PA-18 Empty Weight CG Way Forward

[mfleming]

Our new to us certified PA-18 has an empty weight CG of 9.9” (the best I can figure). This seems to be true due to our inability to get the tailwheel to touch with the mains when doing 3 point landings. This is with a 200 lb instructor in the back seat.

Any ideas why or what to do to correct this?

Cub particulars:

• 160 hp
• PSTOL flaps
• McCauley 1A175/GM8244 (Borer?)
• Battery under back seat (AGM)
• AFA 3” HD Gear
• 31” BW
• AOSS shocks
• Extended baggage
• Leading edge exhaust
• Oil cooler moved to baffles
• AD 30.5 gal wing tanks
• 1187.5 empty weight according to the paperwork.

 

[RaisedByWolves]

Light weight starter and alternator, baby Bushwheel, machine the spacer for the baby Bushwheel out of steel, that gives you 8lbs on the tail.

 

[KevinJ]

Switch to a Sensenich GA composite prop. That’ll net you 10+ lbs off the tip of the nose. Leading edge exhaust is heavy. Not much you can do there. Gap seals on the tail if you don’t have them. Easiest is to experiment with ballast in the tail like Tom said.

Nose heavy cubs fly like lawn darts.

 

[bob turner]

Double check all computations. If you weighed it, be sure you used the correct weighing points and the equation in the AFM.

The usual reason (aside from amphibs) is that the data are mixed. Make sure all computed additions use the same datum. We had just one component (a Wolf filter) using the wrong datum, and it moved the CG well forward of limits.

As to the landings - are you sure the stick is against the aft stop before touchdown? Not against your belly button, but full back?

 

[BC12D-4-85]

What's the wing's reaction and incidence angle as the PSTOL flaps are slowly applied? Does it nose up progressively from 0 to full flaps? Or does it vary nose position from 0 to full flaps?

If the wing's center of pressure moves aft (= more nose down at some point near full flap travel) it can be a challenge to lower the tail. Don't know, just asking.

Gary

 

[supercub83a]

There is almost no way it could be that nose heavy. Does it have oratex fabric? Thats about 25 or 30 lbs off the rear, but I doubt that would account for it either. My last cub was similarly equipped but did include a BBW on the tail and it had a cg of 13.47. The extra 8lbs on the tailwheel is worth roughly 1” aft. I would just re weigh it to verify before i made any adjustments.

 

[mvivion]

Frankly, with no passenger, three inch extended gear and 31 inch Bushwheels, I’d be surprised if you COULD touch all three at the same time.

Re-read what Bob posted, check all measurements and especially datum references.

 

[aktango58]

my experience with the P-stol flaps was that it brought the nose down when applied, which was helpful for visibility.

Seems you might want to begin with maybe the angle of incidence, then I would put it on a scale.

Is it possible that the trim A-frame is upside down?

Please confirm you are not on wheel skis...

 

[mfleming]

There is almost no way it could be that nose heavy. Does it have oratex fabric? Thats about 25 or 30 lbs off the rear, but I doubt that would account for it either. My last cub was similarly equipped but did include a BBW on the tail and it had a cg of 13.47. The extra 8lbs on the tailwheel is worth roughly 1” aft. I would just re weigh it to verify before i made any adjustments.

The last W&B was computed when the PSTOL flaps were installed, 9.95”. The former owner says he never had problems with flying solo with no baggage but for now, I have some weight in the extended baggage just in case.
I’m planning on weighing it during its annual in April and get some good numbers.

 

[mfleming]

What's the wing's reaction and incidence angle as the PSTOL flaps are slowly applied? Does it nose up progressively from 0 to full flaps? Or does it vary nose position from 0 to full flaps?

If the wing's center of pressure moves aft (= more nose down at some point near full flap travel) it can be a challenge to lower the tail. Don't know, just asking.

Gary

The nose pitches up quite a bit when full flaps are pulled.

 

[DENNY]

What is your trim setting?? The stabilizer on PA 18 has a huge effect on the tail compared to a trim tab on a Cessna. Make sure you are trimmed for your touchdown speed on final (42 mph give or take a few). I see a post like this at least once a year and the problem is most often improper trim. I have no problem getting the tail below the mains on final.
DENNY

 

[RaisedByWolves]

There is almost no way it could be that nose heavy. Does it have oratex fabric? Thats about 25 or 30 lbs off the rear, but I doubt that would account for it either. My last cub was similarly equipped but did include a BBW on the tail and it had a cg of 13.47. The extra 8lbs on the tailwheel is worth roughly 1” aft. I would just re weigh it to verify before i made any adjustments.

The worst one I had was 9” empty. There is no empty cg for the -18. But I ended up putting 3” gear baby Bushwheel lw alternator and big tail spring. It came back to 10.5”

 

[BC12D-4-85]

One post #8 says full flaps = nose down pitch, other #10 says nose up. I have no experience with PSTOL, but effective flaps (Beaver/Otter) can lower the nose by moving the lift aft on the wing - and aft of the CG.

This video

describes a nose down pitch with full flaps prior to landing. If so (I assume it's correct) then lowering the tail, especially at forward CG which can make the tail light, can take aggressive aft trim if the pilot runs out of elevator authority as Denny notes in #11. Elevator gap seals can also help. My thoughts.

Gary

 

[mfleming]

One post #8 says full flaps = nose down pitch, other #10 says nose up. I have no experience with PSTOL, but effective flaps (Beaver/Otter) can lower the nose by moving the lift aft on the wing - and aft of the CG.

This video

describes a nose down pitch with full flaps prior to landing. If so (I assume it's correct) then lowering the tail, especially at forward CG which can make the tail light, can take aggressive aft trim if the pilot runs out of elevator authority as Denny notes in #11. Elevator gap seals can also help. My thoughts.

Gary

I’ve also heard the PSTOL flaps give you a flatter approach angle. When I deploy full flaps and I’m trimmed for hands off flight, it takes a couple of turns of the trim handle (nose down) to fly hands off after FF deployment.

 

[bob turner]

Look for a computation using the datum 60” forward of the leading edge. It only takes a couple pounds to really move the computed cg if you normally use the leading edge as datum.

Piper uses both, in the same document, right next to each other - and a mechanic used to Cessnas will often grab the wrong one for a simple change. You discover these things years later when somebody needs a check ride or a 709. In between, nobody bothers to do a weight and balance for a specific flight.

 

[DENNY]

I’ve also heard the PSTOL flaps give you a flatter approach angle. When I deploy full flaps and I’m trimmed for hands off flight, it takes a couple of turns of the trim handle (nose down) to fly hands off after FF deployment.

What is you airspeed and RPM when you trim hands off with full flaps?

 

[FatCub300]

I've read your post a couple times and IF your Angle of attack is pitching up with flap deployment, there's a problem. That is not how a flap should affect the distribution of lift.

Your center of lift should be migrating aft, resulting in a nose down pitching moment, which you re-balance with trim with your horizontal to add additional downward force...the horizontal MUST produce downforce at all times, or you have a serious instability issue.

The whole point of Kellers flap, is to reduce the angle of attack, while maintaining the same net coefficient of lift, at a lower angle of attack, allowing you to fly the same approach speed, at a reduced angle of attack.

If this is opposite of what you are experiencing, I'd be very careful. Adding weight aft, could be a pretty big mistake. It strikes me that either your horizontal/elevator could be picking up wake turbulence rolling off the back of the flap and inducing a downward pitching moment at the tail, due to the long moment arm of stab, versus the short moment arm of the flap...not LIFT...but still FORCE...this is not what you want.

This could induce what you perceive as a symptom of the nose going up, when what you really have going on is a horizontal/elevator losing it's balance with a very disrupted flow, kinda hammering it in the downward direction, instead of any kind of attached flow... Meaning reduced authority.

I'd be very careful deploying those flaps, until I tufted the fuselage sides and stab/elev and could actually see what the flow is doing. Flow separation is weird and can do all kinds of odd stuff.

If you are losing the moment required to balance the negative pitching moment of the flap...then adding weight aft could be pretty catastrophic, with a loss of control, from what most would consider a tail first stall...not good for anyone.

For what it's worth on my Fat Cub...I found that the flap gap, was allowing air to roll like a wave, tumbling out of the gap, which was creating this burbling which was a complete nuisance. Closing the gap resolved the symptom completely.

not trying to be contrary...but I don't want to be a test pilot much these days...and certainly don't want to see a broken plane or person. I'd call and talk to Doug Keller.

hopefully something to consider anyway.


Steve.

 

[BC12D-4-85]

I’ve also heard the PSTOL flaps give you a flatter approach angle. When I deploy full flaps and I’m trimmed for hands off flight, it takes a couple of turns of the trim handle (nose down) to fly hands off after FF deployment.

There's a couple of possibilities here....one is the plane simply climbs w/o a major change in pitch with flap deployment due to added lift. Another is it also pitches up initially and climbs due to additional lift. The problem was noted as "inability to get the tailwheel to touch with the mains when doing 3 point landings." Some of that could be CG related no doubt. But I gather from other sources noted it's normal for the PSTOL to eventually pitch down with full flaps.

If you can't 3-point then:
1. Correct the CG issue but also confirm measurement data
2. Trim more aft than usually done when landing to help the elevator's down force/nose up
3. Don't deploy full flaps which may make 3-point landings easier until the CG is confirmed and corrected
4. There may be others as suggested above

Gary

 

[mfleming]

What is you airspeed and RPM when you trim hands off with full flaps?

45 mph but I haven’t noticed exactly what the rpm is. On the ground with carb heat on, it idles about 480 rpm

 

[skywagon8a]

With a 200 pound instructor in the back seat, you don't have a CG issue. How big is his belly? Does the stick go all the way aft?
3" extended gear, you are trying to land at too high a speed. Try holding the plane off the ground until it stalls on. Are you using any power on final? Get into the habit of CLOSING the throttle on final and controlling your speed with the elevators. Trim out any nose heaviness so that you can almost release the stick without any consequences. You are not getting the nose high enough before touching down. With the 3" gear it needs to be a lot higher than a stock Cub. Your instructor needs to feel as though he is down in a well without being able to see anything except to the sides, as do you in the front. Your described circumstances are those of someone who flies by an airspeed indicator rather than flying the wing. Still having trouble....cover the airspeed indicator so that you can't see it. You don't need any instruments to fly a Cub......NONE!
Edit: I forgot to mention your 31" tires. When coupled with the 3" extended gear you have to be nearly laying on your back when you touch down to get the tailwheel on the ground first.

 

[sj]

As others have said, weigh it so you are not chasing your tail.

sj

 

[wst]

Have you verified the elevator travel is correct? I had an issue with my SC in that the cable was incorrectly rigged and I was not getting the correct deflection with full aft stick. Corrected and flies like a dream now.

 

[sixmile]

how did you rerig the cable to get the correct deflection?

 

[wst]

my apologies in the manual regarding deflection there is a degree setting up and down at maximum fore/aft stick deflection i don't have those numbers available . You can adjust the turnbuckle on the cable or replace the cable if the correct deflection cannot be achieved by adjustment

 

[skywagon8a]

how did you rerig the cable to get the correct deflection?

my apologies in the manual regarding deflection there is a degree setting up and down at maximum fore/aft stick deflection i don't have those numbers available . You can adjust the turnbuckle on the cable or replace the cable if the correct deflection cannot be achieved by adjustment

The elevator is supposed to hit the mechanical stop at the tail. Adjusting the turnbuckles only repositions the stick.

 

[mfleming]

I've read your post a couple times and IF your Angle of attack is pitching up with flap deployment, there's a problem. That is not how a flap should affect the distribution of lift.

Your center of lift should be migrating aft, resulting in a nose down pitching moment, which you re-balance with trim with your horizontal to add additional downward force...the horizontal MUST produce downforce at all times, or you have a serious instability issue.

The whole point of Kellers flap, is to reduce the angle of attack, while maintaining the same net coefficient of lift, at a lower angle of attack, allowing you to fly the same approach speed, at a reduced angle of attack.

If this is opposite of what you are experiencing, I'd be very careful. Adding weight aft, could be a pretty big mistake. It strikes me that either your horizontal/elevator could be picking up wake turbulence rolling off the back of the flap and inducing a downward pitching moment at the tail, due to the long moment arm of stab, versus the short moment arm of the flap...not LIFT...but still FORCE...this is not what you want.

This could induce what you perceive as a symptom of the nose going up, when what you really have going on is a horizontal/elevator losing it's balance with a very disrupted flow, kinda hammering it in the downward direction, instead of any kind of attached flow... Meaning reduced authority.

I'd be very careful deploying those flaps, until I tufted the fuselage sides and stab/elev and could actually see what the flow is doing. Flow separation is weird and can do all kinds of odd stuff.

If you are losing the moment required to balance the negative pitching moment of the flap...then adding weight aft could be pretty catastrophic, with a loss of control, from what most would consider a tail first stall...not good for anyone.

For what it's worth on my Fat Cub...I found that the flap gap, was allowing air to roll like a wave, tumbling out of the gap, which was creating this burbling which was a complete nuisance. Closing the gap resolved the symptom completely.

not trying to be contrary...but I don't want to be a test pilot much these days...and certainly don't want to see a broken plane or person. I'd call and talk to Doug Keller.

hopefully something to consider anyway.


Steve.

Very interesting. I also don't want to be a test pilot.
Flying solo with some weight in the extended baggage, I did a stall series to make sure there were no ill affects. Stalled with no flaps, 1 notch and full flaps. The cub acted the same as when the 200 lb instructor was in the back seat, that's to say very benign. I also reconfirmed how it slips. It likes left slips, right slips I run out of rudder.
I'm going back up today and reconfirm I haven't gotten the pitching forces backwards in my mind when deploying the flaps.

 

[sixmile]

so what's the answer

 

[DENNY]

Very interesting. I also don't want to be a test pilot.
Flying solo with some weight in the extended baggage, I did a stall series to make sure there were no ill affects. Stalled with no flaps, 1 notch and full flaps. The cub acted the same as when the 200 lb instructor was in the back seat, that's to say very benign. I also reconfirmed how it slips. It likes left slips, right slips I run out of rudder.
I'm going back up today and reconfirm I haven't gotten the pitching forces backwards in my mind when deploying the flaps.

Remember to note and test with different trim settings.
DENNY

 

[BC12D-4-85]

I'm going back up today and reconfirm I haven't gotten the pitching forces backwards in my mind when deploying the flaps.

Trim tail in level flight in upper white airspeed arc. With wings level let go of stick and leave power plus trim alone. Use rudder to maintain heading while gradually adding flaps one notch at a time. See how it slowly responds to partial and full flaps. Wait some between flap changes. Does it roll? Does it change pitch per flap setting?

Then do some full flap stalls simulating a landing attitude at safe altitude. See how it responds, note the view of the horizon vs cowling. When on ground note nose position relative to horizon. That's how it should appear during a normal 3-point landing. Memorize that view, it may be different than in the air. Not a CFI so ask one if confused.

Gary

 

[BC12D-4-85]

I asked my CFI buddy who flys a PA-18A with PSTOL. Says his Cub pitches up slightly with each of 3 flap deflections. He re-trims as required to maintain attitude and airspeed. Landing 3-point no problem if trimmed properly...much easier with load aft than when solo and light on fuel. The rest of the story from Fairbanks.

Gary