Author Topic: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)  (Read 8692 times)

Offline Frank N. O.

  • Alpha Rooster
  • *****
  • Posts: 2446
  • Spin It!
Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
« Reply #15 on: October 15, 2008, 09:29:47 AM »
I've sent you a pm Ian.

Frank
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."
Leonardo da Vinci

Offline Baradium

  • Alpha Rooster
  • *****
  • Posts: 1602
Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
« Reply #16 on: January 14, 2009, 07:18:35 PM »
Hey Matt,  when I started flight training they had us do a ground check before shutdown every flight.  Guess they figured they'd rather have to teach students how to do it rather than increase the risk of a hot mag.

What I was always taught and what I did when I was working line service was if you had to move a prop (To hook up a tow bar for example) is to stay out of the arc and use the palms of your hands to move the prop in the OPPOSITE direction that the engine turned.


Brian

I was told that turning the prop backwards (and in turn turning components backwards) can be especially harmful to some components like vacuum pumps, increasing the risk of breaking a vane.  I'd say something if I saw you trying to turn the prop of an airplane I owned backwards... if I owned one anyway...

Also, the method I was taught, and use, is the one I believe Matt was advising.  Palm flat on the face of the prop and push it around, keeping clear of the arc.   


I heard that with some of the larger radial engines, a way you'd "hand prop" them was to put the blade against your shoulder and run in the direction of engine rotation.  If the engine kicked you'd be already getting out of the way.  Never seen anyone do it, but had a couple of old timers mention it.


"Well I know what's right, I got just one life
In a world that keeps on pushin' me around
But I stand my ground, and I won't back down"
  -Johnny Cash "I won't back Down"

Offline ZK Kiwi

  • Cockerel
  • ***
  • Posts: 122
    • Wings Over NZ Aviation Forum
Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
« Reply #17 on: January 14, 2009, 08:32:02 PM »
Regarding you later batch of questions (which are worth asking)
Fixed pitch vs constant speed: fixed pitch is pretty simple, opening the throttle makes the prop go faster, nothing too tricky there. On a constant spped unit, a governor adjusts the prop blades so the prop always maintains the same speed. at low throttle settings the blades go fine to unload the engine and maintain RPM, as throttle settings increase, the governor matches it by coarsening the pitch, loading up the engine and maintaining the same RPM. The Throttle butterfly valve however opens up and more charge is going into the cylinders as this happens, which registers as manifold Pressure (which is confusing in itself as on normally aspirated engines it is actually a vacuum, which decreases as throttle is opened!) hence when you open the throttle, RPM should stay the same but Manifold pressure decreases back towards 30 in Hg.

with regards to low wing vs High wing fuel systems, there is no reason at all to say low wings cant have a "both" position, not any reason why highwings have to have one. it all depends on how the designer decided to plumb everything up. for example, my aircraft has an on - off selection and no individual selection for each wing tank. Both wing tanks gravity feed to a header tank supplying the fuel injection. Cherokees and the like dont have a both selection, but that is more to do with the location of fuel pumps etc in the system  - it just happens to be the way Jim Thorp wanted to do it, there are plenty of alternative methods.
If it was supposed to be easy, everyone would be doing it!

Offline ZK Kiwi

  • Cockerel
  • ***
  • Posts: 122
    • Wings Over NZ Aviation Forum
Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
« Reply #18 on: January 14, 2009, 08:53:00 PM »
Further to boost pump questions - my high wing has 2x High pressure pumps as it is fuel injected, generally carbs require lower supply pressure, and the head from the tanks in the wings is enough to provide this. On low wing aircraft the gravity flow doesnt exist so the engine driven pump provides the feed to the carb - at relatively low volume. Switching on the boost pump is a backup to this pump to provide feed in the event of pump failure. it is used at low altitudes mainly because should the mechanical pump fail the engine will stop and there is less time to attempt a restart. In practice, it is good to put the boost pump on any time when an engine failure would ruin your day (e.g steep manouvers at low speed etc). Injected engines (such as mine) require higher fuel pressures than can be obtained from gravity feed or mechanical pumps, hence even high wing injected aircraft such as Cessna 207s have electric pumps operating continuously. in most cases there is a backup pump used in the same way the boost pump is on carb models - whenever you cant afford to have engine failures! as these pumps move huge quantities of fuel (more than the engine uses) the surplus normally goes back into the tanks. On High wing aircraft this causes problems as it is difficult to put it back into the tank it came out of. this is why header tanks are used (as on my aircraft) as the fuel can go back there, and the main tanks just top up what is used. The other thing you will notice is on injected engines, a reliable electrical supply is essential, as without the electric pumps, things go quiet - injected aircraft generally have more complex electrics to ensure relaibale operation.

superchargers etc:
Aircraft rarely require the full power available from an engine at altitude - especially light planes. the additional complexity and weight of a supercharger system is rarely justified. Unless you live in Bolivia or the likes, most takeoffs (the only time max power is really needed) will be within the altitude band where adequate power is available. There are lots of other factors, such as the fact propellers are inefficient anyway. designers have generally kept to the "Keep it simple" rule to improve reliability, serviceability and practicality, even if it means they are not as efficient.
If it was supposed to be easy, everyone would be doing it!

Offline Frank N. O.

  • Alpha Rooster
  • *****
  • Posts: 2446
  • Spin It!
Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
« Reply #19 on: January 27, 2009, 07:19:11 AM »
Thank you very much for the good and detailed information :)

Frank
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."
Leonardo da Vinci