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Allright, back home with a bit more reading done. The booster coil is indeed activated through the starter switch, as far as I can tell. It is a coil with vibrating breakers, creating a constant flow of high power sparks to one bank of spark plugs*. These sparks are fed to the right cylinders from the distributor, just as the normal sparks from the mags. Normally, the spark is sent to the spark plug around 25 degrees before top dead center (TDC), that is 25 degrees before the piston reaches its top point and begins the downward stroke. This allows the flame fronts time to propagate throughout the combustion chamber and build up to peak pressure in time for the piston to milk the most of the energy out of the expansion of the mixture during its downward stroke. At low RPMs, this is way too early and means the mixture would reach peak pressure way before TDC. The flame front propagation speed doesn't change with RPM. This will at best cause the combustion to decelerate the engine and subtract from the power output and at worst (often, with high-powered aircraft engines) cause kick-back - the engine violently coming to a halt and reversing direction. Kick-back isn't good at all. Lots of stress on engine parts. I guess don't need to point out that you should never attempt to hand-prop anything which can kick back... ? :cry: To solve this, the booster coil sparks are retarded - delayed - at the distributor, causing them to fire the spark plugs later than the normal spark from the magneto. Booster coil ignition should happen right around TDC, even if I have not been able to find the exact figure for the retardation for the P-51 engine. This allows the booster coil sparks to give good sparks during the starting, igniting the mixture and creating a pressure peak during the piston's downward travel, accelerating the engine towards normal RPMs. As RPMs increase, the magnetos start working and providing enough spark to ignite the mixture. When they do, they take over. The second spark being provided by the booster coil will not do anything, as the mixture is already ignited. The engine starts running normally. However, with magnetos off, running off the starter and the booster coil, the retardation of the spark will be way too much. Peak pressure will be reached only once the engine is 30 degrees past TDC. I really can't see the engine running at all well under those conditions, on booster coil alone. Right now, there's no perceptible difference between running the engine on the mags and running it on the booster coil, apart from the fact that it will cut out as you release the starter with the mags off. I don't suspect the operators of current warbirds, including those consulting for ED, will fancy trying to run their expensive and rare engines in any way except that specified by the checklists, so mags off starts is probably not something they have a lot of experience of. Please correct me if I'm mistaken. I do have a few leads to follow on my own though, so with a bit of luck I'll be able to get back with word from people with Merlin experience. It may be a while though. Sidenotes: A booster coil also prevents unburned mixture from filling up the exhaust manifolds, with the potential for spectacular backfires as the engine catches. (An alternative method of providing sparks during startup, more common in modern aviation engines, is to have impulse couplings in the magnetos. At low RPMs, these delay the magnetos working against a spring. As they are released, the spring provides the increased velocity to give a good spark. Simultaneously, they retard the spark by a sufficient amount. As RPMs pick up, flyweights engage and the magnetos return to normal operation.) Cheers, /Fred *) A poster earlier in the thread wondered about exhaus and intake plugs. Aviation engines almost exclusively have completely doubled ignition systems. Two magnetos, two harnesses, two plugs in each cylinder. This is for redundancy, obviously, as otherwise a magneto breaking would kill your engine Having suffered magneto failures in real life, I think this is an excellent design philosophy. It gives the added benefit of more efficient combustion as well, due to the two sources of ignition in each combustion chamber causing two flame fronts. That's why you get the RPM drop when checking mags before takeoff... as you always do, right? ;)

Typo on pg. 68. "Processions" when talking about precession of the gyro. Figure 9 on pg 24 has a remote compass sensor cover on both wingtips.

Talking about two different things here, I think. You are talking about wheel base - distance between tail wheel and mains - while we're talking about the stability provided by the large track width - the distance between the two mains. Tail wheel locked, wheel base doesn't matter a whole lot for stability as long as the tail wheel isn't skidding. Tail wheel unlocked - still doesn't matter, obviously. :) Tail length, empennage/fin area and - first and foremost - distance mains to center of gravity is what does it. Wider track width means increased roll stability and far less chance of damaging something when being ham-fisted in the aircraft. I'll chime in with Smokin' on the process of yaw control on takeoff being almost entirely done visually. The seat of your pants, so useful during smooth flying, is all but useless on the ground roll due to all the "noise" vibrations and bumps caused by rolling down the runway. No real difference between sim and real life there, apart from the little help you get by being rocked back and forth in your harness. On the other hand, that happens due to bumps in the runway as well. By the time you're getting definitive sensory feedback, you're already in an established yaw/turn well on your way into a full-on ground loop. Way too late. FWIW, I've never really had any trouble keeping her straight. My first takeoff wasn't worth posting in the first-takeoff thread, as it wasn't very amusing at all. There's always some drift back and forth across the runway which I definitely don't see even in videos of low-time pilots in poor conditions. Definitely enough, and difficult enough to control, to rule out any thought of formation takeoffs. However, it is conceivable that formation takeoffs are/were never done at full power settings, so I'm giving it the benefit of the doubt for now while exploring the limited-power takeoff - always been the full 61" for me*. Another factor is the fact that we're taking off from concrete runways. Conventional gear pilots, especially of warbirds with more interesting ground handling qualities, shy away from concrete runways preferring grass if given the option - especially with a crosswind. The lower friction offered by the grass makes it a lot easier - that I know from own experience. This leads me to another suspect in the Great Stability Debate - are the tyre slip angles modelled as of yet? If not, it will have a huge impact on stability on the ground. Right now, I don't perceive any differences in ground handling taking off and landing "off field". As I've said before, I'm not ready to call foul on the yaw stability on the takeoff roll. What I do find difficult to believe is the difficulty of catching incipient yaw deviations. Once it starts going, it pretty much is going no matter what you do. To save myself from retyping too much, I'll quote myself from another thread: (The lack of difficulty experienced I attribute to learning to fly dancing on the pedals, and keeping the skill alive through other simulators during recent more boring tricycle gear flying.) *) Remember the list of the most useless things in aviation - altitude above you, runway behind you, etc etc

I believe ED posted a chart comparing in-sim theoretical and achieved performance with real-life data a while ago? Either climb performance or maximum airspeed... can't remember, but either way the match was good.

I grabbed a pair of screenies the other night, showing that there is some caster to the tail wheel pivot axis. Together with the trail, this means part of the reaction force from placing weight on the tail wheel will create a torque striving to align the tail wheel with the fuselage... ...pointing to the rear, one might add. ;)

Finally, flying strict power settings is sim stuff. In real life, different engines and airframes will give you different performance with exactly the same on the clocks in the cockpit. Sometimes radically different performance.

I noticed this effect last night. The term to google for those interested in knowing more is acceleration errors, and I think this is what we are seeing. These are caused by the mechanics of the pivoting arrangement of the compass card along with the mass distribution and inerta of the card in wet compasses. However, the P-51 has a remote indicating compass (with a wet/whisky compass added in some aircraft). This does not contain a compass card. Instead, it is a remote indicator where the needle is controlled by an electric field generated by coils, in turn controlled by the remote compass sensor/transmitter located out on the wing tip (caveat: the ones I'm familiar with - other designs are certain to exist). Such a compass is not affected by acceleration errors! This is also stated on pg. 68 of the DCS P-51D manual. My first thought was that it was an attempt at modelling the effect of the inclination of the Earth's magnetic field, but slip the aircraft at a constand heading and you will find that it only reacts to acceleration and not to the bank itself. Even assuming it is erroneously modelled acceleration errors, they seem way overdone. By the way, remember the flux gate of "Back to the Future" fame? One name for the remote compass transmitter is a flux gate valve, so now you can bore people with this fact everytime you're rewatching that movie. ;)

I don't assume this does away with the sun film as well? I hated the reflections at first but now I've found them a useful tool for maintaining SA while manoeuvring, but I absolutely hate the fact that someone brought my plane to a custom car workshop and had the canopy covered in sun film with the same opacity as my Oakleys... :)

The two connection points with an arc between them? Circuit breaker.

Rotareneg, that prompted further reading. I'm finding documents stating that the booster coil is a high-tension coil with vibrating contact points, hooked up to a separate lead on the distributor to provide 30 degree retarded sparks for starting. I'll wait until I'm home at a computer rather than on a dumbphone before I call a good find and definitely put the issue to rest, but I thought I'd get it in early that I'm beginning to lean towards you (and ED) being right on this. :) Cheers, Fred

Chuck Norris doesn't do missiles. Whatever he is using is becomes hit-iles... ;) And is a small rock a rock-ette? (Sorry for the OT, but it is Friday after all!)

There seem to be some not-so-obvious differences between slider axes and normal axes. I found brakes to be almost an on-off deal with the Saitek pedals set to slider, but good control with them as normal axes. Does anyone know?

The primary function of the rudder is to keep the nose of the aircraft pointed in the direction you are going. There are a number of reasons why the aircraft will not do that by itself, but you really don't need to know all the theory of 'why' at this point (even if it will help, eventually). In real life, it is easier as your pilot-aircraft interface* contains a sensor telling you if you are going sideways through the air. In real life and in the simulators, we have an instrument helping us. On the dash you have a turn rate indicator, a small white spade moving left and right, and below that a ball in a curved glass tube. If you are using the rudder right, that ball will be centered in the tube. If you are going sideways through the air, that ball will drift out to the upwind side. To coordinate the aircraft, you simply need to 'step on the ball' - i e push the pedal on the side where you find the ball. With practise, you start anticipating the need for rudder and doing it automatically. Power changes and aileron use are the big drivers of having to coordinate the aircraft through use of rudder. Uncoordinated flight will mess up your shooting, reduce your speed, acceleration and climb rate and turn stalls into spin entries, so it is rather important. Best regards, /Fred *) Also known as your 'butt' or your 'behind'. You feel what the aircraft is doing by the seat of your pants. :)

Pyroflash, negative dihedral and anhedral are synonymous terms, yes. Neither is more 'proper'. Well done. I meant to say exactly what I said - do not use rudder to line up a shot, as it will cause your bullets to hit somewhere else than where you are aiming. That you can use rudder to e g walk your fire or correct your fire if you didn't manage to line up properly is true, but I didn't mean to say that as I felt it would only confuse matters at this point. Feel free to add it to the discussion though, but do it without attempts at mindreading please.

It's great fun in the A-10 as well. :D

I know I said I didn't have any complaints regarding the explanations given, but there was actually a minor niggle. The friction isn't what does it. Forces and geometry is. If the pivot axis is tilted forward, the tail sits lower with the wheel aligned. Any deflection will raise the tail and generate a centering torque. It's the same as on a pedal bike, where the rake of the steering column makes sure the front wheel aligns with the frame. Part of the weight reaction force will be perpendicular to the pivot axis and will have a moment arm when the wheel is not aligned with the fuselage centerline. There are a couple of pictures in the maintenance manual which I think, even if they are not completely unambigious, shows a definite caster to the tail wheel pivot axis. I think I can have a look at The Real Thing in a bit, unless someone happens to be sitting on better data, e g blueprints. In other words, if there is caster the centering force is there as soon as there is weight on the tail wheel, regardless of whether it is moving or not.

Peak cylinder pressure is raised with lower RPM as the total pressure loss in the induction system is reduced with lower RPM and lower airflow. However, when this is compensated for by the automatic boost regulator which closes the throttle valve and restores the total pressure loss - why would there be an increase in peak cylinder pressure? Especially considering that the ignition is retarded at higher MAP settings in order to avoid detonation. True, high speed is another driver for the retardation of the ignition - but I somehow doubt that the engine engineers would have implemented this poorly enough to allow it to take the engine into the detonation envelope.

'The secondary effect of rudder' is to roll in the the direction of the applied rudder - that's all you need to know. Leto is the one who has it right, for the "interesting facts" part of it. Aircraft are built to roll away from the direction of sideslip. Otherwise, the flight characteristics would be pretty poor. If you started to bank and didn't coordinate it with rudder, you'd sideslip which would increase the bank... The tendency to roll away from the direction of sideslip is called lateral stability. Lateral statility is affected by many factors. High wing aircraft have a higher degree of lateral stability, as the air streaming around the fuselage in a slip will increase the angle of attack of the upwind wing and decrease that of the downwind wing, causing more lift on the upwind wing and a stabilizing moment around the roll axis. Swept wings also contribute significantly to the lateral stability, as the relative wind will be more perpendicular to the length of the upwind wing, creating more lift. Dihedral, the V-shape of the wings, also contributes, as dihedral means increased angle of attack and more lift on the upwind wing. Dihedral is the method usually used to adjust the lateral stability. Thus, low-wing aircraft tend to have more dihedral than high-wing aircraft, and high-wing swept-wing aircraft often have negative dihedral. Depth, have a look at the ball during banking manoeuvres. Might make you want to reconsider your limited use of rudder. :) Never use rudder to line up a shot. You must be coordinated when firing or your bullets will be off laterally. I don't think this is what you said you are doing, but it could be interpreted that way and cause problems for the unwary. Cheers, Fred

He did? Well, that's pretty authorative. I've seen that overboosting is WIP, but that's not exactly the same. Got a link to that post of his? And why would there be, considering the engineering of the aircraft and leaving forum posts aside? :)

I wouldn't consider that a given. After all, the increase in charge in the cylinder will be marginal, only due to the increased time to get air through the intake valves at a lower RPM, and WEP shows there to be plenty of detonation margin at 61". The ignition timing changes with RPM, so that shouldn't be an issue either. As I said, still pondering this so let's work together and speculate endlessly. Why do you think there would be detonation?

Yes. I see raising and lowering of the tail due to power changes, but it doesn't really seem to be in sync with the elevator movements. Edit: Interesting test though - perhaps you or the OP can confirm that it can be/was done without changing the power setting?

That's what I wonder. Without automatic boost, the boost would go through the roof. But with automatic boost, the regulator will close the butterfly valve keeping MP at 61". OTOH, we've gotten a RR limited authority boost regulator in our Mustang - does it have enough authority to prevent an overboost if you drop the RPM from 61"/3000? Please try it and let me know! I'm stuck in a hotel room as usual...

Try full rudder deflection from a standstill with brakes off and then a burst of power though. On the road now so I can't test it again, but I'm fairly certain the effect of rudder in that situation was just about none at all.

"Anticipate the go-around early"... wish you always had the option! :D Still trying to figure out if you can overboost the P-51, or if the MAP regulator has the authority to prevent it. There are no warnings or explicit limitations in the manuals that I can find, which suggests it isn't a concern. There are the limitations specifying 46"/2700, which I interpret as never going above 46" with RPM at 2700, but that's hardly explicit. The automatic boost should limit the manifold pressure, so a serious overboost shouldn't happen as long as the regulator is working. However, 61" at 2700 will fill the cylinders more than 61" at 3000. On the third hand, the engine is rated for 67" without blowing up, so you should still be fine, at least for a bit. Now, what will happen if you foolishly go to 67" without bringing the prop forward... ? Plenty of speculation above. :)

John, absolutely correct. OTOH, I see little reaction to the movement of the elevator.