Camber, Toe-In, and AI

[Bob Turner]

Over on Supercub dot org I got in mild trouble for mentioning that, as the token liberal I was not supposed to either like or use the new AI stuff. Let's see if I can get away with it here - my friends on Emptywheel would be horrified that I am using AI.

But it comes up automatically on the iPad. And for now, it is free (they are trying to suck me in).

I wasn't paying much attention, but then I asked it to do a Mooney take off analysis at Payson, 95 degrees. Took two seconds. A check of the "runaround" charts in the manual, followed by a Koch chart guess, indicates that Apple AI got it pretty much correct.

Then, working on a buddy's wingtip, I took the time to measure his camber and toe-in. I knew toe-in leads to instability (we measure tires front to back; more than 3/4" difference on an 8:00 tire is starting to get dangerous), but had no idea what excessive camber does.

So I asked The Google. First thing that came up was a long AI treatise - they asked for measurements, vehicle type, etc., and I gave them the parameters.

You can check Supercub dot org, or if there is any interest I can re-paste the results here. Stunning answer.

Short answer: zero toe-in, zero camber when loaded. It even asked what type aircraft so it could recommend shims . . .

 

[Bob Turner]

I just realized I did not post the actual results on Supercub.

I measured toe and camber on a Cub, and submitted the actual numbers. The answer is a bit long, and does not include the final note about Piper's recommendation.

Herewith:


A combination of 3 degrees positive camber and 2.7 degrees toe-in will severely compromise your directional stability, making the vehicle incredibly twitchy, erratic, and dangerously unpredictable. While a tiny fraction of a degree of toe-in normally stabilizes a vehicle, settings this extreme cause massive, conflicting forces that violently fight each other across your 6-foot track width.



The Breakdown of Forces

• Outward Camber Thrust: 3 degrees of positive camber tilts the tops of the tires outward. This drastically reduces your contact patch and creates a continuous outward pushing force. [1, 2]
• Severe Inward Scrubbing: 2.7 degrees of toe-in points the front edges of the tires aggressively toward each other. This is more than ten times the normal street setting, causing the tires to drag and scrub sideways against the pavement. [1, 2]
• The 6-Foot Track Leverage: Because the wheels are spaced 6 feet apart, these opposing forces act on a large lever arm relative to the vehicle's center of gravity.

Direct Impact on Directional Stability

• Violent Darting: The vehicle will refuse to track in a straight line.
• Erratic Tramlining: If one wheel hits a slight bump or road seam, its grip will momentarily drop. The opposite tire's massive inward force will immediately take over, causing the vehicle to violently yank or snap to one side.
• Heavy Rolling Resistance: The vehicle will feel sluggish to accelerate, will want to plow straight instead of turning smoothly, and will rapidly destroy the outer edges of the tires. [1, 2]

Ask about
On an aircraft, a setup of 3 degrees positive camber and 2.7 degrees toe-in across a 6-foot track width will cause extreme ground handling instability, making takeoff and landing rolls highly dangerous.
Standard aircraft specifications usually call for near-zero toe (typically 0 to 0.5 degrees) and mild positive camber (1 to 2 degrees) when unloaded to allow the gear to flatten out under the aircraft's weight. Exceeding these metrics by this margin creates severe, conflicting aerodynamic and mechanical forces. [1, 2, 3]

Impacts on Directional Stability and Ground Handling

• Severe Directional Twitchiness: A 2.7-degree toe-in points the front of the tires aggressively inward. The tires are constantly fighting each other to steer toward the aircraft’s centerline. The aircraft will refuse to track straight and will require constant, exhausting rudder corrections to keep on the runway centerline. [1, 2, 3]
• High Risk of Ground Loops or Violent Swerves: Because the wheels are spaced 6 feet apart, any minor runway bump that momentarily lifts or reduces load on one tire will cause the opposite tire to instantly dominate. Its severe inward angle will violently yaw the aircraft to that side. On a tailwheel aircraft, this will almost certainly trigger an uncontrollable ground loop. On a tricycle-gear aircraft, it can cause a severe high-speed runway excursion.
• Massive Takeoff Drag:
  
  
  The extreme toe-in causes the tires to heavily "scrub" sideways against the pavement rather than rolling smoothly. This massive rolling resistance acts like a partial brake, severely hurting acceleration, extending your takeoff roll distance, and potentially preventi

Sent

 

[JackC]

Some of the folks on supercub & BCP can be a little…sensitive lol

Been playing with AI for a while on the paid tiers, there are some things it’s really good at, some things it’s really not, but it 100% needs a “manager” human with common sense to see when it’s out to lunch, for sure a tool to have in one’s belt, but by no means a solution.

Here’s another good AI trick

Record a video of your panel, and upload to Claude for a audio analysis in comparison to your panel

“Here's the full picture on champ 2 in cruise.

The numbers

RPM (audio) 2,094 ± 5 (0.25%)

RPM (tach) ~2,100 indicated

Drift over clip +12 RPM — flat

IAS ~73 mph (read off the Aeronca ASI)

Recording 15.3 s, no clipping


Conditions and loading.
Clip timestamp is 20:23Z today — 4:23 PM local. I could not pull the archived location observation for that hour; the current one (070215Z) is 16°C / A30.16, and mid-afternoon would have been warmer, so figure roughly 24°C. With field elevation 1,421 ft and a typical local-flight altitude, density altitude was somewhere around 3,300–3,900 ft. Weight with half tanks (~6.5 gal), one 176 lb pilot, and a typical 7AC empty weight lands near 1,030 lb against a 1,220 lb gross — about 85%.

Those three numbers hang together well. 2,094 RPM and 73 mph IAS at ~3,500 ft DA and 85% gross is right where a Champ with a metal cruise prop should sit. Nothing about the power output looks deficient for the conditions.

What sounds clean
• Cylinder balance: folded at 720° over 266 cycles, the four events came out 0.972 / 1.062 / 1.013 / 0.953 — 4.2% std. Good.

• No dead or weak jug. The 1/rev line (34.9 Hz) is at 0.0016 of the firing fundamental and the 720° cycle rate (17.45 Hz) is at zero. Both would light up hard if one cylinder were down.

• No prop imbalance. Order 1.0 is buried in the floor. Consistent with a metal prop that's been balanced.

• Harmonic ladder is orderly: 69.8 / 139.6 / 209.4 / 279.2, decaying normally.

• Mechanical noise is unremarkable. 700–2,000 Hz holds 1.65% of energy, 2,000–6,000 Hz 1.18%. No clatter standing out.

The one thing worth naming
There's a strong isolated line at 87.25 Hz — the 2.5 order — sitting at 16% of the firing fundamental. Other half-orders around it are tiny (1.5 at 0.007, 3.5 at 0.026). A half-order means something repeating every two revolutions rather than every one, which is the signature of cylinder-to-cylinder variation.

But I don't think that's what this is, because the direct cycle-averaged measurement says the cylinders are within 4% of each other. Those two results disagree, and the averaged one is more trustworthy. My guess is an acoustic resonance — cabin, exhaust, or airframe — that happens to sit near 87 Hz and gets excited by the firing. Worth noting rather than worrying about. “

 

[JackC]

but here’s where the common sense comes in

GREAT analysis on the engine, but that’s a SLOW champ, so I compared with track logs from another champ



“Restricting both to the initial departure climb only:

mean VS best 30 s climb GS


Champ 1 (1,303→2,336 ft) 391 fpm 492 fpm 66 mph


Champ 2 (1,240→2,660 ft) 383 fpm 583 fpm 64 mph

Mean climb rates are within 2% of each other. The best-30-second figures differ more, but that's a single window and I wouldn't lean on it. Two engines of the same type performing the same — which is the useful result.

Back to the slow-airspeed question

Three data points, none conclusive:

• Champ 2, cruise clip: ~70 mph indicated at max cruise RPM

• Champ 2, KML flight: GPS-solved TAS 70.9 mph

• Champ 1, cruise leg: groundspeed 80–87 mph, wind unknown

If champ 1's winds were light, its TAS was meaningfully higher than champ 2's. That leans toward champ 2 being genuinely slow rather than just reading slow — which is the opposite of an instrument problem and would point at rigging, prop pitch, or airframe drag. But the legs were 12 days apart at unknown relative power settings, and champ 1's wind is unknown, so this is a hint, not a finding.

Its cruise leg was a single heading — course 070°, GS 83.5 ± 2.1 mph at 2,335 ft.

Even the most pessimistic end sits above champ 2's 71. And your comment that champ 1 makes book is consistent with the calm-wind case, ~84 TAS.

Champ 2's actual TAS was ~71 mph. That's slow for a 7AC on its own terms, independent of any instrument. So the data doesn't support "the airplane is fine, the gauge is lying." It's more consistent with the airplane genuinely being slow — which after a restoration usually means rigging: incidence, washout, flying wire tension, control surface rig, fairing alignment. Very common on a fresh rebuild and usually fixable.”