Constant Speed Prop

[poadeleted20]

As to the 50*, that was 50*C . Really, how do you work out 100 rather than 50?

I think if you look at Lycoming's chart, you'll see that peak power is around 125 ROP.

 

[Henning]

I think if you look at Lycoming's chart, you'll see that peak power is around 125 ROP.

Thanks, interesting. Must be a very small fractional difference though because much of that extra fuel will be burning at a rather ineffectual stage of the power stroke, well past TDC. Look at the difference in the steepness of the specific fuel vs. power curve as well. 50 ROP gives you 97+% power with a nearly flat power curve to 100% at 125 ROP. Now look at the fuel curve. That same 50 ROP line translates down near the bottom of the specific fuel curve on a near mirror flat curve back to best economy cruise with a very sharp rise in fuel consumption towards the the 100%125ROP mark. Since drag increases by the cube of speed, at the top like this, 3% power makes much les speed difference than between 65% and 68% power. So now, since you and I are in equal planes and I'm operating 50*rop and you're operating at 125* and we are both maxed out, you will still be in my view when you have to peel off and land for fuel, I'll go a good bit further. So I still say 50* works out best when you gotta get there as quick as you can and you have any real distance to go. Besides, It'll save me lots of money.

 

[gismo]

As to the windmilling prop, It didn't do a thing for me in a Cardinal or a 210. At best glide neither engine was turning over 1700 RPM so the governor had nowhere to seek. Ever since I've reasoned for any next plane will have a feathering prop, preferably reversable. You get to like having that especially if you consider floats.

A reversible prop would be great, especially on a float plane but all the floatplanes I've flown (all 3 of them that is) didn't have one. As to pulling the prop control, the two singles I have the most time in are an E35 Bonanza and a Cardinal RG. Both responded to pulling the prop control with a noticeable increase in glide ratio. Perhaps this only applies to clean airframes that typically have higher speeds for max L/D.

As to the 50*, that was 50*C . Really, how do you work out 100 rather than 50?

Nearly every reference to a "Best Power" mixture I've seen says this is at 80-100°F ROP and most say 100. 50°C would be 90°F so we aren't very far off if you meant 50C.

 

[Henning]

Nearly every reference to a "Best Power" mixture I've seen says this is at 80-100°F ROP and most say 100. 50°C would be 90°F so we aren't very far off if you meant 50C.

Ahhh...best power doesn't necessarily get you there quicker. Look at the graph from Lycoming that Ron posted on the other branch to this post. You gain very little while spending a much greater quantity of fuel. Fuel stops take a minimum of 35 minutes, that's small airport, fuel truck/bowser waiting for you and you can just sign for the fuel. I used to Use Alamosa and Coffey Co Kansas like that. Everyplace else lost me about an hour. I can be at the destination before you're wheels up again. But then, you probably run LOP for this very reason.

 

[poadeleted20]

Thanks, interesting. Must be a very small fractional difference though because much of that extra fuel will be burning at a rather ineffectual stage of the power stroke, well past TDC. Look at the difference in the steepness of the specific fuel vs. power curve as well. 50 ROP gives you 97+% power with a nearly flat power curve to 100% at 125 ROP. Now look at the fuel curve. That same 50 ROP line translates down near the bottom of the specific fuel curve on a near mirror flat curve back to best economy cruise with a very sharp rise in fuel consumption towards the the 100%125ROP mark. Since drag increases by the cube of speed, at the top like this, 3% power makes much les speed difference than between 65% and 68% power. So now, since you and I are in equal planes and I'm operating 50*rop and you're operating at 125* and we are both maxed out, you will still be in my view when you have to peel off and land for fuel, I'll go a good bit further. So I still say 50* works out best when you gotta get there as quick as you can and you have any real distance to go. Besides, It'll save me lots of money.

All true, which is why I fly at peak EGT, and will go even farther than you if you fly at 50 ROP (assuming I don't have to land to hit the head first), but "peak power" is the 100% point, and according to Lycoming, that's 125 ROP. In addition, you'll see that peak CHT occurs about 75 ROP, and that curve is pretty steep. In many planes (including my Tiger), cooling is always an issue, so running at 50 ROP may get you there faster, but "there" may be a valve guide replacement or an overhaul as well as your destnation.

 

[Anthony]

All true, which is why I fly at peak EGT, and will go even farther than you if you fly at 50 ROP (assuming I don't have to land to hit the head first), but "peak power" is the 100% point, and according to Lycoming, that's 125 ROP. In addition, you'll see that peak CHT occurs about 75 ROP, and that curve is pretty steep. In many planes (including my Tiger), cooling is always an issue, so running at 50 ROP may get you there faster, but "there" may be a valve guide replacement or an overhaul as well as your destnation.

Ron. Wouldn't CHT's also peak at peak EGT? I find I have to run ROP to keep my Tiger's CHT's down. Essentially I lean using CHT's not EGT's. (four probe EDM 700)

 

[wsuffa]

For me, it's a delicate balance between cylinder EGT, TIT, and CHT. Running peak TIT puts me really close to redline (at power much above 60%) and often has one cylinder excessively hot on CHT. So, depending on altitude and power, I might well have to run ROP.

 

[gismo]

Ron. Wouldn't CHT's also peak at peak EGT?

Nope. Peak EGT has more to do with burn rates (slower means hotter end gasses coming out of the cylinder as the exhaust opens) than power production, and cylinder heat is rather proportional to power production. And if you think about it, for a given power output, the most efficient mixture should result in the least cylinder heat since extra cyl head heat (for the same actual power output) indicates less efficiency.

 

[nddons]

Wow - all clear as mud!

I start my HP and Complex tomorrow in a new (to the flight school) 182RG.

 

[denverpilot]

LOL! Someone revived this old thread. What's amazing to me is that for a FIRST FLIGHT question, no one mentioned that a properly written CHECKLIST will keep you out of trouble in this regard until you have more time/experience setting power. Shove them both all the way in for takeoff, prop always goes forward first, and the CFI and the checklist will keep you out of trouble from there until you get your head wrapped around it.

 

[Mike Boehler]

Wow - all clear as mud!

I start my HP and Complex tomorrow in a new (to the flight school) 182RG.

I may be the company simpleton, but it seems as though this discussion should have begun with "what's connected to the blue knob and what's connected to the black knob" and progress from there, five years ago

"clear as mud" is a clear indication that most of us need to start from the beginning, and avoid the "set you power here" or "set your blue knob here", we hear charlie browns teacher talking

 

[dmccormack]

Plain and simple, the higher the manifold pressure and the lopwer the RPM, the more efficient you get. It's how Lindbergh crossed the Atlantic which he related to Chenaults Flying Tigers in Burma to get them more mission range and fighting fuel on target. It was then adopted by the entire military aviation sector.

I don't have Spirit of St Louis or Lindbergh handy (two recent reads) but I don't remember a high MAP, low RPM approach -- IIRC a moderate power, low RPM (1750 RPM) with speed just above L/D max and low altitude (e.g. dense air) for most of the trip.

Interesting documents from Donald Hall (Designer): http://www.charleslindbergh.com/images/plane/0010.gif

 

[mikea]

Henning these two small excerpts from your otherwise factual and informative post are a bit in error.

First, pulling back the {prop} handle during an engine out does make a difference on every single I've tried it on. It itsn't that you are significantly reducing the RPM, but rather that you are getting the prop off the fine pitch stops that decreases the drag of the windmilling prop enough to almost double your glide range on a slippery airframe...

As to the windmilling prop, It didn't do a thing for me in a Cardinal or a 210. ...?

... As to pulling the prop control, the two singles I have the most time in are an E35 Bonanza and a Cardinal RG. Both responded to pulling the prop control with a noticeable increase in glide ratio. Perhaps this only applies to clean airframes that typically have higher speeds for max L/D...

I'm with Lance. It does make a difference. Pulling the prop back at idle for simulated engine out in my Cherokee 235 makes enough difference that you'd swear the plane accelerated, and with the Hershey Bar wing and fat gear stuck out it's anything but a "clean airframe." I'll pull it back to get as far as I can as it drops like a brick.

 

[nddons]

Wow - all clear as mud!

I start my HP and Complex tomorrow in a new (to the flight school) 182RG.

OK, THAT was fun! The 182RG can scoot once it leaves the ground. Granted it wasn't a windy or gusty day, but that thing is really stable. In cruise at about 145-155 kts it just didn't move. I felt like I could keep the ball in its cage like a pro today. For some reason steep turns seemed easier than in a 172. Slow flight required good rudder inputs, but otherwise was effortless.

The first landing was a bounce, and I had trouble with my sight picture. However, on subsequent landings once I started looking longer down the runway and grasping the seemingly higher than usual nose attitude, landings were just fine.

And the retractable gear is just plain old fun. Poor bastards flying Cirri don't know what they're missing!

I think I'm going to like that bird.

 

[Greg Bockelman]

And the retractable gear is just plain old fun. Poor bastards flying Cirri don't know what they're missing!

LOL. Fun until annual time. Or Heaven forbid, you gear it up.

 

[nddons]

LOL. Fun until annual time. Or Heaven forbid, you gear it up.

Yea, I know. I'm pretty brave in a rental with a CFI sitting next to me!

 

[denverpilot]

OK, THAT was fun! The 182RG can scoot...

I think I'm going to like that bird.

Ahh, another satisfied 182 SUV-of-the-Sky driver!

The 182 isn't "perfect" at anything, but it fits the "all mission" role pretty nicely! If you drive big vehicles like I do (Yukon), the 182 feels, like Goldilocks said... "Just right!"

Not sure I'd want to pay the money to tuck the gear up unless it had a turbo out West here though. Too much stuff to pay for that's wearing out under there after 30 years. Our straight-legged girl is going to cost us $7000 or so in MX with the avionics work and the bladder failure, minimum, this year.

Only way I'd buy a retract is with partners who all had deep pockets. And it's more likely I'd want to go up to the 210 in that case.

 

[Doggtyred]

Ron. Wouldn't CHT's also peak at peak EGT? I find I have to run ROP to keep my Tiger's CHT's down. Essentially I lean using CHT's not EGT's. (four probe EDM 700)

Nope.. not necessarily. Deakin had some actual data on this very thing. The two temps peaked separately.