Building a Smith Cub

[Bill Rusk]

Christian

Yes, right now I am thinking the Catto is better in my application. I do not have a pull gauge so I don't have really hard numbers. I will try to get more hard data as soon as I can but it may take some time. I will try to do a takeoff test next week but that will only compare the 74-56 to the Catto. I'll have to borrow the borer again to do pull test and T.O. test.

Gpepperd did a comparison for his hotrodded 0-320. Here is his comment

I just installed a Catto propeller on my exp. last night and it is better than sliced bread! Not certified but well worth considering for anyone in the exp. class. It pulled around 8% harder than my Borer 8244 (dynometer scale measured) and yet cruised 3-4mph faster at the same RPM. It is 16# lighter than a Borer as well. Craig now has a nickel leading edge he can install so mine will be going back for that as soon as I get a spare from him. Needless to say, If anyone needs a Borer prop I have two very nice ones for sale. One 8244 & 1 @ 8242.
From this thread http://www.supercub.org/phpbb2/viewtopic.php?t=22054&highlight=catto+borer

CGoldy

I guess that is possible but I think it is not likely. If so it would only be on the LE of the elevators. The stab part is somewhat fixed.

Bill

 

[Bill Rusk]

Amy

Glad you got to fly the Hatz. It is a REALLY fun plane to fly. If you are down this way drop in and we'll go fly the Cub. Hope to get it on skis this winter.

Bill

My sister came up from Houston to visit for a few days (I am Blessed to have a great family) and I took her for a Cub ride to include a little sandbar time. Here is Mona with the Cub on the sandbar.

 

[Steve Pierce]

McMaster Carr sells some very slick thin tape that would eliminate the friction wearing on the fabric.I use it on cowling and fairings.

 

[gdafoe]

Steve what is McMaster Carr's order # on that tape?

 

[musket]

Steve Pierce wrote:

McMaster Carr sells some very slick thin tape that would eliminate the friction wearing on the fabric.I use it on cowling and fairings.

Could that same tape be used to cover sharp edges, screw heads, etc. before covering?

 

[Bill Rusk]

The fun continues. Aero 2000, plastic bottoms, sled dog dollys.

Bill

 

[Iflylower]

Bill, you're having too much fun! Good on ya!

 

[jnorris]

Bill,

Nice shots of your plane on skis. Don't forget to follow the proper "major change" procedure as called out in your operating limitations for approval of the ski installation. Unless you're still in your initial flight-test phase, in which case don't forget to do a separate entry in the logbook for the ski-equipped testing along with the entry for the wheel-equipped testing.

 

[randylervold]

Bill,
Over time, won't the friction of the gap seals wear through the paint and then fabric?
CGoldy

We are concerned about that as well and watching for it while we are in test phase. We may end up with language in the STC that says "remove every 50 hours, clean the dirt out and check for damage". If any of you beta testers see any wear please advise.

 

[cgoldy]

"We started with the air dams on the inlets and after the first couple of flights with CHT's near 450 took them off."

Hey Bill, I didn't understand what you call an air dam? I am seeing 400 + temps and want to work to get them down. I reworked my baffling and they seemed to go up!!!!

CGoldy

 

[Fortysix12]

We are concerned about that as well and watching for it while we are in test phase. We may end up with language in the STC that says "remove every 50 hours, clean the dirt out and check for damage". If any of you beta testers see any wear please advise.

Couldn't this problem could be eliminated by installing 1 inch teflon tape on both inside radius surfaces or where ever the gap seals are installed.

 

[Tim]

JNorris, are you saying to put skis on an experimental airplane one needs to do paper work. It's a safety thing, you can't take off in 2 feet of snow on wheels.

 

[jnorris]

JNorris, are you saying to put skis on an experimental airplane one needs to do paper work. It's a safety thing, you can't take off in 2 feet of snow on wheels.

Yes, putting skis on an experimental airplane is a "major change" in the eyes of the FAA. Changing from wheels to skis (or floats) will require the installation to be tested and properly logged in accordance with the aircraft's operating limitations. It's no big deal, and could be a "safety thing" from the standpoint of aircraft controllability and performance so the FAA wants to see the installation properly tested and documented.

 

[Bill Rusk]

Seats

When I built the kit I built it for the 90% rule, ie most flights are local and around an hour or so. My seats were/are light and very comfortable for about an hour or even up to two. Perfect except that when we flew to the E-Tenn fly-in we used 3 + hour legs and the seats got uncomfortable. So I ordered some of that fancy Confor foam (used to be called tempur foam) and made up some new seat bottoms. That should fix that problem but it did add a pound or two.
With the new prop I lost about 12 pounds so I am around 1140 now give or take. I am still looking for ways to shave weight.

I did a LONG post (4 hrs to put it together) on angles, ie AOI, engine, tail etc and lost it. I think the system timed me out. So.... sorry to Gerald and Colin. I'll put it back together when I get over the frustration later this evening. I feel like I just wasted 4 hours of my life and that is not a good feeling.

Bill

PS Thanks Joe for the reminder. I'll make sure the paperwork is up to speed.

 

[gdafoe]

Oh No, I hate when that happens. Sometimes better to do them in Word or something then paste them into the forum software.

 

[Bill Rusk]

Angles

I spent a lot of time when putting the wings on my Cub trying to get all the angles correct. This includes wing angle of incidence, tail angles, trim and surface throws, and thrustline. Here are some numbers that may help others. I do not claim that these are correct, best, accurate, or anything else. Jason Gerard and I consulted with Mark Englerth of the Thrustline kit/mod (which I have) and with Wayne Axleson of Backcountry Cubs, Steve Pierce and others.

The Backcountry Cub guys will build your forward wing attach fittings to any angle you want. The standard ones they made to correct for early fuselage geometry issues from Nick Smith are 3/4 inch different. This amounts to 1.6 degrees increased AOI.

As mark Englerth of the Thrustline kit will tell you just about every fuselage is different and that will apply to new manufactured ones as well. The engine mount on the PA-18 is 4 degrees nose down but that does not mean that the engine is 4 degrees nose down. On most fuselages the firewall is also canted from 1 to 3 degrees nose down such that the total engine down thrust is between 5 and 7 degrees down. The thrustline kit does not get the engine to zero thrust; it usually ends up being 1 or 2 degrees down.

According to Piper the wing AOI should be 1.8 degrees. The LE of the Horiz tail should go up + 2.5 degrees and down -4 degrees (+/- 1/2 degree) based on the TCDS. This is from level. Piper gives the old plumb line technique to level their fuselages. Nick Smith did not do this so as best I can tell the X brace under the fuselage between the gear attach fittings is level.
This gives me a door sill that is + 1.1 degrees and a firewall at 89.5, 1/2 degree nose up. If I level the door sill my firewall is 89.4 degrees the other way, or .6 degrees nose down.

I used a 3/8" increased AOI wing attach bracket. This gives me...

Wing AOI + 1.8
Engine -1.5
Nose up trim +2.5
Nose down trim -4.3
(you will note that my trim values are reversed from what they should be, the result of a miss-welded attach tower on the aft horiz stab)

I do have an early Nick Smith fuselage and I do have the short aft tower mount for the Horizontal stab. The more you raise the wing AOI the more nose up trim you will get as you basically just repositioned the tail neutral point. Remember the wing will be flying at the same AOA at a given speed, the difference will be the angle of the fuselage, and thus the angle of the tail. It gets confusing. Also as you increase the wing AOI you effectively increase the engine nose down moment as well. These three are all inter-related, engine, tail and wing AOI.

Wayne was adamant that I should use the full 3/4" fitting which was designed to correct wing AOI issues from the early Smith Cubs. I felt that 3/4" was too much as that would have resulted in an AOI of about 2.6 degrees. So I compromised and split the difference. I started out with an AOI of 1.1 degrees with the straight wing attach brackets. According to Mark Englerth the Thrustline Kit does not generally work well when the AOI is below 1.2 degrees. It would be interesting to try different attach brackets and see the performance difference but the hassle factor would be pretty high. Furthermore, the greater the increase in AOI brackets the more problems you will have getting the top of the wing, fairings, windshield, top glass etc to fit up. The fuselage was welded up to match the straight brackets so when you use offset brackets you will have fit issues.

I still feel like I am running out of nose up trim and I intend to re weld the aft horiz attach tower at some point, although it is not a big factor. As the CG moves aft the trim issue goes away. I feel that most guys with 0-360's and heavy Pawnee props are pushing the forward CG limit and that is making the nose up trim an issue. I used the prop face as my datum and my CG range is from 70 to 81. I started at 73.5 and am currently at 75 after putting the Catto prop on and thus removing 13 pounds from the nose. There is a noticeable difference between 73.5 and 75. My trim issue is almost gone at this point but I am still anxious to try moving the CG further aft. I will report the results when I do.

So if you want to play. Level your Cub. Then measure your wing AOI at the butt rib. It should be 1.8 degrees. Measure your engine across the valve covers. It should be 0 to 2 degrees down if you have the thrustline kit. Make the Horiz stab level. From there, the LE should go up with trim (nose down trim) 2.5 degrees and down (nose up trim) 4 degrees (+/- 1/2 degree).

I would be happy for others, Mark Englerth, Jerry Burr, Mike Butterfield, Steve Pierce Et AL to weigh in here and correct me and or add to this data.

Hope this helps

Bill

 

[gdafoe]

Thanks Bill, that is helpful. My TCOW/Smith was their number 2 and came from Smith in one of the jigs to Minnesota and was used to do the early jig corrections that they had implemented. It does have the corrected tower, but not the wing corrections. I have the 3/8" brackets that I have not yet installed. I don't run out of up trim but it comes close. I have a wood prop so am light there. The airspeed in not fast normally slightly less than 100mph. I think the brakets will help the speed and the trim. I also have the Zero Thrust engine mount.

 

[Steve Pierce]

I worked on an early Smith Cub that was already flying from Mexico. The pilot weighs about 90 lbs. and using a 180 hp engine could not three point. Through much schooling from Mark E. I got real educated and we used the 3/4" front wing attach blocks. After performing this change it landed fine in a three point attitude. It was odd however because the nose is way down when the wing incidence is changed by 3/4" since the wing is always going to fly level.

The next step was to install the Thrustline Modification. This, in my opinion made the airplane fly like a Super Cub.

The next modification was to install Wayne Mackey's slats that we got from TCOW/Back Country Cubs. They made the airplane incredible in my opinion. I had a lot of fun with it. This was a 1350 lb airplane and with 70 gallons of gas it performed like a light weight. I was very impressed with slow flight and turns and in my opinion this is where the slats really shined. A lot of people don't like the high angle of attack of slow flight with slats or slots but i thought they were a lot of fun as well.

This whole experience was very educational to me and gratifying not only from what I learned but to see the willingness of people like Mark to educate me and going the extra mile to get it all through my thick skull. All this knowledge obtained by measuring and flying Super Cubs. Everyone has benifited from this if they fly a modified Cub or not because we now know if the measurements are correct the airplane will perform well. This has been proven to me time and time again.

 

[rotto789]

Aoi

HI Bill,
According to TCDS 1A2 on the Faa's web site your numbers are correct on The Rear Horiz. Stab. Here is my question, should'nt you level the Wings at the Spars near the connection points for these numbers to be correct? I just checked mine and I am reading 2.5 degrees up LE/ nose down trim and 4.4degrees down LE/ nose up trim. wings level at spar connection, now when I level the way you describe, by the X brace between the gear attach points ( early 2006 Nick Smith) I come up with a reading almost opposite 4.0 degrees full up LE/ nose down trim and 2.3 degrees full down LE/ nose up trim. I am confused hopefully someone can shed some light on this subject............:roll: Rick

Oh ya I forgot to add that when I level the fuce the way you decribed at the X between the gear attach I end up with an AOI of 1.7 degrees give or take a one or two tenths.

 

[Bill Rusk]

rotto

I recommend using the bottom of the wing root rib for your wing AOI. Use a long level or some other long straight instrument to put your smart level on to take out minor variances.
I do not have a solid, positive way to get level on a Smith fuselage. Perhaps Wayne Axleson can help. At any rate, the important thing is the relationship between the three angles 1 wing AOI 2 tail and 3 Engine.
One option is to call the wing the datum then measure relative to that. IE set the wing to level (or zero out your smart level) now trim your stab til is is -1.8 degrees. Now measure your trim from there. Measure your firewall or engine again from the relative zero of the wing.
You do not want to measure the spar attach points as they are not set at the wing AOI.

PM me for a phone number and I would be happy to talk as much as you like.

Bill

 

[skywagon8a]

I have measured many different locations on my TCOW fuselage with a smart level. There are enough small differences that I can not trust any one place to be the correct leveling point. My plan is, with the wings installed, to use the data from the TCDS 1A2. Drop a plumb bob from the leading edge of the wing, (datum), and measure 2" to the center of the main axle. The plane should then be level. The main wheels are listed at +2". After this is done perhaps I can find a place to use a level, door sill, firewall etc?

 

[mike mcs repair]

I measured on a virgin never flown/assembled -18 piper fuselage... with it in level flight position by plum bob:

wing butt rib was 1 deg trailing edge down
firewall 90 degrees

so thats how i level anything, just put level under but rib, get the 1 deg and then check other side, split the difference and rig.. done...

 

[ohansen]

Thanks so much Bill, for your diligent postings on your project. I'm about a year behind you and have made the same changes you have (except for the prop). Have used the 3/8 brackets though. Your postings have been a great help.
We are currently on an RV vacation in Mexico and hope to meet up with that 90lb pilot that Steve mentioned. I've exchange several emails with her to try to get a handle on the horiz. attach tower issue and she's been very supportive. She mentioned that the changes Steve Pierce made to her cub have made a huge handling improvement. The Alaska Bushwheels apparently made a big difference too.
Ole

 

[Steve Pierce]

Thanks so much Bill, for your diligent postings on your project. I'm about a year behind you and have made the same changes you have (except for the prop). Have used the 3/8 brackets though. Your postings have been a great help.
We are currently on an RV vacation in Mexico and hope to meet up with that 90lb pilot that Steve mentioned. I've exchange several emails with her to try to get a handle on the horiz. attach tower issue and she's been very supportive. She mentioned that the changes Steve Pierce made to her cub have made a huge handling improvement. The Alaska Bushwheels apparently made a big difference too.
Ole

You will enjoy meeting her not only to see her airplane but she is a very interesting lady as well. Tell her Cathy, the kids and I said hello.

 

[gntw]

My son is finishing a early smith cub with stadard width fusalage that was probably built in 1999 or around there would it still have these problems?

 

[Bill Rusk]

Yes.

Bill

 

[ohansen]

You will enjoy meeting her not only to see her airplane but she is a very interesting lady as well. Tell her Cathy, the kids and I said hello.

Thanks Steve, will do. I'm bringing a SC.org calendar for her.

 

[gdafoe]

Angles


According to Piper the wing AOI should be 1.8 degrees. The LE of the Horiz tail should go up + 2.5 degrees and down -4 degrees (+/- 1/2 degree) based on the TCDS. This is from level. Piper gives the old plumb line technique to level their fuselages. Nick Smith did not do this so as best I can tell the X brace under the fuselage between the gear attach fittings is level.
This gives me a door sill that is + 1.1 degrees and a firewall at 89.5, 1/2 degree nose up. If I level the door sill my firewall is 89.4 degrees the other way, or .6 degrees nose down.

I used a 3/8" increased AOI wing attach bracket. This gives me...

Wing AOI + 1.8
Engine -1.5
Nose up trim +2.5
Nose down trim -4.3
(you will note that my trim values are reversed from what they should be, the result of a miss-welded attach tower on the aft horiz stab)
Bill

Bill, I have installed the 3/8" front spar attach brackets now and for what it is worth I will give the resulting measurements. These should compare directly with yours as I leveled using the X brace between the gear attach fittings.

As a reminder, my TCOW/Smith fuselage is serial #2 from TCOW. It was in a Smith jig when TCOW bought the company, and was used to do a series of corrections that TCOW implemented on the Smith jig.

Right door sill is at +1.1 degrees
Firewall is at 89.9 Degrees
Wing AOI +2.3 degrees (was +1.6 w/o bracket)
Engine is at -0.4 degrees
Nose Up trim is -2.8
Nose down trim is +4.2 My stabilizer tower is installed correctly
Washout is set at -1.3 degrees from butt rib (half of what Piper calls for)

 

[ohansen]

Bill, I have installed the 3/8" front spar attach brackets now and for what it is worth I will give the resulting measurements. These should compare directly with yours as I leveled using the X brace between the gear attach fittings.

As a reminder, my TCOW/Smith fuselage is serial #2 from TCOW. It was in a Smith jig when TCOW bought the company, and was used to do a series of corrections that TCOW implemented on the Smith jig.

Right door sill is at +1.1 degrees
Firewall is at 89.9 Degrees
Wing AOI +2.3 degrees (was +1.6 w/o bracket)
Engine is at -0.4 degrees
Nose Up trim is -2.8
Nose down trim is +4.2 My stabilizer tower is installed correctly
Washout is set at +1.3 degrees from butt rib (half of what Piper calls for)

Thanks Gerald
Not sure what this will all mean re: flight characteristics but I hope to find out next summer.
Ole

 

[skywagon8a]

I don't mean to be picky Gerald but don't you mean -1.3 degrees washout? The wash in/out is generally referenced from zero twist with washout being leading edge down given in a negative number. A positive setting, leading edge twisted up, would tend to make the tips stall sooner than the root thereby reducing aileron control at stall speeds. Thus a negative twist is preferred.