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Adjusting Max RPM

DanH

DanH

Legacy Member
Mentor
Tell me about adjusting the maximum RPM setting for a Hartzell CS prop.

Today I tied the airplane, pulled the spinner, and made a few full throttle runs, adjusting the low pitch stop screw each time as indicated in the prop manual. Took about two full turns to get the 2650-2700 RPM range static at WOT.

First takoff was fine, 2710 RPM. However, later when doing T&G's I looked down and saw 2800. Only idea I have is warmer oil, 185F vs 100-120F on the first takeoff with Phillips 20-50.

So what gives? Am I missing something?
 
RPM limit

Dan......Congrats on getting airborne!

No expert here, but I had high rpms as well (IO-360/Hartselle/MT gov). I simply adjusted the MT gov so it would limit. Worked great.
 
Adjustments

On the certified engine, prop, governor installations, the low rpm stop (internal) is left alone. As long as the engine make rated rpm at minimum stall speed or there abouts I let it fly. I think it has something to do with the range that the governor can govern. Adjustments to the governor are with hot oil, because you are either adjusting oil in, or limiting oil out. (oil is flowing through). Another reason the low rpm stop is set at a flying speed is in the event of governor failure. You can get rated power at a flying speed without an overspeed.
I honestly can't say I know this for sure, It's one of those "tribal knowledge tidbits" the FAA is loath to acknowledge without something written from some authority, but it does make sense.
Andrew
AP IA
-9 preview
 
DanH said:
Tell me about adjusting the maximum RPM setting for a Hartzell CS prop.

Today I tied the airplane, pulled the spinner, and made a few full throttle runs, adjusting the low pitch stop screw each time as indicated in the prop manual. Took about two full turns to get the 2650-2700 RPM range static at WOT.

First takoff was fine, 2710 RPM. However, later when doing T&G's I looked down and saw 2800. Only idea I have is warmer oil, 185F vs 100-120F on the first takeoff with Phillips 20-50.

So what gives? Am I missing something?
Click to expand...

Sounds like you've done everthing correctly, now you just need to adjust your governor back to the 2700 limit. The low pitch stop is only for static RPM setting.
 
Dan...

1. FPS. I'm not brave enough to do the FPS via tie-down / full power runs. For ours, and those I test fly, I do it incrementally with aborted takeoffs / short accel runs, and later just observations on takeoff. Look for Max Rpm achieved with an unusually sharp opening of throttle.

2. Governor - does not usually need adjusting.

If I had an aim, I would set 1 to 2600-2650 RPM, and 2 to 2680-2690. By setting 1 a little low, all it does is deny you full power at, say, <20KIAS, by 30KIAS the airflow will allow RPM to get to 2700+.

T&Gs will always be a problem, especially if you do not let IAS decay to almost zero. The rapid acceleraiton of the engine does not give the governor (and the MT ones are probably not the best) a chance to limit to 2700. Solutions are to:
  1. Slow to almost a halt before applying power
  2. Advance throttle more slowly, or in 2 stages - second stage more slowly
  3. Accept there will be overswing, NB Lyc allow 10% (2970RPM) for 3s, check Prop as well.
  4. Problem will be reduced by setting 1 to 2600-50 rather than 2650-2700
This is an area where our fancy EFIS systems do us no good - in the old days, where 2600-2700 was <1/4" on a gauge, usually viewed with parallax, we did not notice this all going on. Now get 2701RPM for 0.1s and red lights, bells, end of world is nigh declared :eek:

Andy
RV-8 G-HILZ
 
This is an interesting topic to me

This is an interesting topic to me. You may know that I try to increase the speed of my RV-6A as much as I can. So far this has been accomplished mostly by drag reduction and thrust increases have been limited to buying a blended airfoil Hartzell C/S prop with F7496 blades to replace my older design with F7666 blades and backing out the prop governor control arm rotation limiting screw to where the arm travels all the way to the structure that the limit screw is mounted in. The prop change did increase the top speed at 6,000 ft density altitude by 3 kts as revealed by Van's tests reported in the RVator. The Governor limit screw adjustment allowed the rpm upper limit to be increased from 2600 rpm to 2720 rpm. I normally cruise at 2450 rpm but I would like to get the additional rpm for racing that is within the "no action required" rpm limit or upper rpm margin (for the prop it is a percentage above the aircraft manufacturer's specification as I recall - 2700 will be my base number). I have not had the time to research this yet but it is a "to do" item in the future.

Bob Axsom
 
I'm not brave enough to do the FPS via tie-down / full power runs.
Click to expand...

FWIW, this rig seemed to work well....

Took a look at the tailwheel spring and rear bulkhead structure, and decided against tying to the tailwheel. First task was firing up a blower and cleaning the asphalt in front of my hangar....no rocks or grit. I have a very big textile strap with loop eye ends, formerly the ballistic parachute bridle for an Eastern Bloc twin-engine lightplane. Formed a noose at each end and cinched them at the top of the gear legs, then ran a large double rope from the bight to my truck. Good solid attach point and much higher than the wheels, the usual noseover pivot. For insurance I grabbed two good buddies and positioned them in front of the stabilizer to keep some meat on the tail. They said it was very windy ;)

Back to the prop. Great comments, more please!
 
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Hey Dan,
Were you only seeing the 2800 during throttle up doing T&G's, or were you also seeing 2800 during a normal TO? I found my Hartzell speed control to be a little unstable during the first couple hrs of initial ops, then it settled in and has been rock stable.
I also had to screw in my low pitch stop screw to limit static / T.O. to 2700. All said and done, I was getting 2690 static. The first couple flights I saw it go to 2710 during climbout, but now it's turning ~2680 during climbout and has remained stable. Airborne, it takes about 1 turn on the Prop knob to get the governor to kick in.
 
rv9av8tr said:
Hey Dan,
Were you only seeing the 2800 during throttle up doing T&G's, or were you also seeing 2800 during a normal TO?.
Click to expand...

Good question. Saturday's two departures from rest showed around 2800, which is why I was adjusting the pitch stop on Sunday. The subsequent departure was in the low 2700 range. Exactly what I frankly don't recall; third flight, so right after liftoff I'm mentally busy as a one-armed paperhanger. The 2800 range showed up later after a touch and go.

Just dug out my A&P textbook; I need to read up on prop governing. Need to read up on GRT data logging too.
 
Thanks Sid, I talked to Bill.

Had some time to read and think yesterday, hopefully more today. Ya'll check me on theory.

The low pitch stop on the prop is just that....a mechanical limit stop for fine pitch, not RPM. Take away the oil system and it is really the same thing as a ground adjustable fixed pitch prop.

With a ground adjustable prop we set pitch for a particular static RPM and expect the inflight RPM to be higher. How much higher is airspeed dependent; a static setting (say 2600) will result in a particular inflight RPM (say 2750) only at one particular airspeed. Lower airspeed will result in a lower RPM, higher airspeed will result in higher RPM.

Ok, now re-incorporate the oil system and governor. In the lower airspeed example governor action can't increase RPM; the low pitch stop prevents blade rotation to a finer pitch. In the higher airspeed example the low pitch stop does nothing. We depend on the governor to increase blade angle.

To my wee brain it appears the low pitch stop screw merely sets the pitch to which the system will revert in the event of pressure loss. To limit inflight RPM with increasing speed we need more pitch, which has nothing to do with the hub pitch stop. I need to adjust the governor itself, which may be as simple as tweaking the stop screw at the arm.....dunno, not that far yet.

Education and recreation, right?
 
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Adjust it just a tad...

Dan, at a time. On our IO-520 Conti's, it only took like half a turn of the stop screw on the governor for 50 revs.

Best,
 
Dan, you are right on. Sorry I didn't see this earlier. Just adjust your low pitch stop on the propeller for 2700 RPM STATIC. Basically, you are adjusting a fixed pitch propeller for max RPM. Now, what will happen as you begin your takeoff roll, the propller will "unload" and increase in RPM if you did not have a governor. But, you DO have a governor, so it usually takes a couple of tries to get the governor set such that it starts governing at 2700 RPM. Just keep making small adjustments as Pierre says, and you should have it in short order.

Vic
 
If it tends to over speed on test flight set the prop control to see 2700 RPM on the tach and leave it there and land. Then screw the governor stop screw in until it hits the arm. Re-test and fine tune if necessary. If it under speeds on the test flight turn the governor stop screw out about 1 turn for each 25 RPM you need. Retest and fine tune as necessary. Normal governor adjustment on any governor I ever worked on was approximately 1 turn for 25 RPM.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
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