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Neutral Trim Hedache on my new Bird

rotto789

Yooper Cub
FOUNDER
Mi. ( Yooper)
Hello everyone,

I have finally commenced flight testing on my newly assembled Smith aviation/ Backcountry Super Cub and I have a problem with attaining neutral trim. let me intoduce my senario, first I will layout all the equipment and configurations, so maybe someone will have an idea of what is going on? here is the aircraft and how it is equiped.

1. 2005 year model Serial number 079 Nick Smith Aviation PA-18 4" wide body Super cub L-21 fuselage
2. Wings, by Smith/Axlesen with 24 gal mains and 10 gal aux, extended flaps, Ailerons and squared
3. Mackey leading Edge Slats
4. Univar struts
5. STOL SPEED Micro VG's placed 3/4 inch behind trailing edge of slats on Wings and Under Horizontal Stab spaced according to there instructions
6. wing tips are the droop tips provided by smith aviation
7. Mackey SQ2 engine mount with 0 thrust line and cowling
8. Aero Sport 0320 B2B, 8.5 to 1 compression with Bart's Mods for supposedly 170+ HP
9. Prop is Catto 84X37 New, designed for this configuration and primarily for climb obviously
10. Duel P-Mags
11. Earth X Batttery
12. light weight starter and alternater
13. Vettermens duel Exhaust with heat muffs
15. the Landing gear is 3" extended with 31" bush wheels, Grove double puck brakes, matco master cylinders
12. floor boards are 3003 .045 diamond decking all the way to back of extended baggage
13. interior is 6061T6 .020 covered with 3M Carbon Wrap
14. Seats are Orgon Aero front and full width back with under seat storage box
15. ABW/Scott 3200 Tail Wheel
16. Rudder and Elevator Gap Seals
17. Panel has Dynon D180, Garmin GTX327, MGL V-10, Uma Airspeed indicator

Weight and Balance comes out at, Right Main 583 pounds, Left Main 591 pounds, Tail Wheel 84 pounds, 6 qrts oil in engine, no fuel, total 1258 pounds Emty using digital Scales with plane in level flight position, reading plumb at firewall and with leading edge of wing at 1.5 inches ahead center of axel.

With me in the front seat at 235 pounds at level cruise speed (which is slow by the way) Im seeing about 75 MPH at 2250 RPM I can not get a nutural stick with full nose up Trim, even at full throttle I still can not get a neutral stick, it is very close but wont get it, I added 80 pounds to the back seat for a flight test and I am now able to get nuteral stick but with not much nose up trim left to spare on the horizontal Stabilizer.

We double checked all the Stab Readings with the plane leveled and come up with 3 degrees down leading edge of stab(Nose Up Trim) and very close to 4 degrees up leading edge(Nose Down Trim) from looking at the TCDS's we appear to be with in range of what is required, at least it appears that way unless i'm reading it backwords, our only other thought was that maybe the rear Stab Tower might be welded on to low,(this was a problem with some earlier Smith Cubs) but it cant be if we are getting the required readings on the Horiz Stabilizer, we also checked the wings for wash, dihedral, and AOI all appear to be within the correct limits with the AOI at 1.7 degrees, dihedral at 3 1/8 inches and the wash at 1.8 degrees from wing root rib to tip rib.

just one other thing I forgot to mentioned, we have not yet installed the flap gap fairings, and we do experiance some shake on tail with full flaps deployed, we will install these as soon as I can find a source for them, anyone? I dont think this would have an effect on our trim issue, correct me if i am wrong.

What can we Do to correct this problem without having to add weight in the tail? or is this my only option? Ideas and/or opinions are much appreciated!

Rick

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Rick,

Glad to hear you are flying that beautiful bird!!!

There was lots of discussion on stock cubs having the trim yoke upside down and causing this very problem. Maybe take a look at that while you are awaiting for the true gurus to reply.

Operationally, before I stuck lead in the tail area, I would be installing some required survival gear in a bag in the back as 'required equipment'. FAA approved that for me when I put AWB's on a cub one time to get me in limits. I was required to have x lbs of survival gear in the back when solo.
 
Rick,

Glad to hear you are flying that beautiful bird!!!

There was lots of discussion on stock cubs having the trim yoke upside down and causing this very problem. Maybe take a look at that while you are awaiting for the true gurus to reply.

Operationally, before I stuck lead in the tail area, I would be installing some required survival gear in a bag in the back as 'required equipment'. FAA approved that for me when I put AWB's on a cub one time to get me in limits. I was required to have x lbs of survival gear in the back when solo.

Thanks George,

we have checked the Trim yoke and indeed it is in the right position, truss up with leading edge of stab bottoming out on longeron, no room for further travel down at all, we also fall easily within the weight and balance range, me in the plane, 24 gallons fuel, end up with a cg of 16.52

Rick
 
Is 1.8° the spec for the wash?

Also double check with a straight edge at each flap hanger that the bottom of the flap is on the same geometric plane as the bottom of the wing.
Two factors contribut to this. The height of the hangers and the flap push rod adjustment. Could be an optical illusion but it appears to be off!
 
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Is the trim bungee spring installed in the tail system. Might increase the strength so that the more you trim the more it assist. I'd make sure that you have the one that assist with trim up is installed. I bought my flap gap seals from Dakota, Univair also has them.

My aircraft runs out of trim forward when heavily loaded (2000lbs). Go figure.
 
If your fuselage was built before Bruce Reed changed the jigs. And per your serial number it is, as he restarted the numbering system, then the tower is wrong. It is easy to interpret stab angles backward. I can find measurements for you if you want.
 
Good point Fortysix12!

If it is installed it looks like it might be too long! With the leading edge of the stabs all the way down the trailing edge of the elevators should be higher! Tie a half hitch in the cable and try it!��
 
If your fuselage was built before Bruce Reed changed the jigs. And per your serial number it is, as he restarted the numbering system, then the tower is wrong. It is easy to interpret stab angles backward. I can find measurements for you if you want.

There was a time way back before we covered the Fuselage (early 2011 I think) that I had a phone conversation with Bill Rusk about this very same issue, I distinctly remember him calling out the measurement since he had, I beleive, the same issues with his first Smith PA-18, I checked my stab mount tower with his repositioned stab tower measurement and we were within I think 1/8th of an inch of his corrected measurment, since I am not the original purchacer of this particular airframe,( I purchaced it in 2010) I assumed at the time I talked to Bill it had been corrected by the original kit purchacer/builder, it did appear like the powder coating was removed and then repainted at the yoke mount and the stab mount locations, I am trying to find my notes from when I talked with Bill so I can verify my memory, if you, or anyone for that matter, has the correct measurements from top of longeron to center of rear stab tube and the required up and down degrees of travel that would be helpfull in determinating if this is the issue, I am leaning more toward this issue as causing the problem, with another degree of nose up I think this problem is solved that is if this is the problem, not sure what the repair will be, as this is a very tight location to try to cut and weld a new tower in with Fabric and paint in the way, Yikes!!

Thanks for all of your input really appreciate it,

Rick
 
Is the trim bungee spring installed in the tail system. Might increase the strength so that the more you trim the more it assist. I'd make sure that you have the one that assist with trim up is installed. I bought my flap gap seals from Dakota, Univair also has them.

My aircraft runs out of trim forward when heavily loaded (2000lbs). Go figure.

Hi Fortysix,

I do have the up elevator assist spring installed and it does help the pressure on the stick, although it could be a little tighter I think, I may try to do like FabMan recomends and shorten the spring a tad to see if that will help, also thanks for the infor on the Flap Gap fairings I think I will call Dakota, I have puchaced needed parts from them before.

Thanks again

Rick
 
Real close to 12.25" from the bottom of the lower longeron to the center of the pivot tube. Measured parallel to the rudder post. If I remember right they were about 1.5" to low. I had to tighten the provided spring on mine to get it right as well.
 
Real close to 12.25" from the bottom of the lower longeron to the center of the pivot tube. Measured parallel to the rudder post. If I remember right they were about 1.5" to low. I had to tighten the provided spring on mine to get it right as well.

Gerald,

Thanks for the information, when I get back out to the airport after work, Im going to check that dimension, I'll let you know how that goes, much appreciated!

Rick
 
Gerald,
Well, I checked the dimension that you gave, she's spot on! Now I'm really confused, I measured from bottom of lower longeron to center of stab pivot tube 12 1/4 inches, give or take an 1/8th, any ideas??

Thanks for the help,

RickIMG_20150901_171352593_HDR.jpg
 

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rotto789, Somewhere on here I saw a picture of a plane which was equip about the same as yours. It had the jack screw pivot shaft attached to the top of the front stabilizer tube giving about an inch more down (nose up) trim. I thought that it was 5rivers airplane, but can't find the picture.

One way to make the flap gap fairing is to get some balsa wood from a hobby shop. Glue it to the trailing edge. Cut, sand to shape and cover with fabric. Works great.

Another way is to use two strips of aluminum. One curved and the other flat, riveted together at the trailing edge. Use flush rivets with the heads against the flap (tails up) to prevent chafing of the fabric.

Both methods should be easy to do in a home shop.
 
rotto789, Somewhere on here I saw a picture of a plane which was equip about the same as yours. It had the jack screw pivot shaft attached to the top of the front stabilizer tube giving about an inch more down (nose up) trim. I thought that it was 5rivers airplane, but can't find the picture.

Hi Skywagon,

If you or someone else could come up with that picture I would really appreciate that,(i'm also looking on here) open to any ideas right now to gain that extra 1 degree of nose up trim with out a huge amout of invasive cuts and welds,

Thank You,

Rick
 
I thought it was the front stab attach tower that was miss welded on the early Smith kits, not the rear. I might be wrong. Also I believe the solution was to adjust the front wing spar attach fitting to increase the wing AOI. Backcountry Cubs should be able to help you work through this. If they can't or won't, Jay at Javron will. He has the wing spar attach fittings in stock on the shelf and is knowledgeable of this issue.

Hope this helps

bill
 
I thought it was the front stab attach tower that was miss welded on the early Smith kits, not the rear. I might be wrong. Also I believe the solution was to adjust the front wing spar attach fitting to increase the wing AOI. Backcountry Cubs should be able to help you work through this. If they can't or won't, Jay at Javron will. He has the wing spar attach fittings in stock on the shelf and is knowledgeable of this issue.

Hope this helps




bill

HI Bill,

yes, you are right about the front spar attach fittings, also the front yoke stab tower was welded on incorrectly from smith aviation, but, I do belive they had a problem with the rear tower as well, all of these corrections appear to have been made on this airframe, (before I purchased it) we have checked all of these dimensions. As far as the front wing spar attach fittings to gain the AOI needed, we have already executed this correction as well with new attach fittings (provided by Axelsen from Turbine cubs at the time) installed and corrected AOI obtained, reading about 1.7 degrees of incidence on calibrated digital scale, we are still at a loss for not being able to get enough nose up trim for a nuteral stick with me solo in the front seat, however, this is a bit of a one off bird in that it has been kind of pieced together with parts from Smith, Mackey, and Backcountry, primarily, the airframe is an early (2005) smith version of the L-21 kit with there wings, Engine mount and cowling from Back Country and leading edge Slats from Mackey, there may still be variables that we are not picking up on. (Yet) I may just Call Jay at Javron to pick his brain.

Thank You for your help, even though we have never formally met, you have been a huge inspiration in my build process over the last five years,

Rick
 
Rick, Fix the gap fairing first. You may find that your trim problem disappears.

Skywagon,

I was questioning this early on in flight testing, A&P at the airport didnt think it would do anything to improve nose up trim, I think we will get this out of the way first, then see what happens, baby steps as they say, one problem at a time, try untill we succeed.

thanks for your help,

Rick
 
That fairing does more than looking pretty. If you would like to learn a little about it, tape on a lot of 6" pieces of yarn on the lower trailing edge of the wing and bottom of the flap including the trailing edge. Place some yarn on the leading edge of the flap and note where it goes. Run the yarn the full length of the flap. Fly with different flap positions and air speeds down to stall. Watch what happens. Put on the gap fairing and look again. If you can take some pictures to study after you land.

When the yarn is flowing in a straight steady line, the air flow is smooth. When it is jumping around it needs some correction. It will be flowing in multiple directions in the prop blast and smooth outboard.

Do this before the fairing is installed so that you can see the before and after of the fairing.
 
Here are some ideas. I myself would install the Gap seals you need them regardless. This will remove the buffet on flap deployment. Second, for testing reasons only, I would see what the effect of strapping down a weight equal to the G35 battery at the same original Piper factory station/location. Third I would adjust the flaps so that they are a tad negative in the full retracted position( not sure how much but keep them the same, somewhere there is a discussion on the subject) And last increase the bungee spring tension ratio. Spring specs are available. First measure/document what you have now by position trim full nose up. perhaps a fish scale type would work measuring what it takes to pull the tail to neutral.
 
46-12, His loaded CG is 16.52". That should be able to be trimmed, since the CG range is 14.0"-20.0". Your battery idea will work, though that is not his answer.

I believe that reflexing the flaps would be counter productive. I do know that some folks shorten the rear struts for more take off lift so am suggesting that perhaps those are the people who reflex the flaps to counter the extra wash in. This is not a good idea since the extra washin reduces aileron effectiveness at stall speeds. I once tried raising the ailerons on my 185 because I had heard that some planes reflexed the flaps for more speed. My drooping aileron system allowed me to do this in flight in order to see immediate results. It did not work, the plane slowed down in any position other than neutral.
 
I'm preliminary testing I've seen between 3 and 5 mph faster with my flaps reflexed starting at 135.

Still working on some more consistent numbers.

Not a lot of gain, but there's a little.

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I'm preliminary testing I've seen between 3 and 5 mph faster with my flaps reflexed starting at 135....
135 what? Miles per hour? If so that is well above normal tricked out Cub speeds and may not be relevant to rotto789's situation.
 
True, it's faster than cub speeds. I haven't tried it around 100 mph yet. In any case, reflexing ailerons wouldn't seem to be a good comparison, as there's already washout in the wing. Reflexing the flaps will reduce the local AoA, and will require a little bit of nose up trim to maintain level flight. But at the tip the local AoA is pretty low already due to washout. Depending on how far, sticking the ailerons up would seem to create a negative camber, increasing induced drag and requiring the inner section of the wing to compensate for the added "weight" of the aircraft, further increasing induced drag. It'd be interesting to see in a wind tunnel for sure.

I think the 185 wing has a different airfoil and washout specs than a piper wing as well, but I'm not 100%.

For anyone that's curious, my flaps reflex 7 degrees. Hardly noticeable to the eye.

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