Max speed at 2700 rpm?

Steve Sampson · #1 02-02-2010, 05:48 AM

Could it be that max speed occurs with a c/s prop at a little less than max RPM in a practical world?

I am prompted to ask this by data off my AFS. I run a carbd O-320 with a 2-blade c/s MT prop. Clearly max power is everything forward, 2700rpm, but I do wonder if at 2650 it goes just a little faster. At first I discarded this because the Lyco curve shows the power dropping, but then I got to wonder if the props efficiency might rise more than enough to compensate. Also, there would be a slight MP rise with the slower pumping.

Anyone know this stuff?

noelf · #2 02-02-2010, 08:20 AM

oops...cant seem to post ...computer problem here.

penguin · #3 02-02-2010, 12:04 PM

Steve,

You're not the first to mention this, especially at altitude. I don't have any specific data, and I can't offer an explanation, but its a definite possibility.

Pete

N8RV · #4 02-02-2010, 12:46 PM

Steve, as a new C/S user, I'm eager to learn as much from others as I can about the proper use of my new prop.

As it was explained to me, pulling back the prop (lowering the RPM) is comparable to shifting a car to a higher gear, thus allowing higher speed. This is certainly noticeable on climbout when I pull the RPM back to cruise climb. And, conversely when slowing in the pattern, increasing the RPM slows the plane.

That's the elementary explanation I was offered. Thus, I'm interested to hear from those with more knowledge and experience what determines that point where, just like with a car, there's not enough power generated to maintain speed in high gear.

I know that the archives contain a lot of info on this subject, but since you started this new thread, I'll hang around and learn something ...

Steve Sampson · #5 02-02-2010, 01:00 PM

Don, well your explanation is right ...and wrong. It is like a car at the start of the takeoff run in that by being in a low gear/ fully fine it allows the engine to rev hard. The difference though is the revs in the car case are directly related to the speed for a given gear. In the aeroplane case the engine holds max rpm all the way for whatever the prop governor is set to. In the car you have to keep changing to higher and higher gears, in the plane the prop by holding say 2700rpm is continuously doing that for you. I hope that helps with understanding the difference / similarity. (Its a bit rough and ready!)

Nominally, at 2700 rpm and full throttle you are developing the max power the engine is capable of. In the car in top with your foot hard down you probably wont be at max rpm. Therefore not max power.

My query is puzzling if in fact right at the top end prop inefficiencies rise faster than the power. That would mean that leaving the throttle wide open, but just pulling the rpm back a little, say 50 to 2650, the loss of power is more than compensated by prop efficiency. I sense this to be possibly true at high speed, though not on takeoff. (Or is it the torque rises?)

Hopefully someone will step in who can explain.

Jumbo · #6 02-02-2010, 01:14 PM

Steven,

my friend Hans has a NSI car engine conversion (2.5 ltrs, 4 cylinder, see www.jodel.com) and I have a Subaru/Eggenfellner with a MTV-18 prop (3.0 ltrs, 6 cylinder, youtube ?PH-ERD? if you like).

We both see the same thing: ie. Hans? plane goes noticably faster actually when he reduces prop RPM after climb with the same MP. We think it goes togehter with with some specifics of the props used and so you can?t extrapolate to other props in general.

As well other MT users report very good efficency ot RPMs around 1900.

Sorry, nothing scientific really however some common observation.

Heinz

SvingenB · #7 02-02-2010, 01:56 PM

Just thinking out load. Even though your RPM and probably shaft power increases, this does not necessarily mean that thrust increases. The efficiency of the propeller should be very flat unless it is starting to go supersonic. It could be that there is no longer enough torque in the engine to both increase RPM and thrust at max velocity (due to propeller friction), only RPM at the expense of thrust by adjusting the pitch.

Tom Martin · #8 02-02-2010, 02:20 PM

It depends on the propellor. My MT prop did not produce much of a speed increase after 2600 rpm but the Hartzell that replaced it shows a noticeable increase from 2600 to 2700.

Bob Axsom · #9 02-02-2010, 05:08 PM

I have a Hartzell C/S prop and the faster I have turned it up through 2730 rpm the faster my RV-6A goes. If a prop is not optimized for an airplane and engine combination this may not be true. Propeller efficiency is unique to a propeller's design and, according to confidential charts provided by Hartzell to help me select the best blades for our RV-6A, the efficiency varies with KTAS. Testing reported by Van's in the RVator a few years ago showed the Hartzell BA C/S prop was faster than the other C/S props tested. It's a specialized art I believe.

Bob Axsom

noelf · #10 02-02-2010, 08:18 PM

...on 2-blade vs 3-blade props. I believe it was an answer by one of the Horton's (sorry, I don't remember which one), and in the answer was a link to an article about propeller efficiency.

Bottom line...it is possible for the prop vs air density vs rpm to reach a transonic region of air flow over the prop blades, and when this happens drag goes up and efficiency goes down. This is one possible explanation for a speed decrease at max engine rpm.