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View Full Version : Just started using CEM... need help for carrier ops.



sledgehammer2
09-09-2005, 07:46 PM
Please indulge my questions here... I understand the concepts on complex engine mgt and I have read the Airwarfare article and Vulgar's article. Great stuff and helpful, but I am still having some trouble.

For example, a carrier takeoff in an F6F - do I set the mixture to 100 or 120%? If the lower percentage prop pitch means the prop is getting more bite of air, then wouldn't I use full lowpitch prop to takeoff, and all through the traffic pattern? If I do that however I cannot get enough spped to take off.

If I set prop pitch to 100% which according to Vulgar's article means it is biting the least amount of air, I have no trouble getting airborne. If I am reading his article correctly then this makes no sense to me.

I also used the first blower stage which seemed to be ok. Any help on my confusion would be appreciated.

Sledgehammer2

sledgehammer2
09-09-2005, 07:46 PM
Please indulge my questions here... I understand the concepts on complex engine mgt and I have read the Airwarfare article and Vulgar's article. Great stuff and helpful, but I am still having some trouble.

For example, a carrier takeoff in an F6F - do I set the mixture to 100 or 120%? If the lower percentage prop pitch means the prop is getting more bite of air, then wouldn't I use full lowpitch prop to takeoff, and all through the traffic pattern? If I do that however I cannot get enough spped to take off.

If I set prop pitch to 100% which according to Vulgar's article means it is biting the least amount of air, I have no trouble getting airborne. If I am reading his article correctly then this makes no sense to me.

I also used the first blower stage which seemed to be ok. Any help on my confusion would be appreciated.

Sledgehammer2

FritzGryphon
09-09-2005, 08:38 PM
The 'prop pitch' control doesn't actually change prop pitch on a CSP plane. It only changes the desired RPM. For most power, you want greatest possible RPM, not the least.

F6F also gets it's highest speed at 100% pitch. There's not much reason to change it (except maybe for economy cruise or something).

I tested 120% mix on a Corsair a while back. Both a speed test on the deck, and a cooling test. In both cases, it has no effect, so I don't know what 120% mix is for.

Tully__
09-10-2005, 06:41 AM
When your aircraft is at low speed you don't want your prop trying to take too big a bite of the air. The propeller blades are just like wings and if the angle of attack (the size of the bite) gets too big, two things will happen:

1. the blades will stall just like a wing. This means a net loss of thrust, not what you want on takeoff.

2. the drag from the angle of the blades going around will be too much for the engine to reach maximum power, resulting in the engine running at reduced power and developing less thrust. Not what you want on takeoff.


Additionally, as FritzGryphon mentioned most allied aircraft pitch controls do not directly control propeller pitch. Instead they control an propeller rpm governor. The mechanism than adjusts prop pitch so that the engine has just enough load to stop it revving faster than the selected rpm. For takeoff you select max power rpm (100%). The propeller will reduce pitch until the engine is able to just drive it at max power, which is exactly what you want for takeoff.

x__CRASH__x
09-10-2005, 02:03 PM
look at the big brain on Tully! http://forums.ubi.com/images/smilies/11.gif

RogueRunner
09-10-2005, 03:15 PM
go read THIS (http://www.auf.asn.au/groundschool/propeller.html)

Willey
09-10-2005, 07:07 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by FritzGryphon:
The 'prop pitch' control doesn't actually change prop pitch on a CSP plane. It only changes the desired RPM. For most power, you want greatest possible RPM, not the least.

F6F also gets it's highest speed at 100% pitch. There's not much reason to change it (except maybe for economy cruise or something). </div></BLOCKQUOTE>

My rule of thumb for any prop plane except 190 and 109/110s (-&gt; CSPs) is: Set Prop just the same as throttle, like 100/100 or 50/50. Works quite well and flying with 50% thr will give you more speed if you have prop at 50%, too. Leaving it at 100% makes it work like a brake, sadly not braky enough. Was better in the very first FB versions.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I tested 120% mix on a Corsair a while back. Both a speed test on the deck, and a cooling test. In both cases, it has no effect, so I don't know what 120% mix is for. </div></BLOCKQUOTE>

It *should* grant better cooling and it once gave higher speeds... and I also remember the better cooling effect for all planes at richer mix settings, but haven't tested it again in any PF versions. At least I know that it never actually reduced the fuel consumption which should be the most noticable effect. Dang it didn't work trying it in a Yak at 8000m or so, mix 20% http://forums.ubi.com/images/smilies/16x16_smiley-sad.gif

As for the blower, I don't try to memorize some altitudes, just roughly and trying out a bit. Just listen to your engine and switch back and forth while climbing. You'll know when it's right, RPM should be higher or at least as high as before after switching. Just wait a second or two for it to settle as it jumps up/down first. Of course you could work out the altitudes this way... just can remember the first switching altitude of the F4Us which is roughly 2200m.

And don't try to mess with the magnetos... they're set to both right off the start adn you'll spool up the engine with [I] anyway.

jds1978
09-11-2005, 09:30 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">And don't try to mess with the magnetos... they're set to both right off the start adn you'll spool up the engine with [I] anyway. </div></BLOCKQUOTE>

so true....so, so true

effte
09-13-2005, 04:46 AM
On many aircraft with automatic mixture control, 120% corresponds to the full rich setting, 100% is auto rich, 80% auto lean and 60% (?) would be fuel cut off.

On the deck at high power settings, auto rich should effectively be full rich, as the mixture management will auto-enrich to prevent detonation, so to not see a difference is not surprising.

To try to find the right switchover altitude by looking at the RPM is futile. As was stated earlier, the CSP governor will keep the RPM constant as long as there is enough power to keep the propeller RPM up with the propeller off the fine pitch stops. It should be pretty much impossible to put the prop on the stops in flight. What you want to look at is the manifold pressure. When that starts to drop during your climb, you are getting close to the switchover altitude where you should go to the next supercharger step.

Cheers,
Fred