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Thread: 1500 rpm why?

  1. #1

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    Default 1500 rpm why?

    Hello,

    so here’s my dumb question of the day and thought I would post to see if anyone knows why. I was thinking about landings and watched some Eaa videos this week in lieu of airventure. I was thinking about how did/do test pilots arrive at standard settings for a new airplane. I thought a standard benchmark for final approach speed was something like 1.3 Vso. So how do they figure out rpm? For just about every ga airplane I have flown in with lots of different engines the answer is 1500 rpm (cub, pacer, champ, 172, Cherokee) Anyone know why? What is 1500 supposed to achieve? 3 degree glide slope? 500 vsi? I was just curious if anyone know where this standard comes from. Just coincidence?

    before someone else says it, at the end of the day of the day, the correct power setting is the one that gets you to the runway. Just curious what’s so magical about 1500 as a standard recommendation for pretty much everything I have ever flown?

  2. #2
    JPerkins's Avatar
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    Default Re: 1500 rpm why?

    Its 1700 for a standard approach in 31P. 1500 gives a pretty steep decent. Full idle, drop a rock out the window and follow its trajectory.
    I think you'll find that all these planes fly at relatively the same speed with a fixed pitch propeller. So they're much more similar than different in that aspect, making approach rpm speeds similar.

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    Jason Perkins

  3. #3
    Administrator Steve Pierce's Avatar
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    Default Re: 1500 rpm why?

    I think I use 1800 in the Clipper, will have to look in the Super Cub, I guess it depens on what kind of approach I am doing.

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    Default 1500 rpm why?

    Quote Originally Posted by Steve Pierce View Post
    I think I use 1800 in the Clipper, will have to look in the Super Cub, I guess it depens on what kind of approach I am doing.
    Thanks. At the end of the day, I think folks will sensibly say you end up using whatever gets us comfortably to the runway. Winds, density altitude, loading all result in adjustments. As a student pilot, I would apply the recipe a cfi gave to me blindly and conditions would favor it working out well or not. Like most, I now just look at the runway and adjust the throttle as needed to get there and don’t really know what the setting is in most cases.

    But I was just thinking about how if you ask someone for their landing “recipe” more often than not the answer for the typical ga airplane seems to be power to 1500, pitch for something (usually 1.3 vso) and some flap setting.

    I was just curious if that works out to be something like 500 vsi, or 3 degree glide slope, a max efficiency point on some curve, so I can be more analytical in my approach to flying. I can apply an input but what am I trying to achieve with the power setting. I guess I am going though my “teen” years as a pilot and asking why?


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    Administrator Steve Pierce's Avatar
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    Default Re: 1500 rpm why?

    I always used the 1800 rpm and 70 mph describing landing the Clipper. Have a good baseline I believe. I know our Tri-Pacer went from 1500 rpm on landing to 1200 rpm when we installed the experimental Dakota Cub slotted wings which told me a lot of what the squared off slotted wing was doing over the stock wing.

  6. #6
    Gilbert Pierce's Avatar
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    Default Re: 1500 rpm why?

    In my 160 Clipper with a climb prop I use 1800 on down wind, that slows me to 85mph, 1650 on base and 1400 after turning final. That gives me 70mph. On short final I pull all the power until just before the stall then I little power to soften the arrival. All of that is just my starting point which I modify for varying conditions. Most of it just plain feel for hitting a spot on the runway at between 55 to 60 mph. I really don’t look at the airspeed after the base to final turn.
    Knowing the rpm vs airspeed makes my landing more consistent under varying conditions and unfamiliar airports.

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    Default 1500 rpm why?

    Thanks for the comments. I was just wondering if back in the 30s 40s if there was some science being applied that explains how we arrived at the benchmarks we use today. The other approach is empirical and it boils down to “this is what works for me.” But the issue I have seen is that those “recipes” tend to be optimized for someone else’s plane, in the weather They tend to fly, with the mods they have, and what they have grown comfortable with.

    When I was new to my pacer I read lots of folks “recipes” of how they land and it is a little exaggeration but the recipes ranged from power off to 1800, 60-80 kts, and zero to full flaps. My pacer with vgs, my washout, lightly loaded, with wingtips flies a lot different than the pa-20 I flew a few years back without vgs, no wingtips, 135 hp, with Bush wheels, and heavily loaded. Using what I was taught in that plane would leave me skipping down the runway like a Pitts and doing what I do now in that plane would probably put the gear through the floorboards.

    Why am I bothering you all? As I fly my pacer more folks are starting to come to me and say help me learn to fly my short wing but it’s a different plane and I am thinking to myself how do I do that? Blindly use the recipe that works for me? Assume it’s more like Gilbert’s or Someone else’s plane and try for the first time a recipe someone else has suggested that I haven’t tried before and I know wouldn’t work well in mine.

    So I was thinking how could I go up test a new to me plane and get into a reasonable ballpark.

    So for air speed, I think I had heard 1.3vso is a good ballpark. That seems to be true in my experience, and makes some Analytical sense the goal is to reduce energy but provide a cushion from stall. So I can take the plane up and stall it in different flap conditions and see where it’s stalling on that day with the loading I have. Multiply those numbers by 1.3 and I have a ballpark speed/pitch to hit.

    Next I was thinking of the rpm. 1500 is a general ball park but what am I optimizing? In my “testing scenario” would it be a good idea to set my 1.3 vso speed and play with power settings to get to what? 500,400,300 vsi? Pitch relative to horizon? I was thinking there may be some old naca studies that defined an optimum power setting. Those guys seemed to have math for just about everything.

    Then again maybe I am just nuts?


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    Last edited by Tnathan; 07-28-2020 at 10:14 AM.

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    Administrator Steve Pierce's Avatar
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    Default Re: 1500 rpm why?

    If I was going to teach someone in their Short Wing I would go up and fly it and figure the nuances of that particular airplane. I have flown a lot of different Short Wings and fly a bunch of different Super Cubs, Carbon Cubs etc and they all have different tendencies. The airspeed indicators are all over the place for one. I would get some time in theirs and go from there. I fly site picture and pitch but I know that is hard to teach so then you have to start looking at rpm, air speed etc.

  9. #9
    Gilbert Pierce's Avatar
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    Default Re: 1500 rpm why?

    I arrived at my numbers by flying the airplane. I would never trust the airspeed indicator in a Shortwing.

    I believe you are over analyzing the issue. Fly the airplane, stall it, fly a comfortable approach and go from there. At 1.3 VSO you will have a pretty good decent rate in my airplane. All my landings are no flaps.

    I flew a Pacer that several people said was a fast airplane. I asked them if they stalled it “no”. I flew it and it stalled at 70 mph indicated. My Clipper stalls at 52 mph INDICATED. That is my starting reference point.
    Last edited by Gilbert Pierce; 07-28-2020 at 11:08 AM.

  10. #10

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    Default Re: 1500 rpm why?

    I dont like to bring my engine to 1500 ever until I need to, like short final. I like to cool it slowly., I am not a mechanic but to me a gradual reduction in power from cruise to approach power is better than right back to 1500, thats almost idle. I have never heard somebody say 1500 you keep saying. 1.3 is simple math formula for v ref in bigger planes, but power to maintain is whatever it takes at the weight you are at.

    My goal is reduce to 1700 to 1800 rpm a few miles from pattern so I give the engine time to cool slowly, again not sure if this is good or bad for the engine, but is is what I do, than abeam the numbers I bring it back to whatever keeps a 500 fpm descent and on glidepath. No flaps will be 1500 or so for me, but flaps will add drag and require more power.

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