Exp cub out of nose down trim....Ideas please?

[mike mcs repair]

I can measure this weekend. Set plane to level flight and then measure at the root across the spars?

bottom of butt rib, but same...

 

[BC12D-4-85]

See page 10: https://www.faa.gov/aircraft/safety/alerts/aviation_maintenance/media/1997/1997_08_Alert.pdf

Gary

 

[KevinJ]

Micro vg’s reduced my spin recovery from seven turns to about one third of a turn....

Do you start thinking about when your chute was last packed after 6 1/2 turns?

 

[DENNY]

I would go to 1.5 on the ski and see what it does (it should help). Mitch is the guy to fix it if you do end up cutting the tail again.
DENNY

 

[fobjob]

Do you start thinking about when your chute was last packed after 6 1/2 turns?

chute?

 

[AkPA/18]

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

 

[Bill Rusk]

Adam

Trust me on this....AkPA/18 above knows what he is talking about. That is your answer.

Bill

 

[Gordon Misch]

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

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!

 

[AkPA/18]

Leading edge up is positive. Stabilizer in relation to wing.

 

[BC12D-4-85]

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

 

[skywagon8a]

I've thought about it but I thought that would intensify the problem wanted to increase the lift moreso….Never flown a slatted cub so school me up..

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.

Micro vg’s reduced my spin recovery from seven turns to about one third of a turn....

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.

 

[stewartb]

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.

 

[fobjob]

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.

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.)

 

[Ron Graham]

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]

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]

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]

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.

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]

It would help to know what your baseline is now. Put the cub in level flight as you would for rigging and get the wing angle at the fuselage and you tail travel up and down.
DENNY

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

 

[adamgrenda]

Mark I posted measurements in the last post in this thread what do you think?

 

[fobjob]

It would have made a lot more sense had you measured with respect to the bottom of the door frame.....by nose up do you mean the “nose “ of the stab or the nose of the aircraft?

 

[adamgrenda]

It would have made a lot more sense had you measured with respect to the bottom of the door frame.....by nose up do you mean the “nose “ of the stab or the nose of the aircraft?

nose up trim so leading edge of stab down.

 

[fobjob]

That makes even less sense. Maybe I have the relationship between the firewall and the waterline backwards?

 

[AkPA/18]

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]

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

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?

 

[fobjob]

Sorry, those numbers still don’t make sense. I’m going to bed now.

 

[AkPA/18]

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

 

[fobjob]

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.....

 

[AkPA/18]

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.....

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.

 

[fobjob]

The long tail moment of the cub ameliorates a lot of problems....the pa-12 has a different(more nose-heavy) weight distribution. That could be a solution, too...is there a pile of bricks in the tail, by chance?
Or, is the engine mounted too far aft?

 

[stewartb]

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.