Lateral balance
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Lateral balance
Elsewhere discussion evolves about lateral balance, (aileron trim) and fuel balance in a 172.
It's pretty common that a light aircraft is not in perfect lateral balance, and could benefit from aileron trim, but is not so equipped. Most of these types allow the pilot to select fuel from one wing tank, and in time, lateral balance can be improved. If the pilot selects one tank only, and empties that tank, the engine will stop. When the fuel is reselected to a tank containing fuel, the engine can be run again. If a tank runs dry while on a "both" selection, the engine will keep running, unless the aircraft is slipped or skidded, which might then cause temporary fuel starvation.
Some aircraft have a metal trim tab which can be bent by hand to improve trim. First - don't play with this, unless it's your plane, or you have permission and instruction to do so. Yes, these tabs will affect trim forces, though it takes some good diagnosing to bend them the right amount. Multiple flights will probably be required to get it right. Bending it following a flight, and hoping the next rental pilot will notice an improvement is not the process. Strut braced Cessnas commonly have cams on the rear wing attach points, which a mechanic to adjust the angle of incidence of that wing a tiny amount to improve roll trim. This will get things close, but perfect is a high achievement. The mechanic doing this has to observe and record the effects. Disjointed efforts can result in one wing being swept very slightly forward, and the other very slightly aft. If that is done, the plane will now have a yaw trim error too. It's correctable, but a dedicated effort is required. The cantilever wing Cessna singles have a screwdriver adjustable aileron tab instead.
Slight damage to a flight control trailing edge can also do this, and may be speed sensitive in a different way. I once flew a Piper Aztec which was suddenly badly out of roll trim, though really only at cruise speeds and faster. An after landing cause inspection found that about 1.5 inches of trailing edge of one flap had been bumped, and curled down. The affect was that of a tiny trim tab on the flap, but enough to move it up within its freeplay range to cause it to cause a roll of the whole plane. Slowing, or extending the flaps removed the affect. Lesson, don't bump trailing edges, and report the snag with precision to the mechanic, when you're saying that it doesn't fly right!
Ultimately, I can report that in some cases, some planes will simply require the pilot to apply a small roll force during at least some of the flight, it's just a part of the joy of flying!
It's pretty common that a light aircraft is not in perfect lateral balance, and could benefit from aileron trim, but is not so equipped. Most of these types allow the pilot to select fuel from one wing tank, and in time, lateral balance can be improved. If the pilot selects one tank only, and empties that tank, the engine will stop. When the fuel is reselected to a tank containing fuel, the engine can be run again. If a tank runs dry while on a "both" selection, the engine will keep running, unless the aircraft is slipped or skidded, which might then cause temporary fuel starvation.
Some aircraft have a metal trim tab which can be bent by hand to improve trim. First - don't play with this, unless it's your plane, or you have permission and instruction to do so. Yes, these tabs will affect trim forces, though it takes some good diagnosing to bend them the right amount. Multiple flights will probably be required to get it right. Bending it following a flight, and hoping the next rental pilot will notice an improvement is not the process. Strut braced Cessnas commonly have cams on the rear wing attach points, which a mechanic to adjust the angle of incidence of that wing a tiny amount to improve roll trim. This will get things close, but perfect is a high achievement. The mechanic doing this has to observe and record the effects. Disjointed efforts can result in one wing being swept very slightly forward, and the other very slightly aft. If that is done, the plane will now have a yaw trim error too. It's correctable, but a dedicated effort is required. The cantilever wing Cessna singles have a screwdriver adjustable aileron tab instead.
Slight damage to a flight control trailing edge can also do this, and may be speed sensitive in a different way. I once flew a Piper Aztec which was suddenly badly out of roll trim, though really only at cruise speeds and faster. An after landing cause inspection found that about 1.5 inches of trailing edge of one flap had been bumped, and curled down. The affect was that of a tiny trim tab on the flap, but enough to move it up within its freeplay range to cause it to cause a roll of the whole plane. Slowing, or extending the flaps removed the affect. Lesson, don't bump trailing edges, and report the snag with precision to the mechanic, when you're saying that it doesn't fly right!
Ultimately, I can report that in some cases, some planes will simply require the pilot to apply a small roll force during at least some of the flight, it's just a part of the joy of flying!
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Pilot DAR a little annecdote; I was reading recently that Hawker Siddeley test pilot Duncan Simpson was asked to test fly the Strathallan Collection's Hurricane after a lengthy refurbishment. It had been acquired by the Battle of Britain film producers in 1968 from a Canadian who had being flying it. Simpson found its roll control absolutely appalling and quite frightening compared with the example he'd been displaying at airshows. The Shuttleworth Collection lent Strathallan the ailerons from their non-airworthy Hurricane and, lo and behold, Simpson landed after the next test flight wreathed in smiles.
It transpired the Canadian chap didn't have any ailerons so had built his own set from scratch and without reference to plans - and had added non-standard ground-adjustable trim tabs in an effort to improve its manners in the air; these had been discarded during the rebuild. An RAF pilot who flew the aircraft during the BoB film confirmed roll control had been diabolical even with the tabs in place. The design of the homebuilt ailerons didn't include vital aerodynamic wizardry employed by Sydney Camm's team; this led to them over-balancing - or something like that.
Sad to say the Hurricane returned to Canada and was destroyed in the Canadian Warplane Heritage's hangar fire a couple of decades ago.
It transpired the Canadian chap didn't have any ailerons so had built his own set from scratch and without reference to plans - and had added non-standard ground-adjustable trim tabs in an effort to improve its manners in the air; these had been discarded during the rebuild. An RAF pilot who flew the aircraft during the BoB film confirmed roll control had been diabolical even with the tabs in place. The design of the homebuilt ailerons didn't include vital aerodynamic wizardry employed by Sydney Camm's team; this led to them over-balancing - or something like that.
Sad to say the Hurricane returned to Canada and was destroyed in the Canadian Warplane Heritage's hangar fire a couple of decades ago.
* flying a very light plane (450 kg MTOW) I am used to correcting all the time anyway - the art being to avoid over-correcting but that depends on wind and a plethora of secondary factors
* what is meant (in this context) by "cam" ? (excuse stupidity)
* what is meant (in this context) by "cam" ? (excuse stupidity)
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The Cessna strut braced wings bolt to the aft spar carrythrough with an eccentric "cam" in the joint. This cam is punched with a mark so the person adjusting knows which direction they are being turned. Their intended function is to raise or lower the aft spar of each wing independently, relative to the fuselage, when turned with the wing attachment bolt loose. However, while moving the aft spar up or down by turning the cam, the biproduct motion can be to also move the aft spar a tiny amount closer or farther from the fuselage attachment (it depends upon how far, and which direction the cams are turned). Turning the cam(s) so that the rear spar is brought closer to the fuselage has the unintended effect of very slightly sweeping that wing back. If the spar is pushed a tiny amount away from the fuselage will sweep the wing a tiny amount forward. As long as both wings are swept either forward or aft, and the incidence is adjusted so that the plane flies level, all is well. It's when one wing is swept forward, and the other swept aft, the axis of the wings is not square to the axis of the fuselage. Thus the plane does not want to fly straight, but the reason is not apparent, 'cause it's not in the tail.
This was all explained to me some time ago by a Cessna rep when I inquired as to why two otherwise identical C150s had differing performance (we tested in formation). One was a little faster, the other would fly just a little more slowly at low speed. The Cessna rep explained that the fast one had the wings swept aft, and the slow one wings forward. When we removed the fairings, and looked at the cams, this made sense. We never adjusted the cams on either plane to experiment more, as they otherwise flew fine. Certainly I have check flown other Cessnas where wing cam adjustment, and reflying was a part of the post maintenance procedure. This is described in 4.7 of the 172 Service Manual, "Adjustment (Correcting Wing Heavy Condition).
This was all explained to me some time ago by a Cessna rep when I inquired as to why two otherwise identical C150s had differing performance (we tested in formation). One was a little faster, the other would fly just a little more slowly at low speed. The Cessna rep explained that the fast one had the wings swept aft, and the slow one wings forward. When we removed the fairings, and looked at the cams, this made sense. We never adjusted the cams on either plane to experiment more, as they otherwise flew fine. Certainly I have check flown other Cessnas where wing cam adjustment, and reflying was a part of the post maintenance procedure. This is described in 4.7 of the 172 Service Manual, "Adjustment (Correcting Wing Heavy Condition).
Sweep
Sweep reduces drag, effectively reducing frontal area. Asymmetric (single wing) sweep would produce yaw, in that the swept wing will want to lead, as the straight wing wants to trail, no? (Due “differential” drag)
Also, forward sweep is no different than aft sweep, related to drag? What was that German forward swept jet? Also “scissor wing”. (Hansa?)
I am tempted to say also that one wing swept forward, the other back, would fly faster than two straight wings. If both are swept aft or both swept forward, no difference in speed. (With equal sweep) All will fly faster than stock. Think F-14.
Also, forward sweep is no different than aft sweep, related to drag? What was that German forward swept jet? Also “scissor wing”. (Hansa?)
I am tempted to say also that one wing swept forward, the other back, would fly faster than two straight wings. If both are swept aft or both swept forward, no difference in speed. (With equal sweep) All will fly faster than stock. Think F-14.
Last edited by Concours77; 6th Sep 2018 at 17:12.
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We had a F172H ( a '69 French-built model) which would either fly half a ball out hands-off or if you adjusted the rudder tab for slip-free flight, needed about 2-3 lb down force on the column. The engineers tried adjusting the cam, then adjusting he incidence of the flaps but with no discernable result. In the end, a Club member solved the problem by landing it on the noseleg. Surprisingly, it didn't snap off, but pushed the firewall back into the cabin, distorting the floor as far back as the front seats. Even that didn't do for the a/c as someone rebuilt the front as a project. What finally did for it was a winter storm that flipped it onto its back.
Sad, really, though it was old it was a good and faithful servant with a lovely 6 cylinder Continental, much nicer than the brash 4-pot Lycoming in my view,
TOO
Sad, really, though it was old it was a good and faithful servant with a lovely 6 cylinder Continental, much nicer than the brash 4-pot Lycoming in my view,
TOO
Jan, I think that the word "cam" is an abbreviation of "camber" and in the context of adjusting an aircraft's wing incidence, it is an eccentric adjuster which can be rotated to change the position/setting/angle of a component. In my aircraft I set the engine idle by means of an adjuster in the cockpit at the rear of the quadrant, using a "penny" washer which I filed progressively more around the edge to get a "snail -shell" effect i.e. decreasing radius from the centre hole. I unlock the screw through the middle, rotate the washer to change the idle speed, then lock with the screw again. The two Bing carburettor idle stops aren't employed and are screwed well out of the way, thereby avoiding strain on the throttle linkage.
Last edited by Colibri49; 6th Sep 2018 at 11:55. Reason: re-think
Thanks for enlightenment! @PD had made the point clear by referring to a camshaft.
I'd love to have a look at those Bings, though, and your idle setting mechanism. A 912, may I assume?
I'd love to have a look at those Bings, though, and your idle setting mechanism. A 912, may I assume?
Yes a 912, so you know what the Bings look like anyway. Just the two idle screws are wound back far enough to ensure that they can never be contacted by the throttle arms. The springs are attached to the arms to pull them towards fully open, if the linkage should fail. My modified penny washer "cam" is at the rear of cockpit throttle lever travel, to act as an idle stop. The aircraft designer indicates in the build instructions that the idle stop should be in the cockpit. Other builders will have devised their own means of adjusting idle speed, perhaps at the carburettors by adjusting throttle cables and having only a fixed stop in the cockpit. My way might be easier; I don't know for sure.
Sorry about this, but I'm an engineer and cannot let things go if I don't understand them, so I looked this up!
The cessna incidence adjustment facility mentoined above is effected by the bush around the rear wing attachment bolt which screw (to put it simply) into the end of the rear spar. The plain (unthreaded) part of the bolt is a close fit in the bush, and the hole in the bush is bored off-centre. The result is that the bush can be rotated (with the wing bolt loose) to move the rear attachment up or down over a total range of about 1/4inch, and then locked in place by tightening the bolt. The bolt and bush look like this:
Cessna wing rear attachment bolt with eccentric bush incidence adjuster
As you can see, the bush itself is not threaded (internally or externally), so rotating the bush to adjust incidence will NOT push or pull the rear spar laterally, so there should be no sweep changes when the incidence is adjusted.
I think someone was pull your plonker...
PDR
The cessna incidence adjustment facility mentoined above is effected by the bush around the rear wing attachment bolt which screw (to put it simply) into the end of the rear spar. The plain (unthreaded) part of the bolt is a close fit in the bush, and the hole in the bush is bored off-centre. The result is that the bush can be rotated (with the wing bolt loose) to move the rear attachment up or down over a total range of about 1/4inch, and then locked in place by tightening the bolt. The bolt and bush look like this:
Cessna wing rear attachment bolt with eccentric bush incidence adjuster
As you can see, the bush itself is not threaded (internally or externally), so rotating the bush to adjust incidence will NOT push or pull the rear spar laterally, so there should be no sweep changes when the incidence is adjusted.
I think someone was pull your plonker...
PDR
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"Cam" - yes, the correct (and Cessna term) is "eccentric", and "bushing" in the parts catalog. "Cam" is used colloquially, because perhaps too much use of the term "eccentric" around pilots and mechanics could be worrying!
Consider the orientation of the bolt and eccentrics. As is the orientation of the forward spar bolts, and wing strut bolts, the aft spar bolts are aligned with the longitudinal axis of the aircraft. Turning the eccentrics has the effect of moving the aft spar up or down as intended. Further to this, turning the eccentrics 'round and 'round, would result in the motion of the rear spar describing a circle relative to the fuselage spar carrythrough, rather than an up and down "slot" motion.
If the "thick" part of the eccentric is set to be up, that rear spar will be positioned down relative to the fuselage spar carrythrough, and vice versa. However, adjustment of the eccentrics may also leave the thick part in or out ('could be in on one side of the plane, and out on the other if careless). If the thick part is in, the rear spar has been positioned a tiny amount away from the fuselage spar carry through, so the effect would be the sweep the wings a tiny amount forward. This effect is very tiny, and unintended in the design. I would have dismissed it as negligible, other than the Cessna tech rep explained to me decades back to look for this error if the plane was not flying as intended. When we flew a side by side comparison of two otherwise identical 150's, the very slight performance differences could be understood to be this difference.
These affects are small, compared to loading, fuel imbalance, or other aerodynamic or flight control defects. But, Cessna wisely built in this adjusting capability so as to keep their clients happy. An experienced Cessna mechanic may anticipate the adjustment. I once maintenance check flew a Cessna 207 whose wings had been reinstalled. The experienced mechanic explained to me that he had deliberately offset one of the eccentrics, as his experience told him that was where to start the fine adjustment from (rather than zero/zero). The plane flew perfectly hands off the first flight. I was delighted to inform him of the wisdom of his work, he saved hours of fiddling, and reflying to deliver a well flying plane to the client.
I once ferried home a friend's 182. It had sat unused at another airport for a couple of weeks, and he wanted it home. My walk around was too casual. I jumped in and took off - wow, it got off the ground really well that day, and climbed crisply! That was pleasing! However, once flaps up, and trimming for cruise, the lateral balance was poor, and the control wheels were not horizontal as they should have been. When I positioned them to be horizontal, the aircraft predictably rolled sharply. I flew it level, and looked around... The ailerons were wrong! Both aileron trailing edges were well below their respective flap trailing edge. Hmmm. When I leveled the control wheel, one aileron was now fine to the flap, and the other well down - 'splained the roll! I assured that the cross wind would favour the side for which I had more roll control available, and landed back at home. The aileron was displaced downward, because the Cessna control lock cocks the control wheel, not the flight control itself. At some point, a tailwind, while the plane had been parked, had pushed down an aileron, and the pushrod was bent. The aileron was otherwise not damaged. Aileron push rod replaced, and the plane flew just fine again! That reaffirmed to me that flight controls, rather than the control wheel, have locks applied. All my planes have external control locks, and I never use the control wheel locks when I park the aircraft. An Air Moorea Twin Otter accident is a much more telling report of why to care about external wind forces on flight controls. https://aviation-safety.net/database...?id=20070809-0 But that's was longitudinal, rather than lateral...
so rotating the bush to adjust incidence will NOT push or pull the rear spar laterally, so there should be no sweep changes when the incidence is adjusted.
If the "thick" part of the eccentric is set to be up, that rear spar will be positioned down relative to the fuselage spar carrythrough, and vice versa. However, adjustment of the eccentrics may also leave the thick part in or out ('could be in on one side of the plane, and out on the other if careless). If the thick part is in, the rear spar has been positioned a tiny amount away from the fuselage spar carry through, so the effect would be the sweep the wings a tiny amount forward. This effect is very tiny, and unintended in the design. I would have dismissed it as negligible, other than the Cessna tech rep explained to me decades back to look for this error if the plane was not flying as intended. When we flew a side by side comparison of two otherwise identical 150's, the very slight performance differences could be understood to be this difference.
These affects are small, compared to loading, fuel imbalance, or other aerodynamic or flight control defects. But, Cessna wisely built in this adjusting capability so as to keep their clients happy. An experienced Cessna mechanic may anticipate the adjustment. I once maintenance check flew a Cessna 207 whose wings had been reinstalled. The experienced mechanic explained to me that he had deliberately offset one of the eccentrics, as his experience told him that was where to start the fine adjustment from (rather than zero/zero). The plane flew perfectly hands off the first flight. I was delighted to inform him of the wisdom of his work, he saved hours of fiddling, and reflying to deliver a well flying plane to the client.
I once ferried home a friend's 182. It had sat unused at another airport for a couple of weeks, and he wanted it home. My walk around was too casual. I jumped in and took off - wow, it got off the ground really well that day, and climbed crisply! That was pleasing! However, once flaps up, and trimming for cruise, the lateral balance was poor, and the control wheels were not horizontal as they should have been. When I positioned them to be horizontal, the aircraft predictably rolled sharply. I flew it level, and looked around... The ailerons were wrong! Both aileron trailing edges were well below their respective flap trailing edge. Hmmm. When I leveled the control wheel, one aileron was now fine to the flap, and the other well down - 'splained the roll! I assured that the cross wind would favour the side for which I had more roll control available, and landed back at home. The aileron was displaced downward, because the Cessna control lock cocks the control wheel, not the flight control itself. At some point, a tailwind, while the plane had been parked, had pushed down an aileron, and the pushrod was bent. The aileron was otherwise not damaged. Aileron push rod replaced, and the plane flew just fine again! That reaffirmed to me that flight controls, rather than the control wheel, have locks applied. All my planes have external control locks, and I never use the control wheel locks when I park the aircraft. An Air Moorea Twin Otter accident is a much more telling report of why to care about external wind forces on flight controls. https://aviation-safety.net/database...?id=20070809-0 But that's was longitudinal, rather than lateral...
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As you can see, the bush itself is not threaded (internally or externally), so rotating the bush to adjust incidence will NOT push or pull the rear spar laterally, so there should be no sweep changes when the incidence is adjusted.
Edit: beaten in great detail by Mr Dar!
