confused about RPMs

[n57flyguy]

while my brothers were telling me I'm wasting all my money on flight training and saying that they could take off and land an airplane if *they had to*, the subject of RPMs came up. How many RPMs does the prop spin at, like, 9,000? no, about 2,500, give or take depending on aircraft and I guess its HP. They didn't believe it, but oh well I tried. So I was thinking my self, why don't props spin faster than 2500, or why can't you exceed that? what does HP have to do with it (I saw that a new G-36 with 300 hp redlines at only 2,700)? Simply, why are they important when your flying the pattern? my assumtion is that it will tell you the rate of climb descent in relation to the amount of throttle you add. I may be confusing terms here, but I really am confused about RPMs. OH, I havn't been able to talk to my instructor about it yet, and I fly a PA-38 which has something like 106 hp. Thanks for your help!

[ananoman]

The reason the prop spins at a fairly low RPM is the tips of the prop have to stay subsonic. If they exceed the speed of sound, shockwaves form that create lots of excess noise and the disturbed airflow will result in a loss of efficiency.

It is usually better to go to a larger prop if you have a more powerful engine, or to add blades. Many of the turboprops have a fairly low cruise RPM, as low as 1200 rpm.

There are some piston engines that have reduction gears, so the engine turns faster than the prop. Most of the big radials were this way. For the later opposed engines they have been less popular, although quite a few big Cessna twins and the Beech Duke had geared engines. Probably the most common one now is the Rotax that is used in light sport and homebuilt planes. They run at something like 5000-6000 rpm.

The problem with the geared engines, is that they usually had more maintenance problems. The radials were very durable, but the geared opposed engines usually don't have the greatest reputations for longevity.

For your airplane, RPM has a direct correlation to power. If you want more power, you open the throttle and the RPMs increase. As you learn to fly, you will learn to set certain RPMs in certain situations, like slowing down to enter the pattern, or to descend on base and final to land.

In the more complex airplanes that have a constant speed prop, there is an additional lever on the throttle quadrant to control the RPM of the prop. So you still set power with the throttle, but you have to look at a Manifold Pressure Gauge (MAP) to set power. The prop governor controls the RPMs, so within limits, you can increase or decrease power, but the RPMs will remain constant. The governor just changes the angle of the blades on the prop with power changes.

If you search around on here, you will find some info on this. You should also go to avweb.com and go to the columns section. Look for articles by John Deakin. He has written quite a bit about this.

[meritflyer]

A compromise of RPMs has to be met. The tip of the prop will have a less of an angle of attack or "bite" of air than the inner portion will as a result of the tip traveling a greater distance hence having a greater resultant velocity. This way a prop blade maintain efficiency along the entire blade and not just one portion. If the prop were to spin to fast and exceed the speed of sound, the shock waves would compromise prop efficiency.

[falconvalley]

Sounds like your brothers are jealous, dude. You'll get that from time to time.

[meritflyer]

N57, just tell them about the GV ILS, hand flown, down to mins, in icing conditions.

It'll convince them.

Okay, I'll stop with the GV thing now.

[n57flyguy]

ananoman, Thanks alot for your response, that is what I was looking for. I'll check out avweb.com to. I think I have more questions that I can't think of right now, I'll post later.




Meritflyer, you are of no help to me because I know your not a pilot and want to have a sleepover in my treehouse. My mom isnt sure yet.

[n57flyguy]

Originally Posted by falconvalley

Sounds like your brothers are jealous, dude. You'll get that from time to time.

Not really, one of my brothers thinks he knows everything and you can't even argue with it. He thinks that by my dad and I paying for flying lessons, I'm wasting all of my dads money so he'll have to pay off his WHOLE car payment. I've played the "it is a career investment" card but the doesn't want to hear it. It gets old. Now I want to fly gliders next Summer, something my dad will support (because he flew gliders). My dad is very supportive about my flight training and wants me to become a pilot, as he works for Boeing. I just need a job, hopfully be this Spring.

[LookoutLoop]

Go dad!