Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Straiga on January 25, 2005, 05:10:56 AM
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In AHII when I reduce prop RPM with the minus key the manifold pressure decreases also. Well this is not right. Just by changing the RPM only changes the oil to the prop govener and prop dome this should not effect manifold pressure at all, which is the throttle.
The manifold pressure shows the given amount of air in the manifold for a given day and density altitude. As altitude increases manifold pressure decreases unless there is a turbo or supercharger keeping the pressure up. The prop RPM will still stay constant as manifold pressure decreases.
Straiga
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Straiga: You are dealing with a supercharged engine. Less RPM causes the supercharger to spins slower, hence less availible mainifold pressure.
HiTech
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Which planes govern supercharger speed via prop rpm? On these planes are the superchargers driven hydraulically via the prop hub? Never heard of this before. Not like I am any kind of expert mind you.
I really just dont know.
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All supercharges are gear or belt driven to the engine. Props are
connected directly to the engine. Hence change Prop rpm, changes engine rpm, which changes the super charger RPM.
HiTech
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Ya OK so I am stupid. LMAO
For some reason I had prop pitch in my mind. Of coarse rpm will effect man pressure DUH what a dope I am.
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In reference to the original post in the thread: Decreasing RPM (with the propeller control) in a naturally aspirated engine will increase manifold pressure.
Edit: At other than full throttle.
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Yep have noted that in my RV casca.
HiTech
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Originally posted by hitech
All supercharges are gear or belt driven to the engine. Props are
connected directly to the engine. Hence change Prop rpm, changes engine rpm, which changes the super charger RPM.
HiTech
Turbosuperchargers (P-47, P-38, U.S bombers) are neither gear or belt driven. They are exhaust driven, and manifold pressure is more affected by load than by RPM. My turbocharged engines will reach maximum boost at less than half of maximum RPM.
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Superchargers are by definition mechanically driven. Turbochargers (or turbosuperchargers if you prefer) are, as you indicate, gas driven. The aircraft you mentioned had supercharged engines that had turbochargers in addition to the mechanical superchargers.
BTW Virgil: Sorry about your dad (saw it in another thread).
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Turbosuperchargers (P-47, P-38, U.S bombers) are neither gear or belt driven. They are exhaust driven, and manifold pressure is more affected by load than by RPM. My turbocharged engines will reach maximum boost at less than half of maximum RPM
So the RPM drop with a large Manifold pressure drop this should not happen.
Straiga
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Originally posted by Straiga
So the RPM drop with a large Manifold pressure drop this should not happen.
Straiga
Correct. To drop MAP in a turbocharged plane you have to cut throttle. Increasing load by increasing the pitch will raise MAP on a turbocharged engine.
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Originally posted by Casca
Superchargers are by definition mechanically driven. Turbochargers (or turbosuperchargers if you prefer) are, as you indicate, gas driven. The aircraft you mentioned had supercharged engines that had turbochargers in addition to the mechanical superchargers.
BTW Virgil: Sorry about your dad (saw it in another thread).
Technically, any mechanical device that increases the amount of air forced into an engine, regardless of what drives that device, is a supercharger. It is what you put in front of the word supercharger that describes the design and what drives it.
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Normal aviation nominclature.
Turbocharge Gas driven.
Supercharge Mechanicly driven.
Capt. But I do agree on you technical definition.
And turbochargers I belive can also effected by rpm.
Basicly the turbo will spin based on the volumn of gas (not fuel) exiting the engine. More Throttle, more expansion, more gas more turbo rpm.
Like you state.
But also, Same gas per stroke, higher RPM,more gas leaving cylinder hence more turbo rpm.
Now some of the planes we have also had a turbo rpm control to not over speed the turbo. Basicly a type of waste gate before the turbo.
HiTech
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Originally posted by Captain Virgil Hilts
Technically, any mechanical device that increases the amount of air forced into an engine, regardless of what drives that device, is a supercharger. It is what you put in front of the word supercharger that describes the design and what drives it.
I agree with you...technically. In common usage a supercharger is mechanically driven and a turbocharger is not.
As far as turbocharging relates to manifold pressure at full throttle operation (part throttle is a different case) on an engine without a supercharger. At lower altitudes part of the induction airflow is sent overboard through the waste gate. The waste gate can be contolled by differential pressures, a simple spring, or occasionally manually in the case of some older turbonormalized equipment. As the plane ascends the wastegate progressively closes. At full throttle the altitude at which the waste gate is all the way closed is known as the critical altitude. Above the critical altitude a decrease in RPM will result in a decrease in MAP (due to the decreased mass airflow though the engine). An RPM reduction below the critical altitude will result in no change in MAP (the wastegate closes some more to make up the difference).
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Casca: AH models it exatly as you describe.
HiTech
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Decreasing engine rpm in a turbo-supercharged US plane should lower manifold pressure just like any other plane.
The turbo-supercharger was setup to provide a certain mass flow of air at SL+ pressure up to the rated altitude. The B series on the P-38 were designed to provide approx 31 in Hg at the diffuser outlet. You will lose some of that pressure in the ducting, so there would be about 29.92 in Hg at the carb inlet on the engine. The integral blower in the engine provide boost above that. The P-38H/J/L engines with their 8.1:1 x 9.5 in blowers did slightly over 2:1, managing just over 60 in Hg at 3000 rpm. If you decrease engine rpm to say 2600 rpm you will no longer be able to get 60 in Hg from them. I'm too lazy to do the math right now, but you would likely only get about 50 in Hg at 2600 rpm.
The turbos could be tweaked to provide higher outlet pressure by closing the waste gate, but that appears to have been much more common on the P-47 than any others. But by design the system was supposed to provide SL pressure to the engine up to the turbo-superchargers rated altitude.
Greg Shaw
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Hi Greg,
>Decreasing engine rpm in a turbo-supercharged US plane should lower manifold pressure just like any other plane.
Wouldn't the exact outcome depend on the exact altitude both for turbo-superchargers and mechanical superchargers alike?
For mechanical superchargers, there's a specific full throttle height for each boost and speed combination. The same should be true for turbo-superchargers.
So reducing rpm above the target rpm full throttle height would reduce boost, while reducing it below target rpm full throttle height would leave it as it is because below full throttle height, boost is limited by the regulator (mechanical) or waste gate (turbo) and not by the supercharger capability.
Does that make sense? :-)
Regards,
Henning (HoHun)
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MAP changes with RPM reduction.
Naturally aspirated:
Full thottle - No change.
Partial throttle - Increase
Turbocharged Only, Below critical altitude.
Full throttle - No change.
Partial throttle - Increase
Turbocharged Only, Above critical altitude.
Both conditions - Decrease
Supercharged Only - Decrease
Supercharged with additional Turbocharger - Decrease
Note: The turbochargers on the class of engines we are discussing, to split hairs, are not actually turbosuperchargers but are turbonormalizers. That is to say they are, as Shaw said, merely providing roughly Sea Level pressure to the induction system at altitude.
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This all very interesting but in AHII at about 5,000 ft take any airplane lower the Prop RPM and watch what happens to the manifold pressure. Also we have a lot of planes in here with two stage superchargers Low boost for low altitude and high boost for high altitude. At 5,000 feet with full power we are not tasking any turbo or supercharger enough to have this notable of a change manifold pressure when prop RPM is decreased.
Also at about 5,000 feet lower prop RPM all the way on any airplane and lower the nose from 100 KTS to about 200 KTS with power at idle and watch the prop RPM climb. This small change in airspeed will not effect the prop in high pitch enough to make the RPM increase at this rate. If this is the case then we would all overspeed abd blow up engines when we desend out of 15,000 with half power at the point of compression to go after and shoot someone down.
Straiga
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No use in talking generalities.
Pick one plane lets take the p51d?.
If you have the critical alts you can do some simple calcs to know the RPM when the manifold will start droping off for each stage of the supercharger.
Basicly take ambient Pressure at critical alt, and max manifold pressure full RPM at critical alt. This gives you a base compresion ratio of the supercharger. Now the compression ratio will vary linarly with RPM and you can then calc the pressure out of the super charger at any alt & RPM.
If This pressure is greater than waist gait pressure , no drop will be seen with RPM change, if it is less, a drop will be seen.
HiTech
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But were not talking high altitudes to have a critical altitude for turbos or superchargers in AHII were lucky if we get to 15 k before we have to come down to fight or drop bombs in a fighter.
If we were at these altitudes we would have to look at the loss of propeller efficience at those altitudes. This also has implacations to the manifold pressure, less mass airflow from the prop because of air density.
Well lets talk about the P-51D at 26,000 you should be able to pull 61Hg 3000RPM and supercharger in high. In auto climb this works, but when I hand flew it I was only able to get 57Hg at 3000 RPM.
Cruise power 15,000, 35 Hg, 2050 RPM In AHII at 35 Hg I could not select 2050 RPM it would not go below 2175 RPM.
Max cruise 15,000, 29Hg, 1700 RPM In AHII I could not get it below 2000 RPM.
But for any flight at 15,000 I should be able to pull 61Hg and have a RPM of 2000.
The P-51 has a automatic maniflod pressure regulator that maintains a .1 inch Hg between .42 and .61 Hg below critical altitude, regardless of the RPM setting is.
Straiga
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Like I said straiga get out a P51d flight manual, compair the numbers. Btw we do also acount for raise in manifold pressure do to ram air.
HiTech
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For nearly all planes in AH2 this is not true, thanks to the constant speed unit.
What is true is that changing the propeller speed will determine the engine RPM (holding the throttle constant) as the load on the engine will change.
What is also true is that one should be operating the engine by controlling both RPM and manifold pressure. There is little point in manipulating just one of the controls.
In particular, lower RPM and moderate manifold pressure should maximize endurance by minimizing friction losses while avoiding detonation.
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Originally posted by hitech
... Props are connected directly to the engine. Hence change Prop rpm, changes engine rpm, which changes the super charger RPM....
HiTech
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ok now that you all have described how manifold pressure works. I would like to know at what alt does the E6B chart ratings derrive from, is there a standard alt used for every plane or does the speed , rpm and manifold pressure ratings come from a different alt per plane on an individual bases.
maybe this is already posted in another thread. but in regards to manifold pressure I have seen it fluctuate over or under the E6B charts in the game from sea level up to high alt 30K or higher......
the E6B does not say best cruise alt, or what not. So one might be at a given alt and never be able to attain the desired manifold, rpm settings the E6B chart calls for.....
if one knew the alt for which the chart is referencing it might would be more beneficial to people that use the E6B chart settings..anyway to include these alts on the E6B chart?
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Every engine in a US aircraft at least has an engine calibration curve that maps RPM and manifold pressure into HP for a given altitude. These curves are hard to come by now, but I imagine there were miles of them in cabinets once.
You can still find these things in aircraft manuals, but the best are found in manuals produced by the engine manufacturers.
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Originally posted by TequilaChaser
ok now that you all have described how manifold pressure works. I would like to know at what alt does the E6B chart ratings derrive from, is there a standard alt used for every plane or does the speed , rpm and manifold pressure ratings come from a different alt per plane on an individual bases.
maybe this is already posted in another thread. but in regards to manifold pressure I have seen it fluctuate over or under the E6B charts in the game from sea level up to high alt 30K or higher......
the E6B does not say best cruise alt, or what not. So one might be at a given alt and never be able to attain the desired manifold, rpm settings the E6B chart calls for.....
if one knew the alt for which the chart is referencing it might would be more beneficial to people that use the E6B chart settings..anyway to include these alts on the E6B chart?
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Forget about the turbo-supercharger for now, just think of the engine as being SL rated. The integral blower is sized to provide 60 in Hg @ 3000 rpm & SL inlet pressure. If you lower rpm, it lowers the tip speed and hence the pressure ratio, so at 2600 rpm vice 3000 rpm the pressure ratio would likely be something like 2600/3000 * 2.0 = 1.73 * 29.92 = 51.85 in Hg max, up to the altitude the turbo-supercharger could provide SL inlet pressure. That is assuming that efficiency stays the same at 2600 rpm as 3000, which is most likely incorrect. It is probably a little better, so call it 52-53 in Hg @ 2600 rpm max.
We could also get into the turbo-supercharger rated altitude dropping off with lower rpm and map due to the lower mass flow through the engine. But lets try to keep it somewhat simple, and just think of it as a SL rated engine, with the turbo-supercharger providing that SL pressure.
(supposed to be a response to HoHun's post, but I took too long and screwed up the quoting)
Greg Shaw
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There is one notable exception HiTech; the DB's. They had automatic variable speed superchargers. As for geared SC sets and turbo's I agree with you; pressure would drop with rpm.
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Hey Scholzie:
How was the transmission. Hydraulic, right?
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I'm not sure exactly how it worked. From what I've been told it was basically just a big clutch.
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GScholz: Auto gear change. Or infinatly varible?
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Hi Hitech,
>GScholz: Auto gear change. Or infinatly varible?
The device used in the Daimler-Benz engines actually was a hydraulic torque converter (infinitly variable, as you point out), with the term "clutch" being somewhat of a mis-translation.
Regards,
Henning (HoHun)
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Originally posted by hitech
No use in talking generalities.
Pick one plane lets take the p51d?.
If you have the critical alts you can do some simple calcs to know the RPM when the manifold will start droping off for each stage of the supercharger.
Basicly take ambient Pressure at critical alt, and max manifold pressure full RPM at critical alt. This gives you a base compresion ratio of the supercharger. Now the compression ratio will vary linarly with RPM and you can then calc the pressure out of the super charger at any alt & RPM.
If This pressure is greater than waist gait pressure , no drop will be seen with RPM change, if it is less, a drop will be seen.
HiTech
Hitech as a retired Tomcat driver I have to ask, which do you know better Aces High, or real A/C, I ask because I Flew I c1 cod before I got into Tomcats
And reading this thread you seem to know more about piston A/C engines than most of the "old" guys that I know LOL
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Sb1086: Thanks for the compliment.
What it comes down to is I have been makeing flight sims for 15 years. And to make sims you have to understand all items on a plane from a system level. Some you choose to detail in the sim, some you don't.
Look at our resent discusion on the torques of jet engines and where it comes from. This is one i had not event thought about before,hence sparcs my interest greatly. Why things work as always been a fancination of mine.
Always looking for new knowledge on how things work. Like why I asked how the varible supercharge worked.
Pyro: Is the knowledge guy on any paticulare plane, I.E. what specific items any plane had.
HiTech
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Try Jet engine thrust:)
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Thanks for the clarification HoHun.
Yes HiTech, infinitely variable. Which is why the DB powered aircraft are the only supercharged planes in AH that has smooth speed and climb curves (almost like the turbo powered rides), and not the typical jagged curves of multi-stage/gear supercharged engines.
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Originally posted by hitech
Normal aviation nominclature.
Turbocharge Gas driven.
Supercharge Mechanicly driven.
Capt. But I do agree on you technical definition.
And turbochargers I belive can also effected by rpm.
Basicly the turbo will spin based on the volumn of gas (not fuel) exiting the engine. More Throttle, more expansion, more gas more turbo rpm.
Like you state.
But also, Same gas per stroke, higher RPM,more gas leaving cylinder hence more turbo rpm.
Now some of the planes we have also had a turbo rpm control to not over speed the turbo. Basicly a type of waste gate before the turbo.
HiTech
P-38s had wastegates AND turbo regulators. Maximum turbo speed was governed by the regulator(s). Wastegates dumped excess boost. Different functions. IIRC, both aircraft had sufficient exhaust flow to spin the turbos to max speed at just 1,800 rpm for the P-38L, and 1,650 rpm for the P-47D (typically). You should not see a drop-off of available boost until engine speeds go below these levels. This is why it was so critical to P-38 engine reliability to increase rpm before increasing throttle, else the engines be severely overboosted and detonate. At least that's what the pilots have told me.
My regards,
Widewing
One of the reservations expressed by pilots being introduced to Lindbergh's low rpm/high MAP cruise settings was concerns about the long term effect of running high boost at low rpm. Most pilots cruised at higher rpm to MAP ratios than Lindbergh advocated.
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Originally posted by sb1086
Hitech as a retired Tomcat driver I have to ask, which do you know better Aces High, or real A/C, I ask because I Flew I c1 cod before I got into Tomcats
And reading this thread you seem to know more about piston A/C engines than most of the "old" guys that I know LOL
Geez, I logged about 1,600 hours in the C-1A as crewchief (CV-60) and it's rare that I run across anyone else who flew or crewed CODs. It's been 26 years since I last flew a hop in the C-1A, but I can still recite the checklists from memory... Must some form of Idiot Savant.... :)
My regards,
Widewing
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Originally posted by Widewing
P-38s had wastegates AND turbo regulators. Maximum turbo speed was governed by the regulator(s). Wastegates dumped excess boost. Different functions. IIRC, both aircraft had sufficient exhaust flow to spin the turbos to max speed at just 1,800 rpm for the P-38L, and 1,650 rpm for the P-47D (typically). You should not see a drop-off of available boost until engine speeds go below these levels. This is why it was so critical to P-38 engine reliability to increase rpm before increasing throttle, else the engines be severely overboosted and detonate. At least that's what the pilots have told me.
My regards,
Widewing
One of the reservations expressed by pilots being introduced to Lindbergh's low rpm/high MAP cruise settings was concerns about the long term effect of running high boost at low rpm. Most pilots cruised at higher rpm to MAP ratios than Lindbergh advocated.
THANK YOU! I was almost certain I was seeing something missing here. I can't seem to get it to work that way in AHII. I also noticed that Levier found the 8th AF doing it wrong when he got to Britain on his trip.
Levier's report, and instructions on P-38 engine management:
Report on P-38s in the ETO, by Tony Levier.
“Having just returned from a four month mission to England on 29 May 1944 where I had been testing and demonstrating some of the new improvements on our P-38J’s, I filed the following report:”
Immediately upon arriving in England, I proceeded to the 55th FG HQ at Nuthamstead, an airbase in East Anglia, north of London, where conditions were pretty grim as far as their P-38’s were concerned. They had just received their first P-38J’s, and had no operational information on them. Their lack of information concerning correct power combinations was appalling.
For combat missions some pilots were using anywhere from 2000RPM to 3000RPM with whatever manifold pressure that would giver them their desired air speed. Some of these pilots were blowing up their engines with high manifold pressure and critically low RPM, while others were running out of gas and failing to complete missions because of such power combinations for continual cruise at 2800RPM with 24” of manifold pressure. Many returned with hardly more than a cupful of gas remaining in their tanks, while others were forced to bail out over enemy territory.
The reason they were cruising at 2600, 2800 and even 3000RPM was that somewhere along the line they had been taught to use high RPM and low manifold pressure. They were under the impression that should they get jumped by a Jerry they could get their power faster if they already had their engines running at high RPM.
The fact is you can get your power quicker if you have low RPM and high boost which gives you a high er turbosupercharger speed. With turbochargers putting out high boost you only have to increase your engine RPM to get your desired power.
Rather than add to their confusion with power curves and range charts we devised the following rule of thumb for their long range missions. It is simple and easy to remember and insures maximum engine efficiency/fuel economy: USE 2300RPM AND 36” MANIFOLD PRESSURE AS THE MAXIMUM FOR AUTO LEAN AND CRUISE CONDITIONS. IN REDUCING POWER FROM THIS SETTING, REDUCE ˝” TO 1” FOR EACH 100RPM; FOR GOING ABOVE THIS SETTING PUT YOUR MIXTURE IN AUTO RICH AND INCREASE THE MANIFOLD PRESSURE 2” FOR EACH 100RPM.
After using this rule the boys marveled at their increased range. Some were returning from 4 hour missions with as much as 150 to 200 gallons of fuel left. Quite a bit more than the cupfuls they had been returning with.
The day after I arrived at another base in England some P-38 pilots who had been escorting Forts over mainland Europe reported a “sort of engine trouble”.
When pinned down they said their engines had been surging and momentarily cutting out while they were flying at altitude under reduced power. The passed it off saying, “it’s probably caused by some extra low octane gas.”
But after further discussion with the boys I suddenly remembered a series of tests we had run back home during flight test operations for proper turbosupercharger settings. As I recalled, the symptoms were very similar.
So I obtained permission from the 55th FG CO to test a P-38 at altitude for proper turbosupercharger operation, and sure enough, the turbosupercharger on the left engine was so rigged that the resulting backpressure and high turbosupercharger wheel speed caused the airflow to the engine to surge, resulting in erratic operation and inability to pull power.
Upon landing, I reported the trouble and recommended that each Lightning driver be given the following procedure for checking his ’38 at altitude for proper turbosupercharger operation: At 30K feet set your RPM at 2600RPM and 37” manifold pressure and back off slowly on the power down to 10 to 15” manifold pressure. While doing this, fix your eyes on the manifold pressure and note if there is the slightest engine failure or surging. Record the exact manifold pressure at which this surging occurs.
Now repeat the process beginning with 2300RPM and 37” manifold pressure , and again record the boost at which surging occurs. (At this lower RPM it should occur 2 or 3” higher.)
If, after you’ve completed this procedure, either of the recorded manifold pressures are above 22”, it indicated the turbosupercharger regulator is set improperly. Tell your mechanic the boost at which the roughness occurred and he will make the necessary adjustments.[/I]
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Originally posted by Widewing
Geez, I logged about 1,600 hours in the C-1A as crewchief (CV-60) and it's rare that I run across anyone else who flew or crewed CODs. It's been 26 years since I last flew a hop in the C-1A, but I can still recite the checklists from memory... Must some form of Idiot Savant.... :)
My regards,
Widewing
Some things you never forget man, I can still go through the whole thing from startup to shutdown at another field. I bet ya I can even stil deck run one on a carrier too.