-up of external draggy items and gaps seals. Trying to determine the best tape to use on hinge seals to be effective, yet be removable with no paint damage (and which gaps to seal...top and bottom of H-stab? Rudder? gear leg and tail fairings? left-to-right seams? longitudinal seams? etc.). I also want to look at my aileron and flap rigging to see if I can clean that up a bit. Also looking at FWF areas, as I can probably clean up my cooling air inlets and maybe reduce my exit area, as I run pretty cool. Tom Martin mentioned smoothing out exit air flow with a ramp back by the firewall, to keep the air exiting the cylinders from slamming into the flat firewall, but that will take some thought, since the firewall is clobbered with hoses, gascolator, and various penetrations. Lots of things to ponder and play with this winter.

As others have said, I read all of your posts on drag clean-up with much interest, and thanks for keeping the racing scene alive for all of us!!

Cheers,
Bob
 
Started working on the molds for new tips

Started working on the molds for new tips today. I had a good size piece of high quality 3/4" plywood made up of 1/16" layers in the garage so that is what I used. I used the base of my 3" tip molds as the the template and drew four outlines. I started to use my small Sears bandsaw to cut them out but it just wouldn't work so I cut the out oversize with a skill saw and will do a fine cut followed by sanding to get them exactly right.

After I get the profile right I plan to glue two together for the right tip mold and two for the left tip mold. After they are cured I will mount them on ~2" diameter steel pipe with thread mounted end plates at each end of the ~6" long pipe. There are two pipe mounts the look like vertical column standoffs for each mold. The top plate will be attached to the base of the mold and the bottom will be attached to a specially made table (I made it out of 3/4" plywool and 2"x4"s). This is a VERY stable work mount and it is needed for the shaping step.

The top 3/4" piece of plywood on each mold will be angled from ~ 3/4" at the top to 0" at the bottom. This will allow me to have a very stiff extension of the tip to cover the end of the aileron. The bottom 3/4" piece of plywood will be left as is to be the shape of the mounting flange. There will be the usual sanding, shaping of the edges, clear polyurethane painting and waxing of the molds. Then I will lay up the tips with three layers of e-glass and EZ-Poxy (24 hour cure for each layer).

Mike Thompson, Tom Martin and Wayne Hadath have flat tips and at least Wayne's are externally mounted over the wing skin. Mine will be angled from <3/4" beyond the wing skin at the top to the wing skin at the bottom (0" extension beyond the end of the wing skin) and they will go inside the wing skin. The same dimpled wing skin holes for #8 flathead screws will be used for mounting the new semi-flat tips and platenuts will be mounted on the tips (same as all my other tips).

This mod will reduce my short race (under 350 miles) wing span from 21.5 feet to 21 feet 1.5 inches - a 4.5" reduction. The angle will cut the frontal area of the new tips in half so the full drag of the 3/4" extension past the end of the wing skins is not 1.5 in X the wing thickness in inches but 3/4 in X the wing thickness.

All of the racers mentioned that have gone to span reducing flat tips have increased their speed so I expect these tips to improve my RV-6As speed above what was achieved by developing 3" streamlined tips that reduced the wing span to 21.5 feet. We shall see.

Bob Axsom
 
dark paint

Hey Bob I am leaning toward a paint scheme a lot like yours. Navy Blue base with red highlights. All of the airport guys keep trying to talk me out of a dark color because of heat.....so would you do it again? Off topic I know, but I had to ask,
Cheers..
Evan
 
Old Wive's Tale

Believe me I am very familiar with the thermal properties of dark paint vs. white paint after working in aerospace for 50 years. "Out there" it makes a difference but it is not a problem at all for metal airplanes. I absolutely love our airplane after five years of flying in California and Arkansas with trips to Baltimore, New York, Alamogordo, New Mexico, Charleston, SC, Asheville, NC, Key West, Florida and many other cities from coast to coast and border to border, heat has never been a problem. Do what you want - you are the one that has to live with the result and you want to be impressed with its beauty every time you see it after an absence. Please don't copy mine though as I worked on it for many years to get the layout just right and I would like for it to stay unique.

One thing you might want to take into consideration is that dark planes are more difficult to see against ground clutter background. It is a collision avoidance weakness.

I guess you can tell I would do it again. NO WHITE AIRPLANES FOR ME!

Bob Axsom
 
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Mold status 12-31-09

The two pieces of the left wingtip mold are glued together, clamped and curing. The right wingtip mold has passed through the glue phase and has been roughly shaped with a plane and a sander (much more shaping to do).

Bob Axsom

Xmaslegflattip050.jpg

Xmaslegflattip055.jpg

Xmaslegflattip057.jpg
 
Bob,

Isn't it true that as you shorten the wings, reducing frontal area and form drag, the wing is forced to operate at a higher angle of attack, increasing induced drag?

At some point wouldn't the total drag increase, lowering your race speeds? How to you plan for this? Just trial and error?
 
Yes there is such a point

I have done my work experimentally so far and I don't believe I'm even close to the point you are talking about. In one of Van's introductory videos Van states something like "I could have made the wing smaller and the plane would have been faster but it would not have provided the all around performance I wanted." With this latest mod I will be at the almost absolute minimum without cutting metal. I am strictly looking for speed so this is the right way for me to go. It certainly is not right for everyone. However, if anyone is interested I'm checking it out and will report the results. This latest mod I resisted because I thought my streamlined 3" tips were just about perfect then three racers with flat tips made it obvious that their way may be better - we will see. When I'm done I will have 8 wingtips for our RV-6A. I'm in this pursuit of speed to win races not create a marketable product.

Bob Axsom
 
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How to plan for it

JHines said:
Bob,

Isn't it true that as you shorten the wings, reducing frontal area and form drag, the wing is forced to operate at a higher angle of attack, increasing induced drag?

At some point wouldn't the total drag increase, lowering your race speeds? How to you plan for this? Just trial and error?
Click to expand...

If you get the drag curve(s) of the airplane correctly to begin with, you can probably predict those changes well. Fortunately, the RV-6A's drag curve is well documented by CAFE. Bob could use my methods (see links below) to compare them and validate the predicted change. If you follow this link you will see that the induced drag at 200 mph for the normal 6A is only about 10% of the total. Shorter wings will raise the stall speed, though.
LINK
 
Right Wing Tip Mold complete

The right wing tip mold is complete but no shaping has been done on the left wing tip mold.

Bob Axsom

Jan13legtip059.jpg

Jan13legtip060.jpg
 
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The botom 3/4" in the photos

In the previous photos you see 3/4" of vertical walls clmpletely around the finished mold. The fiberglass that is formed over this surface is the mounting flange that will extend inside the wing skin at the tip. This will be trimmed as requiredfor a fit that will leave only approximately 1/2" exposed to the wind.

Bob Axsom
 
Bob Axsom said:
I have experimentally modified many things on our RV-6A and participated in over two years in SARL sanctioned cross country air races. My experience and observation of others is the easiest way to increase the speed of an RV is to replace the wing tips with homebuilt shorter ones on the order of 3" span each.

Bob Axsom
Click to expand...

Something you might want to consider, Bob, is to remove about 2' of span on each side, then add a triangular tip to get you to a little more span but at less area to increase your low altitude speed.
 
Right Tip Fiberglass Done Left Tip First Layer Done

wngtip12910003.jpg


The first layer is always the most critical and now that milestone has been passed by both wingtips. The right wingtip has all 3 layers complete and the left wingtip has the first layer complete. These are the 7th and 8th wingtips for our RV-6A and they will be tested in the highest speed configuration - without tip tanks. If they prove to be faster than my 3" streamlined racing tips I will make another set off of the molds for the long range configuration - with tip tanks.

Bob Axsom
 
tips

Bob,
those will be some good looking tips when there done. Now the bad news, when you come up to Canada in August for the race, and see the tips I will have on........well, lets just say your wing tip inventory may grow again !!
 
A-A-H-H-H-E-E-E-E-E-E

C-FAH Q said:
Bob,
those will be some good looking tips when there done. Now the bad news, when you come up to Canada in August for the race, and see the tips I will have on........well, lets just say your wing tip inventory may grow again !!
Click to expand...

... and I don't even know if these will be faster! Oh well the challenge never ends.

Bob Axsom
 
C-FAH Q said:
Bob,
those will be some good looking tips when there done. Now the bad news, when you come up to Canada in August for the race, and see the tips I will have on........well, lets just say your wing tip inventory may grow again !!
Click to expand...

do you have any pictures? Please?
 
tips

Sorry guys,
no pictures available....top secret wingtip design.:D You will have to come to Canada in August to see them. All I can say is they are simplicity exaggerated....now, back to the bat cave.
 
Mating Semi-flat Plate Wing Tips to Wing

drainlegwingtip013.jpg

drainlegwingtip014.jpg


I did the rough mating of the right wing tip today. I almost hope it is slower than my 3" streamlined tips because it is very close to ugly. As Frank Lloyd Wright said "Form follows function" and I'm sure I will come to love it if it is really faster. It will be some time before I am ready to test the new semi-flat tips since the annual needs to be completed now.

Bob Axsom
 
It will be much faster without the Clecos.

Also, it's not really ugly. It just lacks visual appeal.
 
Bob Axsom said:
<snip> I did the rough mating of the right wing tip today. I almost hope it is slower than my 3" streamlined tips because it is very close to ugly. As Frank Lloyd Wright said "Form follows function" and I'm sure I will come to love it if it is really faster. Bob Axsom
Click to expand...

Bob,

They actually look very good. OK, a little stubby, but nice nonetheless. Put a coat of blue, a red and white stripe, and a little "71" on each, and it'll be downright purty!

The only one that will think they are ugly is Gary...when he looks over and sees that wingtip with the "71" on them passing off his right! ;)

C-FAH Q said:
Sorry guys,
no pictures available....top secret wingtip design.:D You will have to come to Canada in August to see them. All I can say is they are simplicity exaggerated....now, back to the bat cave.
Click to expand...

Well now Gary (and this is totally tongue-in-cheek, of course)...if you guys are gonna pull the veil down over the Great White North Skunkworks, we're just gonna have to collaborate down here too...and send spies up there! :D

I've got some ideas on the tips as well, and the foam for the molds just arrived yesterday...so the fun is on! And you wouldn't mind sharing your tip plans with a Sport FX guy would ya? I'm sure Tom and Wayne wouldn't mind! ;) Oh yeah, they are in this with ya!

Finishing up a second TW fairing for my Flyboys TW...did you ever get that done too? Couple other clean up mods, then the wingtips, and some cooling drag experiments (per discussion with Tom and Dan H) in the works as well. All fun stuff!

And by the way, gonna talk to my buds and see if we can re-direct some satellite orbits to get some pics of your work..."ve have vays of making you talk young man" (German accent added). :p

Cheers,
Bob
 
Bob,
I have Vince's new tail wheel on and aslo used Vans tailwheel pant. it was an easy install and does swivel 360 degrees. I really like the new feel of it, ground steering is much easier and gives a little better visual out front while on the ground.


Will be adding lead sheild to prevent and type of radio waves from penatrating the hanger !!

Regards,
Gary
 
C-FAH Q said:
Bob,
I have Vince's new tail wheel on and aslo used Vans tailwheel pant. it was an easy install and does swivel 360 degrees. I really like the new feel of it, ground steering is much easier and gives a little better visual out front while on the ground.


Will be adding lead sheild to prevent and type of radio waves from penatrating the hanger !!

Regards,
Gary
Click to expand...

Concur on the TW. And I guess there goes the X-ray vision...nertz! ;) May have to resort to HUMINT (do you own a big dog?) :eek:

Cheers,
Bob
 
Test soon

21710legwingtip012.jpg


The closure webs adjacent to the ailerons on these tips are fiberglass where the 3" streamlined racing tip closure webs are 0.016" aluminum. The tips are basically done except fo some final fit sanding (a few minutes work at the hangar) and roughly 100 flathead screw installations. I should be able to test fly tomorrow (2-18-10) or Friday. I have some schedule problems but the weather looks OK.

Bob Axsom
 
Bad Luck

I got the tips installed and pulled the plane out of the hangar in the best weather we have had in weeks and it wouldn't start because of a low battery. I brought it home and recharged it for an attempt tomorrow morning.
Bob Axsom
21810tipson003.jpg
 
Good Luck Bob!

Just cut the foam for the molds for my flat tips today. Watching your experiment with great interest. Busy sked next couple weeks, so I'm a few weeks behind you...but workin' it right behind ya.

On those trailing edge closure webs: are they hollow, or did you glass over some sort of stiffener (foam, etc). Talked to Wayne (Race 14) the other day, and he said he used foam and glassed over it for that area. I've been thinking of making a riblet, similar to what is at the trailing edge of my standard tip, and having that stiffen the TE. Hadn't thought of glassing that over, but its a good aerodynamic idea, I'll bet!

Lookin' good! Can't wait to hear how they perform, and how the fast and slow speed flying characteristics are. Go gettem!

Cheers,
Bob
 
At this time the glassed over area is hollow

I have some of the blue foam yet that I bought from Aircraft Spruce and I should have used it to fill the rear end then glass over it. I may still do that. Vertically it is very stiff but latterally there is flexibility that I do not like. If it is faster than my 3" streamlined tips (due to reduced equivalent flat plate area and wetted area) I think I will do that before filling, sanding and painting.

Good luck with your tips.

Bob Axsom
 
Bob,

It's that lateral flex that concerns me too, and Wayne mentioned that strength in that area was a concern of his as well. I'm sure we all concur...keeping the tip's TE from contacting the aileron is the rub.

Thinking out loud here...seems like spanwise flow in that area would be to the outboard, so if it flexes, it should flex out. That's a bit of non AE speculation (so its FWIW) and based on what I think is normal flow, and also on what I heard about others' experience with fences back there, which stopped the spanwise flow, thus not allowing air to spill over the tip, and forcing more air over the ailerons, which loaded up the ailerons, making the control forces very high (stiff) at speed. (nice run-on sentence, eh!)

Have a safe test Bob, and look forward to positive results!!

Cheers,
Bob
 
Hi Bob,

It's really great tip..Will you provide some information regarding the specifications and precautions regarding modifying the wings..

Thank you..
 
rvmills said:
Seems like spanwise flow in that area would be to the outboard, so if it flexes, it should flex out.
Click to expand...
Spanwise flow will be to the outboard on the bottom, but to the inboard on the top. You'll actually get circulation around the trailing edge of that tip, so if anything it will want to twist. But if it was my tip, I think I would add a rib running chordwise, and oriented flat (ie., perpendicular to the closing rib next to the aileron). That would add a lot of rigidity in the spanwise direction, and counteract some of the tendency to flex.

If you wanted to make it really rigid and counteract the twisting as well, instead of the single straight rib in the middle you could form up a "grid" of ribs set at a 45 degree angle to the chordline. But that would be a lot of work, add a lot of weight, and probably isn't really necessary... :)
 
Fill it !

Bob,

It looks like it is hollow now? If so, buy a can of builders foam, fill it and your done!

Regards, Tonny.
 
Better options?

Bob, I am definitely not an expert at this, but will the flat tip not create a lot of turbulence and cause more bad than good, this way?

Last weekend I installed the flap and aileron on my right wing and was really annoyed to see these gaping holes between the aileron and the wing and flap, where the hinges and the pushrod are. I bet you could gain a couple of knots, if you find a good way to close these off (or at least make the holes smaller).

Than there is these ugly flap hinges sticking out into the breeze: six of them as well! If you can make a good fairing for those, you win 6 times!!

BTW, I will be watching your progress, so that I can copy it :p. (Don't worry, I am not interested in racing).

PS: can you do the mods on the Ailerons and Flaps first, I am going to paint mine soon! ;)

Regards, Tonny.
 
Tonny,

I could be wrong, but I think the six ugly hinge brackets out in the breeze are unique to the flaps on the RV-9 (and maybe the -10 and -12?). The RV-6/7/8 flaps use a piano hinge with nothing out in the breeze.

The ailerons, if I'm not mistaken, are deliberately designed to allow airflow from bottom to top over the leading edge of the aileron. I think this is meant to help keep the flow attached to the aileron at high alpha.

Pilottonny said:
Bob, I am definitely not an expert at this, but will the flat tip not create a lot of turbulence and cause more bad than good, this way?

Last weekend I installed the flap and aileron on my right wing and was really annoyed to see these gaping holes between the aileron and the wing and flap, where the hinges and the pushrod are. I bet you could gain a couple of knots, if you find a good way to close these off (or at least make the holes smaller).

Than there is these ugly flap hinges sticking out into the breeze: six of them as well! If you can make a good fairing for those, you win 6 times!!

BTW, I will be watching your progress, so that I can copy it :p. (Don't worry, I am not interested in racing).

PS: can you do the mods on the Ailerons and Flaps first, I am going to paint mine soon! ;)

Regards, Tonny.
Click to expand...
 
Idea/question

Here's a question for Bob A., Bob M., and hopefully some aero guys to chime in:

If you go with a flat wing tip like this with no significant span, why even extend it all the way back to the trailing edge? Does that serve any purpose aerodynamically? Why not just end the wing tip at the rear spar, and make a matching end cap for the aileron, so they're flush with each other?
 
Testing done

This morning I went to the airport, installed the battery and tested the new semi-flat tips. The resulting speed was 181.5 kts. After the flight it took me 2 hours to replace the semi-flat tips with my 3" span streamlined tips and refruel.
rcngtips003.jpg

I reflew the test and the resulting speed was 182.3 kts so I do not intend to pursue the flat tips any further.

Bob Axsom
 
roee said:
Here's a question for Bob A., Bob M., and hopefully some aero guys to chime in:

If you go with a flat wing tip like this with no significant span, why even extend it all the way back to the trailing edge? Does that serve any purpose aerodynamically? Why not just end the wing tip at the rear spar, and make a matching end cap for the aileron, so they're flush with each other?
Click to expand...

roee,

Had that discussion with Wayne (the F1 Rocket pilot that did flat tips last year, and felt he gained 3 knots). He has the same clipped wing I have, and we discussed doing just as you describe. In fact, from what I understand and from our discussion, that is how the EVO wing is constructed (flat tip that stops at the wing trailing edge, flat aileron cap, separate but all lined up). Tom or other EVO drivers will correct me if I'm off the mark on that.

Doing so may very well be a good idea from an aerodynamic perspective (I imagine its cleaner that way), but since I'd only use the flat tips during races, then go back to my standard tips, I don't want to drill holes in my ailerons and add the cap, then have the holes in my aileron when I go back to the normal tip. I guess one could make a flush insert with nutplates that could go in when the normal tip goes back on...but I looked at the ailerons on mine, and there's not a lot of "meat" to work with there. Drillin' & fillin' moveable control surfaces is a bit beyond what I want to play with as well.

Given this is fun racing (sorry for the sacriledge Bob!), the level of complexity that I'm willing to take on is somewhat limited, and I want to be able to have everything reversible.

Bob Axsom said:
This morning I went to the airport, installed the battery and tested the new semi-flat tips. The resulting speed was 181.5 kts. After the flight it took me 2 hours to replace the semi-flat tips with my 3" span streamlined tips and refruel.

I reflew the test and the resulting speed was 182.3 kts so I do not intend to pursue the flat tips any further.

Bob Axsom
Click to expand...

Wow Bob, not what I expected at all...and you put a lot of work into them! I'll pursue it and see what I get. If its a bust, we'll know more. If it works as it appears to have for Wayne, perhaps more testing would then be warranted (that'd be your call of course). Wayne's wing and mine are already clipped (3.5" per side), so perhaps there's more to it than just going shorter, or there's a place where it starts to have a greater effect. That's a bit counterintuitive to me though, as you would think that shortening a longer wing would have a bigger effect, and at some point diminishing returns would kick in...somewhere before the "this wing won't fly any more" point. ;) Of course, there are a lot of things that go counter to intuition in this aerodynamic game, as you know and have taught us from your work! And heck, it could be that your 3" tips are just a really clean design, and going flat created more drag than lopping off 3" from them gained in span reduction. Hmmmm...do you still have those molds and templates? :)

I know you are very methodical in your testing...do you think one flight with each is enough to make the call? I know that's easy to say, as its your time and gas money (and I respect that). Just wondering how I'll approach it, especially if I see the same result (which will lead to many ribbings from the Stead peanut gallery!)

One last Q...did you note any bending, twisting or vibrating in those trailing edge endcaps?

Sorry that did not go better Bob...we'll keep digging in for speed!!

Cheers,
Bob
 
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Some more information - mostly for Bob Mills

I saw and felt no vibration or other adverse effect from the flat tips. I cut down the spanwise gap between the aileron and the tip from 5/16" to ~ 1/8". The take off roll seemed longer with the flat tips and the plane required what seemed like a lot more right rudder to center the ball - like almost standing on it in the climbout and quite a bit in level flight. It was not at all difficult to fly just the ball wanted to rest off to the right more than usual. On the first leg I was feeling pretty good looking at 206.4 kts ground speed tracking 360. Even after I turned to 120 the ground speed was 175.4 kts (5 consecutive 20 sec. interval average). Then when I turned to 240 it dropped to 159.8 kts so I knew it was not going to be a big breakthrough modification. Later at home when I plugged the numbers into the NTPS spreadsheet the result was 181.5. There was some minor evidence that the rear edge of the right tip contacted the aileron (1/16" paint chip on the aileron and some epoxy resin missing from the tip). When I took off the ATIS gave the surface winds as calm but when I returned after the test flight they were 170 at 7. The calculated wind speed at 6,000 ft density altitude was 201.1 at 26.9.

After I went through the change to my streamlined 3" tips the surface wind on ATIS was 170 at 10. When I returned it was 170 at 10 gusting to 18. The calculated wind at 6,000 ft d alt. was 197.6 at 36.9. The leg speeds with the same 360, 120, 240 GPS ground tracks were 217.2, 170.8 and 153.4 respectively.

It is obvious that the wind was building and the most significant change may have occurred during the first test flight. There can certainly be a case made for retesting but my gut, the inflight feel and the NTPS numbers tell me not to waste any more time on this (I can sense John Huft cringing). Time and money resources have to be respected when you believe the mod is a lost cause. I have no doubt that it is faster than the stock tip but what I developed before is better I think.

If you would like to do the short tip and aileron plug mod that may not be as much trouble as you think. I made balsawood plugs for mine in an earlier unsuccessful test that worked very well. I installed the plug into the recess, drilled a pilot hole through it and the end aileron rib, installed it with some automotive silicon sealant, a dimple washer and a flathead sheetmetal screw. It was solid and I was able to dig it out without too much trouble after the test showed a speed loss instead of a gain. I think the speed loss was due to the opening on the high pressure lower side of the wing being left open while the exit low pressure side was being blocked off. I've got an idea for a closure fairing around the aileron mount after which the plug and another upper surface closure mod may go back in. Just thinking at this point.

Bob Axsom
 
Roger all Bob. I'll do some similar testing, and see what I get. I'll carry some of the load with ya!

IIRC, Wayne's tips have small extensions (or drop downs, if you will) that match the outer aileron hinge, giving it a bit of a fairing effect...that's the intent anyway. Trying to decide if I'll do that as well.

Will be interested to see the closure fairings and end caps as you sleuth it out. Will keep you posted on my progress.

Cheers,
Bob
 
Bob,

Speed reduction with flat tip: what I expected!. I would try a flat plate, slightly bigger than the wing, extending an inch or so around the wing and see what that does. Doesn't take a lot of time to make. That way you will reduce the turbulance (air flowing from the bottom to the the top of the wing, over the tip).

Flap hinges: I didn?t know the 7 had a different flap design. I bet that for the 9, a knot or two may be gained with a good fairing around those hinges.

Ailerons: I don?t mean the opening at the top and bottom, along the length of the aileron. It?s those openings where the hinges and pushrod are. Maybe this is a different design on the 7 as well? I was going to close the sides of the flap and the aileron, like I have done with the elevator and the rudder, but I guess it?s no use with these openings that are way more draggy.

Regards, Tonny.
 
Speed vs wingtips

As you know, the equivalent parasite drag area for an RV-6 is about 2.2, and for a -6A is about 2.32. If you increase your wing area, since the 23013 airfoil has about a 0.006 CD, your parasite drag area will increase by about 0.006 X area increase. If you increase the span, the induced drag will decrease by 1/span^2. So if you want to go faster at low altitude where induced drag is minimal, decrease your wing area. If you want to go faster above 7000', increase your wing span. If you cut your span back, say 1' or 2' on each side, and then add triangular tips to end up with a little less area and a little more span, you'll get the best of both! The reason why is that the area, parasite drag, only increases half as fast as does the span, induced drag, with a triangular-planform tip. You will also see a decrease in stall and landing speed and roll-rate, and a better take-off and climb rate. But the tip shape is very important as it determines the Oswald efficiency factor which relates to the effective span of the wing which is where the tip vortex forms affecting the induced drag. A flat tip with sharp edges top and bottom trips the flow around the tip from bottom to top and keeps the vortex near the tip. But if the tip has rounded edges top and bottom, the air flowing up and around will stay attached due to Coanda effect and cause the vortex to form farther inboard, increasing induced drag. The slashed or sharp-edged tip has an Oswald efficiency factor of 0.81, whereas the rounded tip, in planform or as viewed from the front, is down more toward 0.75, and since this factor is in the denominator of the equation for CDI, lower values are worse. Have any of you seen Jim Smith's tips on his -6?
 
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