PDA

View Full Version : 1500 rpm why?



Tnathan
07-27-2020, 06:42 PM
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?

JPerkins
07-28-2020, 06:20 AM
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.

Sent from my SM-G930V using ShortWingPipers.Org mobile app (http://r.tapatalk.com/byo?rid=95463)

Steve Pierce
07-28-2020, 06:51 AM
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.

Tnathan
07-28-2020, 08:33 AM
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?


Sent from my iPhone using Tapatalk

Steve Pierce
07-28-2020, 08:37 AM
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.

Gilbert Pierce
07-28-2020, 08:54 AM
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.

Tnathan
07-28-2020, 10:09 AM
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?


Sent from my iPhone using Tapatalk

Steve Pierce
07-28-2020, 10:33 AM
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.

Gilbert Pierce
07-28-2020, 10:54 AM
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.

SMO22
07-28-2020, 12:44 PM
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.

sierrasplitter
07-28-2020, 02:45 PM
Sounds like my Technic is completely wrong, or completely different. When I have the numbers in my strut V on downwind I cut power. Completely. Put in One notch of flaps and set trim at 80 . One more notch of flaps on base brings me to 70 . By the time Im over the fence Im at 60-65 . Then I try to make my spot without adding any power. Sometimes I may have to kick it back up to 1200. Full Flaps and sometimes I even have to slip at that .

thebeerdedpilot
07-28-2020, 03:25 PM
I think it's just an easy number to remember that can be applied to multiple small GA aircraft. But as with anything, you just give it more or less as needed. Personally, I chop the power when abeam my landing point and glide her in. That seems to work the best for me. But there are times when I need to give her some throttle to hit my mark. I find that all the tailwheel airplanes I've flown (Pacer, Cub, Stearman) are done mostly by feel rather than hitting specific RPMs and speeds like a C172 or Warrior.

Tnathan
07-28-2020, 03:54 PM
Sounds like my Technic is completely wrong, or completely different. When I have the numbers in my strut V on downwind I cut power. Completely. Put in One notch of flaps and set trim at 80 . One more notch of flaps on base brings me to 70 . By the time Im over the fence Im at 60-65 . Then I try to make my spot without adding any power. Sometimes I may have to kick it back up to 1200. Full Flaps and sometimes I even have to slip at that .


I wouldn’t say wrong. If it is working, it is “right” for you. And if it ain’t broken don’t fix it.

I am an engineer, so it’s a fair criticism that I may be overthinking it. It just gives me something to do when I go out to try and improve rather than blindly rely on some recipe someone told me. Thanks to everyone for your comments.

Gilbert Pierce
07-28-2020, 05:54 PM
Sounds like my Technic is completely wrong, or completely different. When I have the numbers in my strut V on downwind I cut power. Completely. Put in One notch of flaps and set trim at 80 . One more notch of flaps on base brings me to 70 . By the time Im over the fence Im at 60-65 . Then I try to make my spot without adding any power. Sometimes I may have to kick it back up to 1200. Full Flaps and sometimes I even have to slip at that .
If it works for you, you don’t have it wrong. Stick with what works.

ClippedWing
07-28-2020, 06:13 PM
I agree, Not wrong Ray.

Each new airplane, I take it up and stall it in a few configurations, especially landing, with power off to find out what she does and at what indicated speed so I know what to expect during landing.

And I’ve flown a few and they’re all different! My plane stalls with almost no indicated airspeed with power on.

One thing that I maintain in common is that I try not to ever put the aircraft in position where it won’t make the runway if the engine quits at ANY point. That means, long, extended downwinds, or even a stabilized 3° glide slope is not a normal approach for me in a Pacer.

All that being said, that’s a matter of preference after 37 years of flying, so your mileage may vary. And certainly there are circumstances where a flatter approach can’t be avoided.

In my 135 hp Pacer, it’s typically about 1900 rpm on downwind to get to the top of the white arc for flaps. Then base is about 1300-1400 rpm to maintain 75, turning final, reduce just a bit to maintain 70 until landing is assured then full flaps and idle to touchdown. Most of my landings are wheel landings because I think the tail has more authority in the air and I think it’s easier on the tw assembly to lower it slowly to the runway. Again, just personal preference.

If I needed to be short, I’d fly a flat approach with power at about 55 and three point but even with my normal procedure and wheelies, I’m clear of the runway at the 900 ft mark.

Long story short, (too late!) Use what you need and try to do it similarly each time.

ysifly2
07-28-2020, 09:43 PM
Sounds like my Technic is completely wrong, or completely different. When I have the numbers in my strut V on downwind I cut power. Completely. Put in One notch of flaps and set trim at 80 . One more notch of flaps on base brings me to 70 . By the time Im over the fence Im at 60-65 . Then I try to make my spot without adding any power. Sometimes I may have to kick it back up to 1200. Full Flaps and sometimes I even have to slip at that .

I’d say that my technique is similar, most of the time. Works when at uncontrolled airports when you have the pattern to yourself... like my home airport.
But... I also try to learn and practice using different approaches. Maybe someone else in pattern and you have to extend downwind. Maybe controlled field and you are directed to enter on base with no downwind. Occasionally will fly into an airport with very very active skydiving.... need to sometimes use non standard pattern.
Key for me is learning various techniques and approach views.... and learning the feel of your airplane.

Bryan


Sent from my iPad using Tapatalk

Tnathan
07-29-2020, 08:57 AM
I’d say that my technique is similar, most of the time. Works when at uncontrolled airports when you have the pattern to yourself... like my home airport.
But... I also try to learn and practice using different approaches. Maybe someone else in pattern and you have to extend downwind. Maybe controlled field and you are directed to enter on base with no downwind. Occasionally will fly into an airport with very very active skydiving.... need to sometimes use non standard pattern.
Key for me is learning various techniques and approach views.... and learning the feel of your airplane.

Bryan


Sent from my iPad using Tapatalk

That's my approach. I dont want to dredge up some of the old eternal arguments, but I think it is wise to have as many tools in my bag as I can. If you are good mechanic, it always pays to have the right tool for the job. I think that applies to pilots too.

akflyer
07-29-2020, 05:41 PM
what works for pavement pounders wont work for those who are hitting small strips with tight tall trees etc. Every approach is different and all are fluid. I am like Steve and fly the approach the situation calls for and its solely by sight and feel. Why bother looking down when the place you want to hit is out in front of you and the sight picture will tell you what you need to do. 2 days ago I was hitting a strip that comes in over a gorge and I was anywhere from a dead idle to full power to stop the sinkers and lock to lock on the yoke.

LarryV
10-10-2020, 09:18 PM
You need to have more than one arrow in your quiver and you acquire those in more or less a certain order.

Super Cub Super Cruiser, Citabria or short wing Piper, my default has always been power off abeam the touch down point and then adjust the rest of the downwind leg, the base leg and final leg as needed to make your landing spot. In terms of private and commercial maneuvers it's an accuracy landing with +/- 200' and -0 /+200' criteria respectively. It works great on uncontrolled airports, when the pattern isn't chock full of aircraft.

It doesn't work so hot when you've got people flying 2-3 miles downwind before turning base, or when you have a air traffic controller calling your base leg. IN that case you need to carry some power on your base and final legs to make up for the now unnecessarily long legs you are forced to fly.

That said, in each airplane I've flown, I also initially landed them with some power on until I got to know the airplane. Carrying some power and some excess speed extends the time in the round out and flare and gives you more time to develop a feel for where the wheels are relative to the runway. You can also practice a low pass with the wheels 6" off the runway, and when you get good at that you can Bob Hoover it down the runway, placing 1 wheel and then the other wheel on the runway, while keeping the airplane tracking straight with the rudder. It'll greatly speed the process of getting consistently good cross wind wheel landings. Let's call all that Phase 1.

Phase 2 is to do an approach at the same speed (let's say 80 mph) all the way to the flare, but power off from the perch position abeam the numbers, and then use that extra speed/energy in the flare to give you more time to position it just above the runway for a full stall landing or to fly it onto the runway in a wheel landing.

Phase 3 is learning to do the power off approach at 70 mph slowing to 60-65 mph over the fence, which still lets you do either a full stall or wheel landing, but gives you a lot less time to get everything just right, especially with full flaps.

Phase 4 is carrying some power and hanging it on the prop on short final just short of a stall so that you can drop it right on the target when you need maximum short field performance.

Eventually you should be able to do all of the above when and where required.

Other than the power off approaches, I couldn't tell you how much power I carry in a given situation as it will vary with the wind, the temp, whether air is rising or descending on final, the load in the aircraft, etc. Quite frankly I'm not looking at the rpm, I'm looking at my touchdown point to see where I'm coming up long or short relative to that and slipping, or when needed adding power to hit that touchdown point.