Run Up RPM

[FlyingMonkey]

Hey All,

 

Eddie's thread about roughness on mag checks got me thinking,and I didn't want to hijack his thread...

 

What is the best RPM to run up for mag checks? The CT manual and the checklist sticker on my panel says 3000rpm, but I was taught to use 4000rpm, so that's what I have been doing. From Eddie's posts I gather he's using 3000-3500rpm. I imagine any RPM will work, it's probably just a matter of what is best for plugs, brakes, fuel, etc. 4000rpm might show more high RPM stumble if present, but might just be unnecessarily wasting fuel and making heat.

 

What say you?

[Roger Lee]

The Rotax manual says to use 4000. So what's the difference between 3500 and 4000? Just a bigger number for the drop and a possible wide discrepancy so you might catch something wrong. If you use too small an rpm then you can miss small irregularities in a mag drop or between the 2 ignitions. I usually use 3500, but if you like 4000 go for it. You could use 3400 or 3700, ect....

3000 would be my absolute bottom because lower than that may cause you to miss subtle changes or misses between the systems. So the actual number isn't a concern, but the difference between the two ignition systems is what's important.

[FlyingMonkey]

Thanks Roger! After already flying one day and stopping for a while, when I taxied for takeoff I did an "abbreviated" mag check while taxiing at about 2300rpm. I was confident in the ignition since I had just flown the plane, but based on what you said in the future I'll just stop and do a proper mag check so I don't miss a problem.

[Roger Lee]

If I fly out to breakfast or some short hop I don't always do a second mag check. I don't think any of the guys with CT's here at the field do a second one for short hops. Certainly it's a users preference and I'm sure many do it before every single take off. Do what ever is comfortable for your peace of mind.

[FlyingMonkey]

Thanks.

[FastEddieB]

Actually, my POH calls for 3,850 rpm.

 

That's hard to see on my smallish analog gauge, so I use 3,800.

 

In my video, I just wanted to take it "off idle" to get it out of the idle circuit, so I felt about 3,000 rpm would do that.

[Roger Lee]

Hi Eddie,

 

One last thing to double check is make sure both chokes are returning all the way to their zero point and one isn't hanging up.

[Jim Meade]

In a conflict between a manufacturer's POH/AOI and a component spec, who wins, according to the FAA?

Rotax calls for 4,000. Eddie's plane calls for 3850. The CTSW calls for 3200.

Here's why this is an important question. If you look at an engine by itself, the manufacturer may give certain operating parameters. If you look at a propeller the maker may say run it such and such a way. But if you put them together, there may be harmonics that are detrimental. Many of us have flown airplanes that have a yellow mark that says don't operate in a certain range.

So, when Rotax says one thing for an OEM engine that can be put in anything and the airplane manufacturer gives differing instructions, what is the mechanism we have for the tie breaker?

I'd have to think that the aircraft manufacturer would be responsible for the whole system and therefore a lot of credence should be given to his guidance.

Does anyone have any FAA rulings on how this works out?

[Ed Cesnalis]

Checklist on my panel says 3,000 RPM and my POH says 4,000.

[Jim Meade]

CT, I was looking at the CTSW operators manual and it said 3200. What date is your POH and what model/year was it issued for?

I'm still interested in the question of how the FAA breaks ties when there is conflicting guidance.

[Ed Cesnalis]

I'll check next time I'm at the airport.

[Rich]

A mag check before each take-off on the 912 ULS at 4K may provide enough suction to draw that unsuspecting particle from the float bowl into the main jet catching the problem before departure.

 

If not a mag check, then a high rpm run-up will serve the same purpose.

 

I'm a believer, after having a substantial RMP loss after take-off which resulted in an immediate return to the airport. I now do a mag check prior to each departure.

 

It's not fool proof, but one more chance to catch something that may cause a problem.

 

Rich

[Roger Lee]

Personally I would stick with the MFG of that particular item i.e. Rotax unless FD or which ever MFG has a reason to change it. The example is that FD has been wrong in their documentation and recommendations several times in the manuals and settings, but have made corrections. The one big example was back in 2005, 06 and 07. FD set and wanted the max WOT in flat and level flight to be only 5200 rpm. Which of course was totally wrong and now it is set up to at least 5500 rpm.

There are several examples where FD had made a mistake and had to change later. Rotax would be better qualified in this case unless FD could justify the change and then justify why it is different in different manuals. There are still some errors in the FD manual and in the new Rotax FI manual there are several mistakes that should be corrected on the next manual re-issue.

 

If you go to Rotax school you can ask this question which almost always gets ask during a service level class. The actual real rpm for the mag drop isn't important so long as it isn't too low. The higher the rpm during the mag drop the bigger the mag drop number. The rpm is just a reference point so you can compare each ignition system to each other at that rpm. If you pick 3400, 3500, 3600 or 4000 it doesn't make any difference, you're just comparing the drop numbers.

[Roger Lee]

Hi Rich,

 

The 3500-4000 rpm isn't enough to check for a debris issue most times. I have had to deal with this a couple dozen times. You can run that engine to full rpm 2-4 times without any issue and then take off and have it shut down from debris. Ask Bob Corey (FlyingCfarms here on the forum). The problem with sitting on the taxi way and running up is any fuel particles are setting in the bottom of the bowl. There is not enough fuel movement or agitation to get the particles to float high enough to get into a suction stream of the main jet, plus 4000 rpm isn't enough fuel flow. Once you take off you have full throttle and plenty of fuel flow, but more importantly you have the fuel in the bowl being bounced around and agitated and the particles start getting stirred up off the bottom. Sooner or later they may get high enough in the fuel level and pulled into the suction.

[Rich]

Thanks Roger------Rich

[Jim Meade]

Roger's discussion of why he prefers Rotax runup numbers is persuasive. In saying that he prefers Rotax over FD he says it is because FD has a history of being wrong. I don't disagree with that. That doesn't mean the FD is wrong this time, though, for the airframe and propeller being used.

It looks to me like we have a case where there are two iimponderables:

1. it useful for us all to know what the regs require us to do if push comes to shove (whether we choose to do that is a different thing). Does anyone know if FD or Rotax wins in the battle over which is authoritative? (Note, I didn't say which is right.)

2. Is there any information on whether there is any reason for FD to have set the 3200 rpm number? Any history or documentation? Was Rotax calling for 4,000 when FD said 3200? Good grief! They're just over the border and they speak the same language (well, Austrians and Germans might have a jocular argument on that, just like British and American English )

[Tom Baker]

I think you would have to go with the aircraft manufacturer unless they defer to Rotax.

[Rogerck]

I wonder if the 3200 recommended by FD has anything to do with the significant shaking of the stabilator at higher rpm static run up. Exposing the stab to heavy shaking on a regular basis may not be prudent. Also, if you're demoing a CT to sell, seeing the stab shake doesn't inspire confidence. I hate conjecture and am not trying to start yet another wrangling discussion here, just a thought that we will not know the answer to unless FD reveals this.

Roger Kuhn

I wonder if the 3200 recommended by FD has anything to do with the significant shaking of the stabilator at higher rpm static run up. Exposing the stab to heavy shaking on a regular basis may not be prudent. Also, if you're demoing a CT to sell, seeing the stab shake doesn't inspire confidence. I hate conjecture and am not trying to start yet another wrangling discussion here, just a thought that we will not know the answer to unless FD reveals this.

Roger Kuhn

[FlyingMonkey]

That's a good thought Roger...I have never seen a CT run up from the outside, so the stab shake did not occur to me.

[Roger Lee]

It shakes on the ground at any thing above 3000. It won't hurt the stab. It truly doesn't matter what rpm you pick so long at it isn't too low. That's why you see so many different numbers in prints. Pick one and just check the mag drop. I have talked to many a CT owner and 3400-3600 seems to be the winning rpm. Too low and you can't hear, feel or see any significant drop if there is a real problem especially with analog gauges.