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RPMs at takeoff gradually increasing

vinzer

Well Known Member
Hello all!

As some of you know I recently finished building and have flown new to me RV10. So far everything looks good but there is one moment which I am little bit unsure about.
On takeoff roll when I set throttle full forward and while rolling my RPMs are 2550-2580. As soon as I take off and climb to 100-200ft RPMs gradually rise up to 2700 and stay there while limited by governor.
My engine is a standard IO-540 and my prop is Hartzell two blade model (standard one recommended by VANs).
Since I am getting RPMs at a right number (2700) and they stay there during climb I assume my governor does its job correctly limiting max RPMs. However why don't I have full 2700 when throttle is full forward on the roll ?
Governor which I have is old Woodward (155 model I think). It is set for 2700 and it works well in the air for all RPMs range (55% to 75% settings).
Governor test during runup goes well, RPMs drop when knob backwards is noticeable at 1900RPM or higher.

Do you think it may be a problem ? How your RV10 demonstrates RPMs at t/o roll ?
I used to fly Cherokee 235 with similar O-540 engine. Gross weight there was 2900lb and takeoff RPMs 2550 max for that engine. I assume since I am able to reach that it is already safe for takeoff but....
 
I suggest you watch the new Baseleg YouTube video on how to set prop RPM (at the prop and on the governor).

It sounds like your governor is restricting prop RPM on take off, and then the prop RPM limiter is taking over after takeoff to keep the prop below 2700.

Carl
 
I suspect it is just that even at the fine pitch stop, the engine doesn't have enough torque to pull the maximum RPM at low forward speeds.

I fly a 235, with a fixed pitch prop, and the engine max RPM is 2575, but the max static RPM is 2300, with a min static RPM of 2125. There isn't a max/min RPM for a constant speed in the TCDS, just a High and Low pitch setting.

My plane is in spec for the static RPM, and on takeoff roll it starts at about 2200RPM, and increases as the forward speed builds up.

Mark
 
THANK YOU VERY MUCH EVERYONE !
It pretty much explains what happens in my case.
I will readjust things properly then.
 
The governor has a hard stop adjustment for the amount of cable travel. Full RPM at static should be set around 2550. Then test fly it. On ground roll, since now the prop is getting clean air, it should hit 2700 RPMs. If it is too high or low on ground roll and climb out, you just make a fine adjustment to that stop. Make sure stop nut is tightened properly.
 
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During take of / approach, the prop is limited by the high/ low pitch stop in the hub. While in the air the governor is maintaining the set rpm.

Good luck

https://hartzellprop.com/MANUALS/115N-0000-A.pdf
https://www.aircraftsystemstech.com/p/propeller-governor.html

This^^^
You need to adjust the hard low pitch stop by pulling off the spinner, loosening the nut and adjusting the (Allan head, iirc) bolt on the very front of the prop hub. See the prop manual to get the direction and about how much of a turn you need on that bolt to bring the static rpm up to about 2680 or so.
 
THANK YOU VERY MUCH EVERYONE !
It pretty much explains what happens in my case.
I will readjust things properly then.

Vlad

You’re pretty close at 2550-2580, so don’t over do it. If you target 2650, you’ll be at 2700 before rotating for takeoff. At least you don’t have it the other way around where the governor is doing all the work and the low pitch stop is not involved. This will occur when the low pitch stop is set at too fine of a blade angle. In this condition you will often see and hear the RPM surge above 2700 when power is first applied during takeoff or a touch and go. It’s like nails on a chalk board to me when I hear it from and improperly set up airplane!

Listen to Bob Turner, watch the Base Leg video and read your prop manual and you’ll be set.

P.S. For those that have your low pitch stops set too low (opposite from Vlad), not only might you see and hear a prop surge, but you will be giving up some engine out glide performance due to the blades being too fine and the prop windmilling at a higher RPM.
 
All above are correct. You want to adjust the low pitch stops on the propeller to about 2650 static. That will give you more power on your roll and be close to 2700 at rotation.
 
Thank you to all of you for this valuable information.
I watched the video and it explains my exact problem.
Tomorrow I plan first release governor to over max RPMs, then adjust my Hartzell low pitch stops (got manual, learned how to do that), and then after having static 2650 limited by prop I will adjust my governor during next 1-2 flights so I will have proper setup. Shame on me I did not know all this before, I was under impression that the prop comes with pre set blades low pitch which can not be adjusted by my self and governor is the guy to take care of all rpm controls….
 
.... I was under impression that the prop comes with pre set blades low pitch....

Well, usually the factory does pre-set that stop, if they know the airframe/engine it's going onto.
Since you reported 2700 rpm in cruise it sounds like your governor is already set correctly. And as long as you plan to set the mechanical stops to 2650 there is no reason to touch the governor at all.
 
Well, usually the factory does pre-set that stop, if they know the airframe/engine it's going onto.
Since you reported 2700 rpm in cruise it sounds like your governor is already set correctly. And as long as you plan to set the mechanical stops to 2650 there is no reason to touch the governor at all.

Bob,
I am a bit unclear here then. So when I set full power on takeoff roll I see RPMs 2550-2580. Airplane takes off and in couple hundred feet of altitude I already have my 2700.

Now lets assume that my governor is correctly set, so then why on take off RPMs are not 2700 ? My impression is that my governor is set to lower than it should be and it in fact limits my RPMs on takeoff.

In regards to my low pitch stop on prop settings: assume prop is set below 2700 at this moment, for example it’s set to 2580. In that case I should never see 2700 because governor can not override prop low pitch stop as I understand ? In my understanding low pitch stops on prop are last resort to limit the prop speed in case if governor goes south. So what I think, after reading and watching video, is that my low propr pitch stops are set to some unknown position now, but definitely allowing 2700, while my governor likely set too low on RPMs and kicks in immediately on takeoff roll.
My plan is:

1. Release governor a bit to higher end of RPMs, test static run and see RPMs on the ground reaching say 2700
2. Adjust low pitch prop stops to set 2650 static RPM max
3. Do runup and see where prop lever kicks in, means see how far I should pull prop backwards before governor start to change pitch. Adjust governor limiter accordingly.
4. Fly airplane and test in the air.

After above I expect that prop low pitch setting will always ensure that no matter what prop will never exceed 2700. With the same time I expect on takeoff roll 2650 and gradually increase to 2700 on rotate.
In the air prop governor should work normally and allow adjust in standard range or RPMs.

Correct me if I am wrong please.
 
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Vlad,

Looks to me like your governor is set perfectly. I wouldn't touch it.

The low pitch stop is just a mechanical prop angle limit.

Your governor wants to give you 2700 RPM on takeoff, but can't get the blades to a low enough pitch because the low pitch stop is preventing it from doing so.

The mechanical low pitch stop will never ensure that no matter what prop will never exceed 2700.

As you accelerate on takeoff, at some point, the blades come off the low pitch stop and the governor is able to control the blade pitch and give you 2700 RPM. Then, the faster you go, the further away from the low pitch stop the governor positions the blades. It must keep increasing the blade angle as you go faster to meet the ever faster oncoming air. If the blades didn't come off the low pitch stop, the RPM would continue to increase past 2700.

My advice: do what Flyguy25 and BobTurner said. Adjust the low pitch stops on the propeller to about 2650 static and go fly.
 
Bob,
I am a bit unclear here then. So when I set full power on takeoff roll I see RPMs 2550-2580. Airplane takes off and in couple hundred feet of altitude I already have my 2700.

Now lets assume that my governor is correctly set, so then why on take off RPMs are not 2700 ? My impression is that my governor is set to lower than it should be and it in fact limits my RPMs on takeoff.

In regards to my low pitch stop on prop settings: assume prop is set below 2700 at this moment, for example it’s set to 2580. In that case I should never see 2700 because governor can not override prop low pitch stop as I understand ? In my understanding low pitch stops on prop are last resort to limit the prop speed in case if governor goes south. So what I think, after reading and watching video, is that my low propr pitch stops are set to some unknown position now, but definitely allowing 2700, while my governor likely set too low on RPMs and kicks in immediately on takeoff roll.
My plan is:

1. Release governor a bit to higher end of RPMs, test static run and see RPMs on the ground reaching say 2700
2. Adjust low pitch prop stops to set 2650 static RPM max
3. Do runup and see where prop lever kicks in, means see how far I should pull prop backwards before governor start to change pitch. Adjust governor limiter accordingly.
4. Fly airplane and test in the air.

After above I expect that prop low pitch setting will always ensure that no matter what prop will never exceed 2700. With the same time I expect on takeoff roll 2650 and gradually increase to 2700 on rotate.
In the air prop governor should work normally and allow adjust in standard range or RPMs.

Correct me if I am wrong please.

You aren’t understanding what the governor can do. Sounds like your setup is close and you shouldn’t do much, maybe decrease the low pitch stops a little. Let me see if my explanation makes sense. On the ground static, you should see 2650 rpm. That’s with the governor not kicking in yet. The low pitch stops on the prop don’t allow the blade angle to go any flatter, so static rpm is 2650. Then rolling down the runway, at 50 to 60 knots, the rpm increased because the air load is slightly reduced, just like a fixed pitch equipped plane sees an rpm rise in motion vs static. Then at 2700rpm, your governor starts to increase blade pitch to prevent the rpm from going any higher. Basically the governor is only increasing blade pitch at this point (2700+ rpm). The fact that you only get 2580 static means either you need to allow your prop pitch to go flatter, or you could be down on power.
 
Bob,
Correct me if I am wrong please.

Okay, I think you’ve lost sight of the main reason for using a constant speed prop. We tend to think of the prop as spinning really fast, but in fact the speed - especially near the hub - is very comparable or even less than the forward speed of the airplane. That means that as the plane moves forward faster and faster, the speed and angle of attack of the air over the blades changes. In particular, the angle of attack goes down. Remember flying a fixed pitch prop, on a 152, 172, Warrior, etc.? At the start of the takeoff roll you’d only get 2300 rpm at full throttle. By the time you were up to 70 kias, maybe 2400 rpm. If you leveled off down low, still full throttle, the rpm may have gone over red line. A CS prop is designed to avoid these issues. At the start of the takeoff, the governor will command ‘lower blade pitch’ (no oil pressure in the hub) but the prop hits the mechanical low pitch stop at 2650 rpm. So that’s what you get. As the plane gains forward speed, the angle of attack on the blade goes down, less drag, so the rpm’s go up - just like in a fixed pitch prop. But now, when the prop hits 2700 rpm, the governor kicks in, and increases the blade angle, as needed, to hold 2700 rpm. You might note that you could set the hard stop to 2700 rpm (static) and then the hard stop would do nothing in normal operation. It’s sole function is to keep the blades from going to zero degrees if oil pressure fails. Typically, if you reduce power until the rpms get below 2700 following a governor failure, you’ll have just enough power to limp home at 75 knots or so. So why set the stop to a bit less, like 2650? (1). Following a governor failure you can use a bit more throttle without going over red line. But mostly (2) it’s so you know the hard stop is set correctly, by checking full power rpm at the start of the takeoff. So adjust the bolt on the front of the hub, check with a full power runnup. If it’s over 2650 you went too far; less, you need to adjust more.
 
Oh, now I understand! So basically looks like my governor is set to more or less correct RPMs. With the same time it looks like my prop is set to lower then required RPMs (say 2520-2530). On the roll I am getting my 2580 and that’s all my engine can do with these blades angle and power given.
In flight RPMs go obviously higher because we move through the air and gets limited by governor at 2700. Ok that’s make sense now.
I will simply work on low pitch stops tomorrow without touching my governor settings at all and then test fly it to confirm the changes.
Thank you very much for very detailed explanations to all of you guys.
;)
 
Okay, I think you’ve lost sight of the main reason for using a constant speed prop. We tend to think of the prop as spinning really fast, but in fact the speed - especially near the hub - is very comparable or even less than the forward speed of the airplane. That means that as the plane moves forward faster and faster, the speed and angle of attack of the air over the blades changes. In particular, the angle of attack goes down. Remember flying a fixed pitch prop, on a 152, 172, Warrior, etc.? At the start of the takeoff roll you’d only get 2300 rpm at full throttle. By the time you were up to 70 kias, maybe 2400 rpm. If you leveled off down low, still full throttle, the rpm may have gone over red line. A CS prop is designed to avoid these issues. At the start of the takeoff, the governor will command ‘lower blade pitch’ (no oil pressure in the hub) but the prop hits the mechanical low pitch stop at 2650 rpm. So that’s what you get. As the plane gains forward speed, the angle of attack on the blade goes down, less drag, so the rpm’s go up - just like in a fixed pitch prop. But now, when the prop hits 2700 rpm, the governor kicks in, and increases the blade angle, as needed, to hold 2700 rpm. You might note that you could set the hard stop to 2700 rpm (static) and then the hard stop would do nothing in normal operation. It’s sole function is to keep the blades from going to zero degrees if oil pressure fails. Typically, if you reduce power until the rpms get below 2700 following a governor failure, you’ll have just enough power to limp home at 75 knots or so. So why set the stop to a bit less, like 2650? (1). Following a governor failure you can use a bit more throttle without going over red line. But mostly (2) it’s so you know the hard stop is set correctly, by checking full power rpm at the start of the takeoff. So adjust the bolt on the front of the hub, check with a full power runnup. If it’s over 2650 you went too far; less, you need to adjust more.

Very clear and simple explanation! I wish I know all this before!
Thank you Bob.
 
In regards to my low pitch stop on prop settings: assume prop is set below 2700 at this moment, for example it’s set to 2580. In that case I should never see 2700 because governor can not override prop low pitch stop as I understand ?

Bob in Post 17 explains it best so far.. here’s another way to think about it.. my RV-4 with a fixed pitch prop gets 2200 rpm static. So does that mean that 2200 is the most rpm I’ll ever see? No, WOT in flight, it can go to 2840rpm if I let it. That’s over a 600 rpm increase. Now if I had a way to add prop pitch, I could maintain 2700 or whatever setting I told my governor to hold.
 
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