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semi permanent RPM limitation at prop lever

KayS

Well Known Member
Hi All, have an RV7 with IO-375/Vetterman 4-2 crossover and Hartzell BA 72" CS in phase 1 and think to limit the forward travel of the blue knob to reduce the max RPM to let's say 2400. maybe sounds idiotic but let me explain...

we have some strict noise regulations in germany (people seem to be more noise sensitive than in north america). as i understand there are three noise categories when it comes to registration of GA aircraft... bad, medium and ok. i'm probably in the catastrophic category. i love my setup but she's loud. something will happen on the exhaust with mufflers but that will likely not be enough.

lately i made a takeoff with 2400 RPM, noticeable difference but still runs and climbs like crazy. with 2400 she's still overpowered and i'm ok with running a bit oversquare as lycoming doesn't state MAP limitations at specific RPM for the -M1S (which is an M1B with cold air sump) and the guys with fixed pitch setup for cruise seem to be happy.

some limit the max RPM directly at the governor... i will not do that. in the air force i was a technician for the tornado. i remember the pilot was able to move the thrust levers freely to "max reheat", but there was an further stage (combat) with even more afterburner. that combat mode had to be avoided as it was very bad for the engines/taxpayer, not so important if the MiG is chasing you.

there was some kind of a seal at the thrust levers that the pilot could push over to reach combat and i am thinking to do something similar at the blue knob. a latch or something that is semi permanent and one could apply max RPM quickly if it's really needed. I have the standard Vans blue vernier cable between MAP and mixture.

any ideas how to do that?

Best Regards
Kay
 
I remember my uncle talking about throttle limiting devices that could be broken when the need presented itself. Maybe a thin wire that could be broken with hard push?? Same principle for the governor. And I agree with your idea. You need everything available if you get in a bind.
 
I remember my uncle talking about throttle limiting devices that could be broken when the need presented itself. Maybe a thin wire that could be broken with hard push?? Same principle for the governor. And I agree with your idea. You need everything available if you get in a bind.

War Emergency Power (WEP).

There was a stop on the throttle and a warning mark on the manifold pressure gauge that warned the pilot not to exceed those settings, as well as a specification in the manual for the longest permitted time in War Emergency Power setting. To engage this setting, the pilot either had to manipulate the throttle lever in an unusual direction (pull up, push sideways) or break an actual wire with a push strong enough not to be done by accident, in order to bypass the stop. In aircraft where it was used, the wire served to notify ground crews that WEP had been used.
 
Mickey: thank you but i contacted Klaus already and going to install his mufflers. I would like to focus this thread on the RPM limits.
 
Take a look at the sliding travel limiter on almost any small ag tractor's three-point lift arm control lever.
 
First thought: https://www.mcmaster.com/detent-springs/press-fit-ball-nose-spring-plungers-6/

Build a bracket on the engine side that holds this perpendicular to the prop governor arm. The bracket should be designed so that the arm hits the ball at 2400 rpm. You can then push through the ball to get full rpm. They have different spring strengths so you can play with break-through forces. There's probably different physical packaging of this concept, so look around for some sort of spring detent that could be easily adapted.

You could also put a spring plunger inline with the control arm. Once you hit the plunger the force will increase, but it won't be a hard stop. You can adjust how much extra force it needed by adjusting the spring in the plunger.
 
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I think the trouble with your idea is that it will change with altitude and or density altitude. Just manually set it for each take off and save yourself some trouble.
 
Want low tech?

Set the prop control at 2400 RPM while in flight and leave it there through landing and shutdown. Make a removable stop block to match the exposed control rod length behind the back of the knob. Tether this to the panel. You be able to replicate the 2400 RPM control knob setting anytime you like, and not have to bother with anything to "push through" if you find yourself needing full power.

Cheers, David
RV-6A KBTF
 
I think the trouble with your idea is that it will change with altitude and or density altitude. Just manually set it for each take off and save yourself some trouble.

Not true. The prop governor directly senses RPM. One position-stop on the prop control should limit rpm to, in this case, 2400, regardless of altitude or temperature. The trick is to make the stop (wire, whatever) strong enough that you feel it during normal prop operation, but weak enough that you can push thru it in an emergency.
 
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Seems like a lot of effort for something that will likely never happen...what sort of "emergency" do you envision getting yourself into which requires this solution?

I'd just learn to set 2400 on my takeoff roll and be done with it.
 
If it were me, I would NOT limit rpm with any kind of stop. .if the airport is so noise sensitive, how about a technique similar to what the airlines doat KSNA.. they take off full powrr, climb steep, then by the airport boundary, you quiet it down with rpm and tiptoe away until 3000 feet or so many miles. .would that be an option? I just feel that partial power takeoffs in piston airplanes may not be the best way to go. .
 
several asked for what all that fuss is about... the noise certificate is part of the final registration process and there will be an official noise measuring event by the authorities. if it would be just me i would publish in the POH (review of POH is also part of that process) "limit RPM at takeoff and climb to x RPM" and then pull the prop lever to a somewhat lower RPM and just take off.

but the government requires indeed some sort of physical limitation of the RPM if you wanna go that route. some did that directly at the govenor so you could never apply full power... something that is complete nonsense in my view.

so i try to find an intermediate way... a mechanical limitation that the governmental officials are fine with, but that would still give you the option to apply full power if needed. for example if you are deep in that alps valley and you have to get out with a steep climb. i live around 3 miles away from the alps and did some flying in these mountains, you can never have enough excess power when you fly in the alps. i guess people flying in the rockies agree with that.

i talked with the guy at the LBA (german FAA) about that and he said physical limitation of RPM is required if you wanna lower the noise via RPM reduction. but it would also be great to find a neat solution for a semi permanant limitation. "why not?" he said.
 
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A shaft collar.

Full bolt-on for inspection, then get one you can loosen on the fly.

41lNWfbfOyL._AC_SY200_.jpg


s-l225.jpg
 
I don't think so Tim.

I think the trouble with your idea is that it will change with altitude and or density altitude. Just manually set it for each take off and save yourself some trouble.

Please say exactly why you think this will happen.

It is a governor, flyweights, RPM, it never knows the altitude. Oil pressure does the work and there is nearly always excess pressure so it won't change with air density.

The pitch will change, and only not work when it hits the pitch stop, that happens with speed. He might have that issue in Leadville but it sounds like the issue is at noise sensitive airports.

How about a collar on the shaft of the blue knob . . . would it be repeatable? A friend and I use a spacer for ground ops in warm weather for the MIX. (another story). Just a short aluminum bar on the mix shaft. Now, if it is repeatable, then a cylinder with a slot and a push pin to physically limit. This has not been tried, and may not be acceptable, and needs some development, but it is simple and compact and removable for emergency or high altitude airport takeoff. ??
 
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I have a RV-4 with a 200+ hp IO360 and CS prop. I normally take off at 2,500 rpm and 25". I put a mark on the throttle quadrant for 2,500 rpm on the prop and get 25" as I open the throttle. Simple is better.
 
Set the prop control at 2400 RPM while in flight and leave it there through landing and shutdown. Make a removable stop block to match the exposed control rod length behind the back of the knob. Tether this to the panel. You be able to replicate the 2400 RPM control knob setting anytime you like, and not have to bother with anything to "push through" if you find yourself needing full power.

Cheers, David
RV-6A KBTF

+1

David and BIll saved me typing. A bit of rubber or foam inside the spacer should help it from falling off.
 
The prop is constantly changing pitch to maintain the set rpm. If you are at sea level and set the pitch for a set rpm it will be different pitch at a higher altitude. High rpm at a finer pitch is much noisier than at a course pitch with the same rpm. That is why I said I didn't think a stop on the prop control was a good idea.
 
First, let me apologize for my very crude drawing. I was trying to do it in under 10 minutes to convey my thoughts..

How about some kind of spring that you simply push out of the way when you need more RPM? I was thinking something similar to a spring clip at the end of a pen.

One end would be sandwiched between the panel and the end of the control and the other end would nest around the rod at the base of the knob. The whole thing could be made adjustable if necessary by making it in two pieces with slots and small screws. Making the base of the clip like a “C” clip would allow you to install it without removing the control. This is just a thought.
 

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first: thank you all for the inputs! i really appreciate.

before christmas i made a short summary of your suggestions and presented it to the aviation authority and they liked the idea of a simple spacer between the knob and the bracket most. one that can be pulled downwards to apply max RPM. buddy of mine is already 3D-priniting one and we'll see how that works out.

Thanks
Kay
 
Hi Kay

Here is a photo of a T6 throttle quadrant which I am struggling to embed, you can see to the right of the black throttle ball is the slot that the throttle moves in, and on the right the metal liner is bent slightly inwards, it provides a "soft stop". You can feel when the throttle hits this stop and a little extra effort allows you to push through it. You should be able to make something similar for your aircraft using a strip of brass perhaps.

Gary

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no photo

Hi Kay

Here is a photo of a T6 throttle quadrant which I am struggling to embed, you can see to the right of the black throttle ball is the slot that the throttle moves in, and on the right the metal liner is bent slightly inwards, it provides a "soft stop". You can feel when the throttle hits this stop and a little extra effort allows you to push through it. You should be able to make something similar for your aircraft using a strip of brass perhaps.

Gary

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Hi Gary, the photo does not work for me. Perhaps try with the little paper clip icon above and attach the picture that way.
 
Here is an example of a detent on a lever, on RV-12. When you want full power you move the lever slightly to the right.

RV-12iS_Throttle.jpg



Note it is a lever and not a push-pull knob on a shaft.
 
Clothes Pin

I've heard of people using a clothes spin for this. Although 3D printed and labeled "pull for max RPM" looks much nicer.
 
Here is an example of a detent on a lever, on RV-12. When you want full power you move the lever slightly to the right.

RV-12iS_Throttle.jpg



Note it is a lever and not a push-pull knob on a shaft.

Why would you limit power on a 914? Lol
 
Hi Gary, the photo does not work for me. Perhaps try with the little paper clip icon above and attach the picture that way.

Trying again Mickey, thanks for the tip.
 

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