mike mcs repair
Registered User
chugiak AK
bottom of butt rib, but same...
Exp cub out of nose down trim....Ideas please?
- Thread starter adamgrenda
- Start date
bottom of butt rib, but same...
BC12D-4-85
SPONSOR
Fairbanks, AK.
Do you start thinking about when your chute was last packed after 6 1/2 turns?
fobjob
MEMBER
Salt Lake City, Utah
Do you start thinking about when your chute was last packed after 6 1/2 turns?
chute?
AkPA/18
MEMBER
Big Lake Ak
Adam,
Fuselages can be a pain to measure for angle of incidence. Just an idea if you want to simplify it. Measure your wing angle across the bottom of the spars, then put your trim full flight nose down trim. Measure the stabilizer angle. You should have a positive number greater than the wing number. This will give you an idea of the trim relationship to your wing. I would do both wings
Take my advice with a grain of salt. No pedigree IA or A&P
Fuselages can be a pain to measure for angle of incidence. Just an idea if you want to simplify it. Measure your wing angle across the bottom of the spars, then put your trim full flight nose down trim. Measure the stabilizer angle. You should have a positive number greater than the wing number. This will give you an idea of the trim relationship to your wing. I would do both wings
Take my advice with a grain of salt. No pedigree IA or A&P
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Gordon Misch
MEMBER
Toledo, Wa (KTDO)
Could you please clarify "positive number greater than the wing number"? Which direction is "positive", and in reference to what? The Horiz Ref Line? or ? Thank you!
BC12D-4-85
SPONSOR
Fairbanks, AK.
Do a rigging survey of the airplane versus stock Piper. Info above and via TCDS. Report. If Experimental determine the difference between Piper and yours.
Gary
Gary
skywagon8a
MEMBER
SE Mass MA6
Before you start adding slats or VGs or ........ fix your issues. IF adding other "enhancements" "cures" the problem, they are only masking the real issue. Band aids only cover the wound, they don't fix it.
If your plane could not recover from a spin in no more than one turn, something is very wrong. That is the certification requirement. The VGs are only a band aid covering up the real issue.
A 150hp Cub was certified at 20" at 1760#, right? It isn't unreasonable to think the tail won't support 20" at 2300#. In my experience the tail is always weak when heavy with aft CGs. At 540# over design gross? Maybe you need more tail surface?
The change between skis and tires is probably more about aerodynamics than CG. I like skis rigged flat.
The change between skis and tires is probably more about aerodynamics than CG. I like skis rigged flat.
fobjob
MEMBER
Salt Lake City, Utah
The VGs saved my life (improved my aileron response, too) until I could figure out the problem and get it through the jig in Belgrade, Mt. twenty some years ago....the a/c was crashed on the airport(while jackassing around), repaired ( no paperwork on any of this) and the wings actually rewelded at zero degrees angle of incidence...musta thought it was a PA-12.(?)... then they sold it to me after it was flying line patrol for three years....amazing.( not mentioning any of this to me; I wondered why they rushed to take my first bid, and never returned my phone calls.)
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Ron Graham
Registered User
My tail ski caused problems like this. It was acting like a trim tab in flight. I re rigged it to my advantage and fly trim more or less neutral now.
Gordon Misch
MEMBER
Toledo, Wa (KTDO)
Good point in post #42. I'd think that in the absence of data to the contrary, the maximum CG moment for any weight would be best limited to the certified aft CG moment. This would fundamentally change the shape of the W&B graph at the high-weight end. Definitely keeping that in mind in our E-AB build.
Fortysix12
Registered User
RanchAero Grand Vista, Brooksville
Have you thought about installing a bungee on the trim system. You can increase the ratio by increasing the spring tension spec. as required until you reach perfection. easy fix i would think.
adamgrenda
Registered User
King Salmon, AK
I have the top elevator spring installed nothing on the bottom. I’m sure it’s an angles thing after everyone’s comment. Also Stewart makes a great point going up to 2300 and leaving things close to stock this would be expected is my guess.
adamgrenda
Registered User
King Salmon, AK
Denny I just went out and measured with a smart level. Pretty sure AOI is around 1.0 degree I’ve done multiple measurements from firewall to bottom of wing.
I think tail is out of spec by 3 degree or so since full nose down trim gives me less that the bottom of wing which it should be 2.5 degree about it from tcds.
also reference picture as longerons are not flat as I believe the should be which would make sense.
Firewall zero to butt rib is 89.2-88.7
Full nose down trim top of stab is 87.6
Full nose up trim 85.85
Stab up to down total travel 6.5 degrees
Attachments
adamgrenda
Registered User
King Salmon, AK
Mark I posted measurements in the last post in this thread what do you think?
adamgrenda
Registered User
King Salmon, AK
AkPA/18
MEMBER
Big Lake Ak
First glance it looks fine. Fobjob, put a plus in front of the nose down and a negative in front of nose up to realize these numbers are on either side of the firewall 90 to get the 6.5 range I believe? Buy the way,nose down trim should be called stab up as in tcds. No big deal. That is just the way it is supposed to be referred to, to prevent confusion although doesn't always. Measured movement is the direction of travel of the control surface
adamgrenda
Registered User
King Salmon, AK
mark I guess I thought based on the numbers that the stab angle nose down trim would be a greater number that the bottom of wing. What about longerons coming up and 3/4” gap at tail post?
AkPA/18
MEMBER
Big Lake Ak
Not sure if this explains or not
The straight lines would be your digital level
It was set to zero at the fw vertically to start.
The horizontal ref is 90 to the fw for reference.
The other 3 lines are the wing and stab measurements.
I am assuming that is the way you measured Adam to come up with numbers like that?
Your up stabilizer has a value of 2.5 which is greater than a wing value of 1.0
Attachments
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fobjob
MEMBER
Salt Lake City, Utah
Thanks Mark, the low angle of incidence of the wing appears to be the major culprit, the lack of pitching moment will require up force on the tail, which is BAD.... unstable .. stall/spin hazard...the twist in the wing tips might be excessive also, effectively lowering the pitching moment. If I had an experimental cub, I would make the twist in the wings zero and put some aluminum angle iron on the inboard leading edge to control the stall.
All this is assuming that the firewall is actually ninety degrees to the waterline....
Any measures applied to the tail isn’t going to solve the main problem.....
All this is assuming that the firewall is actually ninety degrees to the waterline....
Any measures applied to the tail isn’t going to solve the main problem.....
AkPA/18
MEMBER
Big Lake Ak
I wouldn't think that would be a problem with all the pa12s flying around at zero AOI and fair number of pa18s with less than perfect AOI. Scratching my head a little here.
This thread seems to be mixing two issues. The perception of inadequate trim and trying to quantify it with what appears to be a suspicious W&B. I'd start with a plumb line and verify the W&B.
The other way to figure it out is to run a few different CGs to establish the pilot's useful aft limit, then when loading it up try to stay within that limit. Which is one reason belly pods are so popular.
Food for thought. As the Exp Cubs have been fitted with bigger wings, slats, improved flaps, and bigger motors guys started installing bigger tail feathers and now the trend is to extend the fuselage. The purpose of the extension is to improve tail authority. They still fly better with a neutral CG. One of the E-AB phase 1 tasks is to establish CG limits. Forget what a Supercub TCDS says. At 2300# it doesn't apply.
The other way to figure it out is to run a few different CGs to establish the pilot's useful aft limit, then when loading it up try to stay within that limit. Which is one reason belly pods are so popular.
Food for thought. As the Exp Cubs have been fitted with bigger wings, slats, improved flaps, and bigger motors guys started installing bigger tail feathers and now the trend is to extend the fuselage. The purpose of the extension is to improve tail authority. They still fly better with a neutral CG. One of the E-AB phase 1 tasks is to establish CG limits. Forget what a Supercub TCDS says. At 2300# it doesn't apply.
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