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Better airfoil for Christavia Mk IV

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Streffpilot

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Mukwonago, WI
Hi, I have the plans for the Christavia Mk IV on order. Hopefully they arrive this Wednesday. I am planning on trying to reduce drag a little bit and am also looking for a lower drag airfoil. The main goal of this is to get a little more cruise speed out of the airplane. I don't have any runways less than 1500' close by, so I don't mind raising the stall speed a little bit to get a higher cruise speed. The "plan" is to use the airplane to explore our state (Wisconsin) and the neighboring states.

So, any "better" airfoils than the stock airfoil??? To be honest, I don't even know what airfoil it uses yet, I am just throwing ideas around in my head on ways to lower drag and increase speed......
 

TFF

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Memphis, TN
I always though the plane was fast for a CLT, Cub Like Thing. Are they metal wing or wood? Fabric covered wing?
 

Streffpilot

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Jul 12, 2011
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Mukwonago, WI
It is a 4 seat tube and fabric with wooden fabric covered wings. I plan to clean up the landing gear a bit, and some other little things to make it a tad faster.....i know it will never be a Glassier or something, but Im a guy......more speed is more gooder..... :)
 

TFF

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Being fabric covered you are not going to get much of anything, I'm betting. It has a custom airfoil already. Your best bet is matching it up with its closest relative . I bet its a modified Taylorcraft airfoil which is probably what I would do if trying to go faster without reinventing the wheel and having a bad set of wings. I would think all the knowledge homebuilt would be in Wisconsin?
 

Tiger Tim

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Thunder Bay
Do a search, there was a recent similar thread on doing the exact same thing to a Pacer. I believe the end verdict was there was a lot more to be gained by fairings, cooling drag, etc than you would ever get by changing airfoil.

If it were me I'd tidy up one thing at a time once it's flying, saving the wing for last. Then, if I was really ambitious I'd look into the feasibility of doing something like the Wilson Ellipse's wing:
 

dcstrng

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VA or NoDak
Always liked the Christavia family and perhaps naively considered the MK IV as an alternative to the WagAero 2+2... Being from Tailwind country I assume you've already considered and rejected Callbie Wood's CF-4. Four-Runner -- a somewhat faster bird on similar power (yep, I know -- the last thing someone wants to hear after they've committed to a particular bird is some nut telling them to consider another altogether, sorry...). You didn't say what your projected flight envelope is, or what power you are considering -- but along with the Bearhawk, that is about the customary universe of tube and fabric birds, from backwoods/utility to speedier CX. Not sure how clean the Christavia is, so even a successful airfoil change may not gain as much as one might hope (pure speculation from out here in the peanut gallery).
 

Streffpilot

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Messages
318
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Mukwonago, WI
I did look at the other airplanes, and although each is great on its own, I just didn't care for them for some reason or another.

So far my "speed" mods planned are:

172 landing gear

Wheel pants

Internal antennas

Make my own cowl

attempt some fairings


Anything else i should consider??
 

Streffpilot

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Messages
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Mukwonago, WI
I would think all the knowledge homebuilt would be in Wisconsin?
Actually, Just spent the day up at the EAA museum, since my 2 year old is Airplane nuts, and has been for at least a year now. My main reason for building right now is to keep that fire kindled. She can already point to a picture and point out the propeller, landing gear, wings, tail, cockpit, and stuff like that. (as you can tell i might be a tad proud of her)
 

Victor Bravo

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A different lower drag airfoil on the Christavia will be the least benefit on that style of aircraft IMHO.

Take out the air gaps and surface discontinuities from the existing "airfoil" and you will see a lift and drag benefit.

Address the turbulence at the wing root, strut attachments, and control/hinge attachments and you will see a noticeable drag benefit.

Make sure the wing is mounted on the fuselage at the best possible angle and you will see a noticeable drag benefit.

Reduce the cooling drag from the airflow through the engine compartment and you will see a much bigger benefit than changing the airfoil.

Use well designed wheel fairings ("pants") and you will see a significant cruise speed benefit.

Run the aluminum or plywood "sheeting" from the leading edge to 33% chord line and you will see a lift and drag benefit.

Use the "pressure recovery" principle (Taylorcraft "pumpkin seed" fuselage) when designing the fabric forming stringers and formers, and you will see a drag benefit.

Use a Grove aluminum landing gear, with drag reducing "semi-airfoil" machined shape, and you will see a drag benefit.

Use streamlined flying wires on the tail instead of round wires, and you will see a benefit.

All of these things are a lot less effort, risk, and re-engineering than changing the airfoil.
 

Streffpilot

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Joined
Jul 12, 2011
Messages
318
Location
Mukwonago, WI
A different lower drag airfoil on the Christavia will be the least benefit on that style of aircraft IMHO.

Take out the air gaps and surface discontinuities from the existing "airfoil" and you will see a lift and drag benefit.

Address the turbulence at the wing root, strut attachments, and control/hinge attachments and you will see a noticeable drag benefit.

Make sure the wing is mounted on the fuselage at the best possible angle and you will see a noticeable drag benefit.

Reduce the cooling drag from the airflow through the engine compartment and you will see a much bigger benefit than changing the airfoil.

Use well designed wheel fairings ("pants") and you will see a significant cruise speed benefit.

Run the aluminum or plywood "sheeting" from the leading edge to 33% chord line and you will see a lift and drag benefit.

Use the "pressure recovery" principle (Taylorcraft "pumpkin seed" fuselage) when designing the fabric forming stringers and formers, and you will see a drag benefit.

Use a Grove aluminum landing gear, with drag reducing "semi-airfoil" machined shape, and you will see a drag benefit.

Use streamlined flying wires on the tail instead of round wires, and you will see a benefit.

All of these things are a lot less effort, risk, and re-engineering than changing the airfoil.
THAT is the list i have been looking for!!!!

Thank you very much!
 

Victor Bravo

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I could have just said "go find your nearest glider operation and talk to several of the competition type pilots" :)

Since that is apparently the type of information you were seeking, I'll elaborate a little...

The "cooling drag" and gap sealing and smoothing the existing wing and "parasite drag" stuff should be at the top of the list, it's the "low hanging fruit".

If you are feeling adventurous, look into "up-draft cooling" as opposed to the standard "down'draft" type that almost all airplanes are using. If you're ready to travel to futuristic worlds in this area of clean-up, research Peter Garrison's Melmoth 2, the cooling system in particular. The big key is that you need to let the air into the cooling system at the highest pressure point on the cowling during climb and cruise, not just cruise. Then you need to let the air out of the system at the lowest pressure point during climb and cruise. You will quickly see that most every production airplane has it wrong. This was originally for aestheitc and sales reasons. Understand that the very top front of the cowling is often under lower pressure than you thinkk... when you are climbing the airplane. Some airplanes are so bad that the cooling flow reverses, because the bottom of the cowling is in high pressure and the top front of the cowling is in lower pressure. If you do not want to be that adventurous to build an up-draft system, then use a GOOD down-draft system, let the air in at a lower part of the front cowling, and let the air out of the system on the sides of the cowling instead of the bottom (worst place).

Since you are building from scratch, definitely install moveable louvers or "gills" that you can open and close in flight. This will give you the ability to cool your engine safely on a warm day, yet reduce the drag in reduced power cruise flight.

When you cover the wings and tails in fabric, learn how to do the "racing stitch" with the rib lacing cord. It only takes a little extra effort but it does yield a smoother surface. Better yet, use the Aeronca style pop rivet option, and take the time to "dimple" the rib holes so you can use countersunk rivets. This will yield a really smooth low drag surface.

Your project is an ideal candidate for wheel pants because you mentioned you will be flying from decent quality airstrips. So you can go to a 6.00 x 6 wheel and tire and use a good pressure recovery style wheel pant. There is a fair amount of drag that can be reduced with wheel pants, because reducing the wheel drag also reduces the nose-down pitching moment, which requires "trim drag" to overcome.

You can build up some wooden "false ribs" that go onto the stabilizer and elevator, taking it from a flat section to an airfoiled section. You will need to put some wooden "sheeting" on the front half of this as well for best results.

It's not really a speed mod, but since you are building the airplane you should definitely use a better wingtip shape than what is usually found on these type aircraft. Take a look at the modern transport from the Dornier company for the best visual example. The trailing edge should remain straight and the leading edge should be swept back sharply. This improves low speed handling, and increases the climb rate (through drag reduction). At the very least use the Wittman style trapezoid tips .

If you have room on your forward fuselage "boot cowl", extend the lower edge of the windshield forward to yield a shallower, more raked angle at the front of the windshield. The front of the windshield is one of the higher pressure points on the airplane, so reducing that ram air pressure will help a little.

When you make those little removable strips that cover the gap between the wing root and fuselage, make them out of fiberglass and find a way to incorporate a smooth, moderate radius between the lower wing surface and fuselage. Any leaks, gaps, or holes in this area will create a drag-producing jet of air coming up from the bottom to the top. Tape these gaps up like a glider pilot does, or take the time to install foam weatherstripping to prevent the leaks.

A fiberglass fairing that creates smooth flow around as much of the tailwheel and tailwheel spring as possible is like free speed. You cannot cover it all of course. But that spring and all the bolts and brackets is just sitting there causing drag 100% of the time.

If you are feeling frisky, adapt the RV system that uses a small round steel rod instead of the big leaf springs. If you decide to keep the leaf springs, use fiberglass leaf springs. Saves a bunch of dead weight, rust, etc.

Your cruise speed and your climb rate will both be measurably improved by the use of a variable pitch propeller. True "Constant Speed" is not necessary, you can adjust it manually. Do not pass go, proceed directly to WhirlWind Propellers in El Cajon, CA.

Likewise, use at least one LightSpeed Engineering electronic ignition system to replace one of the magnetos. You will get more power (speed, climb), better fuel economy, higher cruise altitude, and more miles per gallon. Do not pass go, proceed to LightSpeed Engineering in Santa Paula, CA... "go see Klaus".

After all this about the Christavia, if a faster cruising speed is high onyour list, before you start building anything in earnest or buying materials, make a side-by side assessment and comparison between the Christavia and the Wittman W-10 Tailwind.

OK, rant switch off, remove soap-box....
 
Last edited:

dcstrng

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Oct 17, 2010
Messages
913
Location
VA or NoDak
Kent Paser's "Speed with Economy," although not generally for tube and fabric birds, gives lots of ideas -- mostly in line with Victor Bravo's list...
 

Pops

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Jan 1, 2013
Messages
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USA.
I could have just said "go find your nearest glider operation and talk to several of the competition type pilots" :)

Since that is apparently the type of information you were seeking, I'll elaborate a little...

The "cooling drag" and gap sealing and smoothing the existing wing and "parasite drag" stuff should be at the top of the list, it's the "low hanging fruit".

If you are feeling adventurous, look into "up-draft cooling" as opposed to the standard "down'draft" type that almost all airplanes are using. If you're ready to travel to futuristic worlds in this area of clean-up, research Peter Garrison's Melmoth 2, the cooling system in particular. The big key is that you need to let the air into the cooling system at the highest pressure point on the cowling during climb and cruise, not just cruise. Then you need to let the air out of the system at the lowest pressure point during climb and cruise. You will quickly see that most every production airplane has it wrong. This was originally for aestheitc and sales reasons. Understand that the very top front of the cowling is often under lower pressure than you thinkk... when you are climbing the airplane. Some airplanes are so bad that the cooling flow reverses, because the bottom of the cowling is in high pressure and the top front of the cowling is in lower pressure. If you do not want to be that adventurous to build an up-draft system, then use a GOOD down-draft system, let the air in at a lower part of the front cowling, and let the air out of the system on the sides of the cowling instead of the bottom (worst place).

Since you are building from scratch, definitely install moveable louvers or "gills" that you can open and close in flight. This will give you the ability to cool your engine safely on a warm day, yet reduce the drag in reduced power cruise flight.

When you cover the wings and tails in fabric, learn how to do the "racing stitch" with the rib lacing cord. It only takes a little extra effort but it does yield a smoother surface. Better yet, use the Aeronca style pop rivet option, and take the time to "dimple" the rib holes so you can use countersunk rivets. This will yield a really smooth low drag surface.

Your project is an ideal candidate for wheel pants because you mentioned you will be flying from decent quality airstrips. So you can go to a 6.00 x 6 wheel and tire and use a good pressure recovery style wheel pant. There is a fair amount of drag that can be reduced with wheel pants, because reducing the wheel drag also reduces the nose-down pitching moment, which requires "trim drag" to overcome.

You can build up some wooden "false ribs" that go onto the stabilizer and elevator, taking it from a flat section to an airfoiled section. You will need to put some wooden "sheeting" on the front half of this as well for best results.

It's not really a speed mod, but since you are building the airplane you should definitely use a better wingtip shape than what is usually found on these type aircraft. Take a look at the modern transport from the Dornier company for the best visual example. The trailing edge should remain straight and the leading edge should be swept back sharply. This improves low speed handling, and increases the climb rate (through drag reduction). At the very least use the Wittman style trapezoid tips .

If you have room on your forward fuselage "boot cowl", extend the lower edge of the windshield forward to yield a shallower, more raked angle at the front of the windshield. The front of the windshield is one of the higher pressure points on the airplane, so reducing that ram air pressure will help a little.

When you make those little removable strips that cover the gap between the wing root and fuselage, make them out of fiberglass and find a way to incorporate a smooth, moderate radius between the lower wing surface and fuselage. Any leaks, gaps, or holes in this area will create a drag-producing jet of air coming up from the bottom to the top. Tape these gaps up like a glider pilot does, or take the time to install foam weatherstripping to prevent the leaks.

A fiberglass fairing that creates smooth flow around as much of the tailwheel and tailwheel spring as possible is like free speed. You cannot cover it all of course. But that spring and all the bolts and brackets is just sitting there causing drag 100% of the time.

If you are feeling frisky, adapt the RV system that uses a small round steel rod instead of the big leaf springs. If you decide to keep the leaf springs, use fiberglass leaf springs. Saves a bunch of dead weight, rust, etc.

Your cruise speed and your climb rate will both be measurably improved by the use of a variable pitch propeller. True "Constant Speed" is not necessary, you can adjust it manually. Do not pass go, proceed directly to WhirlWind Propellers in El Cajon, CA.

Likewise, use at least one LightSpeed Engineering electronic ignition system to replace one of the magnetos. You will get more power (speed, climb), better fuel economy, higher cruise altitude, and more miles per gallon. Do not pass go, proceed to LightSpeed Engineering in Santa Paula, CA... "go see Klaus".

After all this about the Christavia, if a faster cruising speed is high onyour list, before you start building anything in earnest or buying materials, make a side-by side assessment and comparison between the Christavia and the Wittman W-10 Tailwind.

OK, rant switch off, remove soap-box....
I increased the speed of my Falconar F-12 by 30 mph back in the 1980's by doing most of the items you listed. The 1 and 2 mph increases add up. The biggest increase was gap seals on everything, next was cooling drag, next was a more raked windshield.
 

Streffpilot

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Joined
Jul 12, 2011
Messages
318
Location
Mukwonago, WI
Thank you to all who submitted Ideas. I have printed them off, with the thought of attempting to implement what I can where i can.
 
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