What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

The Back-Side of the Power Curve (or lack thereof)

N91CZ

Well Known Member
This article popped up in a Google search and was ironically linked back to this site. There are some fundamental errors I wanted to point out. These errors are not unique to this article, but have been around for a long time and unfortunately keep getting repeated.

VansSnip-comment%20(700x473).jpg


The article describes the make-up of the drag curves and shows a plot of these. The error comes in when these curves are equated with power curves. These are not at all the same. In the text it states: ?We know that in steady, non-accelerated flight, Thrust equals Drag, so the Thrust (power) curves must be the same.? This is not true. While drag does equal thrust, it does not equal power. Power is obtained by multiplying the drag curve by velocity. This has a dramatic effect on the curves. It pulls down the left side of the curve. The left side can be pulled down so far that the upward hook disappears altogether. The consequence is that there is no region of reversed command in the power curve for the aircraft.

Now if we were flying jets and we had thrust levers, then the thrust/drag curves are the ones to use. A piston/prop aircraft, on the other hand, has a power lever and so we need to look at power curves. Below is an example of an actual thrust and power curve (my apologies, happens to be a Lancair) both in the clean and landing configurations. You?ll see the hook on the left completely disappears. What this means is that there is no back side of the power curve for this aircraft.

Cruise-Lndg-Thrust-Power.JPG


What drives this phenomenon? For a constant power input, propellers produce higher thrust as velocity is reduced. This automatically compensates to the increase in drag shown in the drag curve. If the increase in thrust keeps up with the increase in drag, the power required curves does not climb.

It is absolutely critical to separate jets (thrust lever) and props (power lever) because you need to look at different curves to determine if there is a region of reversed command.
Here is a link to a video showing the lack of a back-side in the Lancair. If power required was on the rise as the aircraft is slowed, sink rate would increase. Instead it either decreases (landing config) or remains the same (clean config) as stall speed is approached.

https://www.youtube.com/watch?v=RfEv4EiIA4c&t=338s

I have talked to many pilots who thought they had experienced the back side of the power curve in aircraft that did not have one. There are dynamic events (not steady state) that can easily lead one to falsely believe one was on the back-side of the power curve.
It is a simple thing to check in any aircraft. Replicate what was done in the video and see if sink rate starts climbing.
 
Slowing my 7 down on final has a noticable effect on sink rate. Nose up to go down faster works every time.
 
what you are looking for

This is the chart you are looking for:


power_curves_zpsajocoakw.jpg
[/URL][/IMG]

As you can see from the generic chart, the bottom of the power required curve is relatively flat. This explains your empirical result in the video.

You will find that if you go even slower than the 65 Kt in the video, you WILL begin to see the region of reversed command, as the increase in power required is exponential in nature. The question then becomes, "Will you perceive the rise in power required prior to the occurrence of the stall?" That will be airframe dependent, and with a little speedster like the Lancair, it is likely possible to reach the stall speed while still on the flat portion of the power required curve...
 
Slowing my 7 down on final has a noticable effect on sink rate. Nose up to go down faster works every time.

Would you mind generating a plot of speed vs sink rate? Power off, flaps down. I have data for five different aircraft models that all show no back-side. -am looking for one that does.
 
This is the chart you are looking for:




As you can see from the generic chart, the bottom of the power required curve is relatively flat. This explains your empirical result in the video.

You will find that if you go even slower than the 65 Kt in the video, you WILL begin to see the region of reversed command, as the increase in power required is exponential in nature. The question then becomes, "Will you perceive the rise in power required prior to the occurrence of the stall?" That will be airframe dependent, and with a little speedster like the Lancair, it is likely possible to reach the stall speed while still on the flat portion of the power required curve...

If a particular aircraft can't generate the CL required to reach the left end of the curve, then the region of reversed command is just theoretical and of no consequence. These generic chart would leave you to believe the bottom will fall out from under you.
Perhaps the video could have nibbled a little closer to stall (it was within 4kts). It was certainly far below normal approach speed.
I am looking for aircraft to document that can reach reversed command in the normal approach region.
 
Curious

I would be curious to know what five different models have no backside, and your testing method...

The speed at which the power available and power required cross in the low speed regime is the minimum level flight speed, where they cross in the high speed regime is the maximum speed in level flight. I would suggest running your experiment at a zero rate of descent and monitor your power setting for each airspeed, up to and including the power on stall. You will likely find that the generic curve is accurate...

It is also a false assumption that an "increase in thrust as velocity decreases" compensates for the drag increase equally...

It is very easy to demonstrate in most training aircraft, however, in order to get a good demonstration, you must be well below the normal approach speeds
 
Last edited:
I would be curious to know what five different models have no backside, and your testing method...

The speed at which the power available and power required cross in the low speed regime is the power on stall speed, where they cross in the high speed regime is the maximum speed in level flight. I would suggest running your experiment at a zero rate of descent and monitor your power setting for each airspeed, up to and including the power on stall. You will likely find that the generic curve is accurate...

It is also a false assumption that an "increase in thrust as velocity decreases" compensates for the drag increase equally...

It is very easy to demonstrate in most training aircraft, however, in order to get a good demonstration, you must be well below the normal approach speeds

Stall depends on angle of attack, not engine power. But otherwise I agree with above. At low enough speeds, drag scales as 1/V^2, and power required scales as 1/V. So at low enough airspeed, the curve will turn up. But most if not all of this region will be below stall speed.
Cessna 152s can demonstrate this region of reversed comand: Carefully adjust power and trim for level flight, at a speed just above where the stall horn comes on. Remove hands from yoke, use rudder only to keep wings level. Do not touch power. Apply a small amount of nose up trim. After an initial pitch up, the plane will settle into a steady state descent. (The nose up trim trims the airplane for a lower airspeed. But the power required for level flight increases, not decreases(?reverse command?). Since you left the power unchanged, the plane will descend.)
 
Cessna 152s can demonstrate this region of reversed comand: Carefully adjust power and trim for level flight, at a speed just above where the stall horn comes on. Remove hands from yoke, use rudder only to keep wings level. Do not touch power. Apply a small amount of nose up trim. After an initial pitch up, the plane will settle into a steady state descent. (The nose up trim trims the airplane for a lower airspeed. But the power required for level flight increases, not decreases(?reverse command?). Since you left the power unchanged, the plane will descend.)

Bob,
Was this clean of flaps down?
Higher drag configs tend to push the min power point to the left.
 
other models

I would be curious to know what five different models have no backside, and your testing method...

.....

It is also a false assumption that an "increase in thrust as velocity decreases" compensates for the drag increase equally...

I didn't mean to imply compensation was -or remains- equal, just that in the region near the bottom of a very flat power curve the two effects can cancel each other out within the range of interest. Yes, many non-linear things working together, but the end result is a nearly flat power required curve from ~100KIAS down to stall (clean config).

Aircraft recorded were: Lancair 235, 360, Legacy, IV, and PA28-180
When others are flying I simply have them record steady state descent rate at idle and full flaps across the entire flap extended speed envelope - down to as close to stall that individuals feel comfortable. Increments vary by aircraft, but enough points to generate a good trend line. What you end up seeing is that the slope at the slowest point is still rather steep indicating the aircraft will stall before reaching minimum power speed.
For the 360, I have full drag polars, a propeller map, and engine map. Those are from a prior airframe characterization project.
 
more

I have no experience with the Lancair series of aircraft, however, I have many, many hours in the PA28 series...and demonstration of reverse command in this series of aircraft is easily accomplished.

If your data indicates that the PA28 does not indicate an area of reverse command, I submit that your method or assumptions are flawed...
 
Aircraft recorded were: Lancair 235, 360, Legacy, IV, and PA28-180
When others are flying I simply have them record steady state descent rate at idle and full flaps across the entire flap extended speed envelope - down to as close to stall that individuals feel comfortable.

Perhaps none of your test data subjects were comfortable getting very close to stall which would make your data invalid.

I have personally flown many aircraft that do exhibit an increase in decent rate with a decrease in IAS, and I imagine Van (author of the article you posted) has as well.
 
Good post, Chris. I was one of the ones who originally thought that sink rate increased at lower speeds in my Lancair, but you (and 200 hrs. of flying) have convinced me otherwise. I'm glad you're here.
 
"...Stall depends on angle of attack, not engine power..."

Never said it didn't. The generic power available/power required chart has two intersections. The low speed side defines the point at which the power on stall occurs. t.

I don?t think so. Repeat the experiment, but limit the power to 60%. The power available will go down, the intersection with the power required (low speed end) will move to higher airspeed - but the stall speed will remain unchanged. This intersection is just the slowest airspeed where level flight can be maintained. (Assuming the wing hasn?t stalled).
 
I have no experience with the Lancair series of aircraft, however, I have many, many hours in the PA28 series...and demonstration of reverse command in this series of aircraft is easily accomplished.

If your data indicates that the PA28 does not indicate an area of reverse command, I submit that your method or assumptions are flawed...

Have you demonstrated reversed command in the landing configuration?
Not all data that comes back from folks is pristine, for sure. They are not all flight test engineers. For the PA28 it was the owner and his CFI/aerospace engineer son that gathered the data, so I am resaonably confident they followed instructions.

Here are some of the curves. They still show a significant downward slope at the last point taken.

235-360-Leg-PA28%20descent.JPG
 
Perhaps none of your test data subjects were comfortable getting very close to stall which would make your data invalid.

I have personally flown many aircraft that do exhibit an increase in decent rate with a decrease in IAS, and I imagine Van (author of the article you posted) has as well.

Scott,
The popular GA media/authors would have you believe that anything under approach speed gets you to reversed command. I just posted a plot of curves from different aircraft. The margin to stall varies, but are generally just a few knots away. Does the trend look like any of the curves are about to reverse and increase?

In my plane if you actually compute the CL required for min power in the landing config, you see that it is unachievable. Others may vary.

I am a little surprised that Van himself wrote the article. He should no better than to mix drag and power, jets and props.
 
Stall depends on angle of attack, not engine power.

Bob,
Here is an interesting segue on the topic of power vs stall AoA.

While mapping out Pitot-Static errors and the extreme low end of the speed envelope, I spent a lot of time in the buffet region at different flap and power settings. What emerged was a trend that showed how stall speed and stall AoA were affected by the prop wash as it transitioned from driving to back-driving. If the prop is driving it is adding energy to the air stream and it is relatively well behaved. If back-driving the air coming off the prop is low energy very turbulent. This affected the inboard section of wing. One could actually feel the difference sitting in the plane.

Note the units are KIAS and not KCAS

CritAoA.jpg



StallSpeed.jpg
 
I would suggest running your experiment at a zero rate of descent and monitor your power setting for each airspeed, up to and including the power on stall. You will likely find that the generic curve is accurate...

Bob,
The original drag and power curves were actually obtained in this way. The aircraft was instrumented with data acquisition and calibrated test equipment. With help from Lycoming and Hartzell we were able to get shaft power and then thrust values at each data point. Engine and prop efficiencies really start to fall off at these extreme low power settings.
 
Chris, I think what may be confusing people is your use of the term "reversed command". I'm not sure what you mean by that. In my plane in the landing configuration, pitch controls airspeed and power controls sink rate. I demonstrate that on every landing. What am I missing?
 
Have you demonstrated reversed command in the landing configuration?
Not all data that comes back from folks is pristine, for sure. They are not all flight test engineers. For the PA28 it was the owner and his CFI/aerospace engineer son that gathered the data, so I am resaonably confident they followed instructions.

Here are some of the curves. They still show a significant downward slope at the last point taken.

235-360-Leg-PA28%20descent.JPG

It's been ages and ages since I flew spam cans, but the lowest points on these graphs seem to be considerably higher (in IAS) than I recall being able to achieve in either Cessnas or Pipers.

Not sure what the debate is here...it's pretty easy to demonstrate this "back side of the power curve". Just slow down and hold altitude, reducing power, until you can't hold altitude anymore. Continue pulling back *and adding power back in* to hold altitude. Voila.

I've had 172s, e.g., practically hanging on the prop, showing something like 15 or 20 kias IIRC (but in any case WAY below stall speed) and full power, holding altitude.

If that isn't the back side of something, I don't know what is.

Come to think of it, I've only done it a time or two with an instructor on board in my RV, but guess what? Same thing happens.
 
It's been ages and ages since I flew spam cans, but the lowest points on these graphs seem to be considerably higher (in IAS) than I recall being able to achieve in either Cessnas or Pipers.

Not sure what the debate is here...it's pretty easy to demonstrate this "back side of the power curve". Just slow down and hold altitude, reducing power, until you can't hold altitude anymore. Continue pulling back *and adding power back in* to hold altitude. Voila.

I've had 172s, e.g., practically hanging on the prop, showing something like 15 or 20 kias IIRC (but in any case WAY below stall speed) and full power, holding altitude.

If that isn't the back side of something, I don't know what is.

Come to think of it, I've only done it a time or two with an instructor on board in my RV, but guess what? Same thing happens.

There is only one spam can in the list and it is about 3 kts above stall. 52 kts flown 49 kts stall. The only curve with any significant margin is the 235 curve.

It really wasn?t so much a debate as just clarifying that there were errors in the article shown where drag curves where equated to power curves. Many articles in the likes of AOPA and Flying magazine warn against getting slow on final approach. The claim is that a high sink rate can develop as one is supposedly flying on the back-side of the power curve. The question posed is this: Is a piston/propeller aircraft susceptible to the back side of the power curve on final approach ? and this of course assumes configured for landing.

From the discussion it appears some have managed to fly on the back-side of the power curve in the clean configuration. While this is certainly possible it is not the configuration that is relevant to the warnings in those articles. The physics of the situation would tend to make it less likely to be achievable considering the relationship of CL and CD at min power. For many aircraft it places the required CL too high to be flyable.

A number of aircraft were qualitatively checked to see if this assertion is generally valid. During engine idle descents, gravity is the propulsion and descent rate is an indicator of magnitude. If power required was increasing at progressively lower airspeeds, one would expect sink rate to start increasing. Instead, in all cases the trend is that sink rate decreased throughout. The curves all begin to flatten but one would be hard pressed to argue that with another 3 kts they would reverse and look anything like the hypothetical curves being published.

With respect to the published articles, when you look at the author bios, you will see lots of jet time, typically retired airline pilots. I suspect they end up transferring the characteristics of jets onto GA aircraft without understanding the differences. They all describe the increase in drag at low speed correctly and then make the mistake of equating that with power.

I would love to see a video of a 172 in level flight indicating 15-20 KIAS. Unfortunately, once you hit buffet or stall you are not operating on the power curve being discussed.
 
Chris, I think what may be confusing people is your use of the term "reversed command". I'm not sure what you mean by that. In my plane in the landing configuration, pitch controls airspeed and power controls sink rate. I demonstrate that on every landing. What am I missing?

John,
The Clift Notes version.
Generally when one wants to fly slower (assume no altitude change) a power reduction is needed. In the case of a jet, a thrust reduction.
Flying in the region of reversed command means that flying slower requires more power, or in the case of the jet, more thrust to maintain altitude.

This is a common event for jets. This is because jets have thrust levers and their Vref approach speed can be very close to minimum thrust required. i.e. going faster or slower requires more thrust.
In propeller aircraft we have power levers. Our reversed command region starts once we get below minimum power required speed. This occurs at a much lower airspeed than typical approach speed. In fact, it is close to or can even below stall speed.
 
I will check my 8 in 20 flaps next time I have a chance.

For the poster asking about backside, another myth commonly believed is you add power to slow down (or a region of reverse command). Typically most aircraft will have more thrust or power available to get out of this region. If you are straight and level at some point you’ll find the least power or thrust required setting. To go slower pull some power, add drag, or g and slow a little more. If now you want to maintain straight and level the power/thrust setting will be higher than the minimum power setting you found before that was at a faster airspeed. And again you have almost always you have more power available than required to fly at this condition, firewall it and you speed up; you don’t slow down with added power (not reverse command). I have found backside in turbo props, I know it exists; haven’t tried a piston GA aircraft yet.

I am aware it is called a region of reverse command, but this is a misleading term. Command is something you do, like add power or pull aft on the stick (command more thrust or command more g/aoa). A region of reverse command should be used when it actually means what it says. For example the mig15 exhibits regions of reverse command at elevated g, you push on the stick at some point to get more g (and vice versa); or many none fly by wire aircraft when transonic exhibit reverse command in roll (stick left and aircraft rolls right; true also at very slow speeds in some due to adverse yaw).
 
Last edited:
The only curve with any significant margin is the 235 curve.
Nice of you to say ;) My 235 stalls around 60 KIAS (depending upon weight) in the landing configuration. I guess I need to re-perform that sink rate test to get you another data point on the lower end.

The only time my 235 develops a high sink rate is when I am in the flare and pull the power. Unless I'm doing a short field landing, I usually leave a little power in and ease it out when stabilized 6" over the runway in order to gently set down. Like so (Note: There's a 1 second encoding lag in the displayed ground speeds):

https://www.dropbox.com/s/5kfrgmuds4xfdoi/2018-12-30 Flare.mp4?dl=0
 
Last edited:
Glider guider

In my sailplane days, I would thermal at minimum sink speed, which was just before the onset of the stall buffet. And as it sounds, this is the speed with the minimum sink. As Paul Mcready proved with the Gosemer Condor, this is also the speed of least power to maintain level flight.
I think there is a nomenclature issue here in that many power planes can fly straight and level below this minimum sink speed if they are hanging on the prop. Helicopters do this all the time. And even in the 172, this is easily demonstrated by maintaining level flight with full power in the stall buffet region. JMHO
 
Video

"I would love to see a video of a 172 in level flight indicating 15-20 KIAS. Unfortunately, once you hit buffet or stall you are not operating on the power curve being discussed."

You are never going to get a video of a 172 flying at 15-20 knots.

If I get out to the airport this week, I will try and get a video of the 172 requiring more power to fly at a slower airspeed AND maintain level flight...
 
The term has been hijacked for sure. Should be more like "reversed response"

It isn?t even reverse response, if you are in the region and addpower you will accelerate to the other side of the region past the minimum required point. One of the reasons without DLC or low slung motors flying the ball is a difficult task.
 
"I would love to see a video of a 172 in level flight indicating 15-20 KIAS. Unfortunately, once you hit buffet or stall you are not operating on the power curve being discussed."

You are never going to get a video of a 172 flying at 15-20 knots.

If I get out to the airport this week, I will try and get a video of the 172 requiring more power to fly at a slower airspeed AND maintain level flight...

I think y'all missed my point. I did say it's been years (at least 15) since I flew Cessnas, and maybe it was a 152, but the point was that you CAN get the airspeed WELL below stall speed (in clean or dirty configuration) and into the so-called "back side of the power curve). So much so that I remember being shocked at how slow we were actually flying the first time my instructor demonstrated it to me. Then it became a challenge...see how slowly you can get the plane to fly (and hold altitude).
 
Other interesting bits from the Van's article

Interesting and thought provoking discussion. The OP's primary point is that Power is Thrust times velocity (N m/s) is worth noting. It will flatten the plot but does not eliminate the existence of the backside behaviors we all noticed early in our private training days, that to maintain altitude during slow flight with flaps extended, a significant amount of power had to be added and that increasing pitch caused a descent and decreasing pitch caused a climb.

Van goes on to say in the OP's first post:
?Back Side? operation is primarily for lower power aircraft of aircraft with high lift systems which reduce stall speeds but produce very high drag in the process. If the total thrust available is equal to or less than the drag of the airplane, it will be unable to climb or accelerate out of this condition. ? Most RVs have enough power that they, at normal operating altitudes, can ?power out? from behind the power curve.​

Perhaps the root of the discussion in this thread is that the Lancair 360, I assume optimized for cruise, does not have enough lift to exhibit a "backside" behavior because it stalls at or near the minimum sink speed, as represented by the descent rate vs airspeed plots for that aircraft.

My 9A will drop out of the sky if I lower the speed below 60 knots, to 55 or even 50 knots on final, but I must confess, I don't know if if the sink rate increases. The glide slope certainly gets steeper as the speed decreases. This technique is as effective as a slip when high on the approach.

It's a good excuse to go flying and collect power vs airspeed curves for constant altitude in my 9A. It does take full power to stall the airplane during power on stalls. That, by definition, is a backside behavior.
 
... increasing pitch caused a descent and decreasing pitch caused a climb...
Oh now you've gone and done it! I've never flown an RV but in every other plane I've flow in slow flight, pitch controls airspeed and power controls altitude (or sink rate). Here's the cockpit view of the same landing I posted earlier. You can see all the instruments and hear the power changes. I was using pitch to keep my airspeed at 75 Kts and power to control my rate of descent. As I pitched up in the flare, you can see my airspeed bleed off but my descent rate didn't increase until I eased out the power. Grabbing some popcorn...

https://www.dropbox.com/s/p14jwd07nsu95f4/2018-12-30 Flare cockpit.mp4?dl=0
 
You can fly front side and still use pitch to maintain airspeed and throttle/power for glide slope.

I flew some test points today. 30.17, OAT 16, 2300MSL. Wind was rather smooth. Fastback RV8, 180hp io360, constant speed blended airfoil prop. Fuel flow is broke. Mixture prop full forward for all points.

Technique for front side was set power let aircraft settle into speed maintaining zero on the VSI (looked outside for this mainly to see trends quicker), use feet to center the ball.

Backside, use slight G to increase drag to get slow and then set zero vsi and gradual power to maintain airspeed and zero sink/climb. Again feet for the ball.

KIAS/%power/mainfold/rpm
20 flaps
67/31/15.2/2030
65/30/14.9/1990
58/35/16.4/2060

Full flaps
67/37/16.8/2190
54/39/17.3/2110

Appears in both configurations I got backside. This isn?t professional quality data, just a quick eval. I probably got on conditions for about 20-30 seconds. I suspect I am close.
 
Good data!

I flew some test points today.
That was good data! We can all see that the slower you got, the more power you needed to hold altitude - especially with full flaps.

I went up today and did a brief flight test to confirm some of the Lancair 235 numbers I gave Chris a couple years ago (Gray line on Power Off Descent Rate - Landing Configuration chart) . Conditions were kind of gusty today so I was reluctant to go below 65 KIAS lest I fall out of the sky. At 90 KIAS my descent rate was still 1,200 fpm, but at 70 KIAS I was getting 950 fpm. Maybe Chris can adjust his gray Lancair 235 line accordingly to show it a little flatter on the bottom end.

https://www.dropbox.com/s/c3kginhloyhh79g/2019-01-01 Power Off Descent.mp4?dl=0

Everyone should do this stuff to learn the numbers for their own plane.
 
Nice landing Snopercod!

As I mentioned, I am guessing that the Lancair does not have high enough lift in the wing to fly on the backside of the power curve, which is what I think is at the root of the discussion. It appears to be stalling near the minimum sink rate, of the bottom of the U shaped drag vs airspeed for constant altitude plot in the first post. Van's referenced article specifically mentioned that this was "primarily for lower power aircraft of aircraft with high lift systems", which I suspect does not describe the Lancair.

Time to go fly and test the -9A
 
Well - I was not expecting this!

Just got back from flight testing on a perfectly still air day in the PNW. I got the same flat power curve that has been shown for the other aircraft.
1560 lbs, 40°F OAT, IO-320, FP Sensenich GA, Density altitude between 1350 and 2580 ft, full flaps

For constant altitude I got:
IAS HP RPM MP
80 67 1830 19.5
75 59 1750 18.8
70 52 1690 18.2
65 44 1620 17.5
60 40 1570 17.2
55 40 1560 17.3
50 41 1555 17.3
45 42 1560 17.4

6cqZOc8Q6cBCdjg5DSjYH_uGqu2BDwUf1ovKgyxjXiujMQePD_CDV7-Z8rtTtrWMXB-CRbF2zGW83sDXoaXflbKIJIjaiUasVxMgeCHhooLt5lxRKVQ493WSp4jCYCWQo-NLa4o_diCWHoq4I_v7RKvCICxQdSF95cU-KbIizK2-xsfQWZf3hks-WaJhAgLAQgbuTxc2krQXsICw-OGJ-psF8C4RlnilxdNKhja96IudG7qHvnz4ALr1kgAweYzIZ9_sPo1pOzS39iMuVvR7tx1dnR5VE-hdLAjMM6tTmu5jCBTplEsw9IxrhubFbaFxefjYvwp_siFw74DHkypr4z3VJmiirnjPVz2V7LULjfBpqWdoHbdVVzyI3qnJDjwgr7J_lY79-QiNFdDxEgpsACxr25vvmWFQajdEh8MCTeV7FH1xl2Q3H135OJletCNEItghQ_MDWHRn_RXFONzAm9LoDHmlDhkvJouBeoCEiG38Al5C01jGHPtpQEjyCfqMknp9c33Jwwa3tAQFw5lmdBsSp1Z4CmAOyp-ceJmY-M4tsMwDuaWPkJDrWZnqTlOPK2Ovlf_72nf4LBGURvtXNe1vefKNuBpjss-uWdFfSLoKQacgONR9ZrUvNKeEHjP0n9VR22N22h-MdY7k8EcV2uxDKG-rPHBxeyFAKTjyh0B6vNuu7zE9EGnNcq4dRCz2mMzbaZOTQk8EI7ZDbQ=w483-h290-no


For constant airspeed with full flaps and 9 inHg MAP I got:
IAS Descent Rate
80 1400
75 1090
70 900
65 730
60 600
55 550
50 500
45 450
42 300* 1 knot above stall - Verified from Skyview data log

DomgEoXST_jKCnNRqQrwWpH3QkxK9X8fnMnCqV09hpoS_M5HWEIJ_0H9xIJSFocVo23iHxOlI5Ka-v5J8WZ2GHm747sPAv1VA_M_W8Zw4wlSGONl7Snj4R_Uwgx6iwvudUr_gnxZ8b1NAiJUBFqTpfjZktHwZcYB8ifQDXSFiMxrLTxCzOPSu3Z7tw2RSYksKBYRuUnlVcgUxpv-aabDaaYFao2BEcdlc3ekLsWK4i5v-EUC0NQHKNzhxU_aLHgHHAovr4oYbOOxWXfzIz9q1bsf8R9GgY0aEVvgVNz40vXln8p88Y_HODU_1jAEYZvtO5OzLvZRzBXTZAF5VigNppzH9zWKY1XdjqsZWChSwkqP-v1Yf9uKGzRY47NfzgaGR4unVk-hKIOTEsfOmBYHfyUt00V4qQPP8nR0vdISfel2Tt58hEe6TQwU-RQWi5sFmuzlozJuXGr6sUG5NSoypkGiaOVOFYSgsF7PqA8QaFIED4b4F9nezpflEuAriBD-2fwlhF8WUdDzVRsm0wekwkednEj2RjX3iduQZxlFOk-15aOfB_oqfMhX4bUHsPz5dvOlVbFixbLwfL19gAO1fn4rolAd9qdo-A80k2vm2IBmWECQ7-_cYSHatopFapdgEonha4zNf-CR516Sms3B-WXiAFNeBBxQOaEBDZb1xAYqawBHtLZMvdQuodIfPDz6iV0PdEXVmPRzBhDOWw=w484-h289-no


For the constant altitude test I was sure I would have to add power to fly at slower speeds. The data shows a slight increase, but that is only 2 HP.

The power curve at high lift was nearly flat just as Chris originally posted. I do only get 30 degrees of flaps out on the -9A. But still it was pretty amazing to slow down the plane and essentially not have to add power between 60 and 45 knots.
 
Last edited:
RV-8 Data

I ran my own set of tests this afternoon in my RV-8.

Tests were run at 3500' +/- 30', 47F OAT, 30.15 in Hg Baro.
The tests were flown in extremely smooth air over the ocean, with very low winds aloft (< 8 kts).

The airplane configuration was:
Weight: 1610 lbs
Flaps: Full
Prop: Full Forward (Hartzell BA, 74")
Mixture: Full Forward (IO-360-A1B6 Angle Valve)


IAS MAP Eng. Spd. FF
KTS in Hg RPM GPH
_____________________
85 17.5 2490 10.4
80 17.1 2400 10
75 16.5 2300 9
70 16.3 2220 8.5
65 16.3 2150 8.3
60 16.5 2140 8.3
55 16.8 2140 8.5
51 18.1 2230 9.8

I ensured that the plane was completely stabilized in speed and altitude at each test point before recording the data. In my plane, at least, there is quite a noticeable increase in power required just above stall speed. I will try to run these tests again someday closer to gross weight. I won't be able to fly the 51 knot test point at heavier weight but I'm pretty certain that in my plane at gross the speed at which increased power is required moves up to somewhere around 60-65 kias.

Skylor
 
Last edited:
A couple of these tests seem to stop just when it's getting interesting...just a tiny bit below Vs0 or Vs1, when they had to add a touch of power to slow further. Why not keep going? Increase pitch further while adding power, and continue until either you're uncomfortable with the pitch attitude (it'll be very nose up) or run out of power to give.

It's an interesting flight regime, just keep your turns coordinated and don't stall a wing!
 
Speed

A couple of these tests seem to stop just when it's getting interesting...just a tiny bit below Vs0 or Vs1, when they had to add a touch of power to slow further. Why not keep going? Increase pitch further while adding power, and continue until either you're uncomfortable with the pitch attitude (it'll be very nose up) or run out of power to give.

It's an interesting flight regime, just keep your turns coordinated and don't stall a wing!

50-51 KIAS was about as slow as I could consistently fly whithout stalling. This was already at the point of buffet (which is much less pronounced with flaps than without) and at one point I got a hair under 50 and had a clean stall break and had to restart the test.

Please note that I updated the data with a correction in the 55 kias run.

Skylor
 
50-51 KIAS was about as slow as I could consistently fly whithout stalling. This was already at the point of buffet (which is much less pronounced with flaps than without) and at one point I got a hair under 50 and had a clean stall break and had to restart the test.

Please note that I updated the data with a correction in the 55 kias run.

Skylor

Now I gotta go do this again (been a few years since I did it during a BFR) and see what I can get! :) I'll give it a whirl this weekend, weather permitting. Gives me another excuse to go flying LOL!
 
A couple of these tests seem to stop just when it's getting interesting...just a tiny bit below Vs0 or Vs1, when they had to add a touch of power to slow further. Why not keep going?

I was thinking the same thing. It?s going to be a bit tricky maintaining zero sink at the edge of stall with full flaps.

Does anyone have experience with power on stalls with full flaps? I imagine it?s going to pretty high pitch angle with lots of power.
 
A couple of these tests seem to stop just when it's getting interesting...just a tiny bit below Vs0 or Vs1, when they had to add a touch of power to slow further. Why not keep going? Increase pitch further while adding power, and continue until either you're uncomfortable with the pitch attitude (it'll be very nose up) or run out of power to give.

It's an interesting flight regime, just keep your turns coordinated and don't stall a wing!


I could keep going, my intent was to show my RV8 has typical backside behavior. The trend was obvious to me, and from qualitative analysis (flying the plane in slow flight before) I suspected this to be the case. The data does not lie, but sometimes our perception is not reality (specifically verifying my qualitative analysis and assumptions matched what was actually going on). I have done plenty of backside flying, when I began flying and even for years into flying it would always make me nervous; it felt like the plane was going to fall out of the sky. At some point it didn?t bother me anymore and I do find it interesting and fun. I?d suggest everyone get comfortable with it and listen to what the plane is saying in those regimes. It has made me a better pilot, and hopefully helped me recognize when the plane is getting there. I am a believer that you should always expand your own envelope, that way when you need to do something a little bit crazy you are more likely to know what to expect. However when expanding, use build up, don?t go right to the end points.
 
The power required curved has nothing to do with engine power. Gliders have power required curves associated with them, just like airplanes, even though they have no powerplant. Power can be considered as a force times velocity; therefore, the power required at any given velocity can be determined by multiplying the drag times velocity. The power required curve is somewhat similar in shape to the drag curve, but skewed a bit due to additional velocity term that it contains. Like the drag curve, the power curve has a minimum point. More power is required at speed more than and less than the speed associated with minimum power; however, the speed at which minimum power is required is not the same as the speed for minimum drag. The term power required actually refers to the power required for level flight.

The important thing that Van is trying to point out is that you don?t want to unknowingly get behind the power curve. There is an airspeed that at full power you will not be able to climb and if you are low enough, you will most likely impact objects that are in your direction of flight. If you continue to reduce speed without having more power available, you will stall, spin and most likely kill yourself. I have seen this happen many times. This is especially important on twin engine airplanes where there is a point that you will not be able to maintain directional control (the blue line) even though you may have enough power to remain airborne on one engine.

If you have enough power available, you can actually climb at zero airspeed. In this situation the power curve would go straight up on the aft side. This is demonstrated quite nicely with high performance and what they call ?3D? aircraft flown by very experienced and talented Radio control pilots. These aircraft have thrust in excess of the weight of the airplane. Yes, propeller airplanes have thrust also but just at a percentage of brake horsepower. The power available curve in the diagram above is curved as a result of propeller efficiency. A very good example operating an aircraft below the power curve is demonstrated below. If you listen close enough you will hear the pilot adding power when he approaches a stall. He will also pulse the engine throttle. This is to maintain airflow over the control surfaces of the airplane which are very large compared to most aircraft. These large surfaces are affective at very low airspeeds. If you hang around to the very end of the video, you will witness a very unusual short field landing. Much like the STOL full scale aircraft use except the inverted part, he uses the aircraft power while operating behind the power while operating below the normal stall speed.

Enjoy.

https://www.youtube.com/watch?v=moQlvtHItg8
 
"Departure Stalls"

I was thinking the same thing. It’s going to be a bit tricky maintaining zero sink at the edge of stall with full flaps.

Does anyone have experience with power on stalls with full flaps? I imagine it’s going to pretty high pitch angle with lots of power.

Yes, I practice "departure stalls" with flaps in my -8 several times a year. At full power, the pitch is indeed quite steep. I'll have to do a couple and add them as a data point to my test data.

Skylor
 
Last edited:
OK, the data don't lie :).

Experimented with a co-pilot today. Half tanks, two big guys, around 40 pounds or so in the baggage area (steaks, smoke oil, what have you :)).

Data logging via Dynon Skyview, observation by co-pilot on D6. I'll talk about calibration errors in a moment. 5500' altitude, RPM set to 2500 and unchanged throughout, full flaps.

Slow flight 1, into the stall buffet:
IAS 46 knots, observed IAS (D6) 41/42
AOA 72 %
Pitch 19.1 degrees
% Power 59
MAP 20.3"

Slow flight 2, full stall break:
IAS 46.2 knots, observed IAS (D6) 41
AOA 72 %
Pitch 19.2 degrees
% Power 47
MAP 18.1"

Looks like I could have given it a little more during the second run and gotten a bit slower, but the two are consistent.

My latest Pitot/Static/XPDR cert shows the Dynon reading 3 knots high < 50 knots (+1 at 50), and the D6 4 knots low < 50 knots, so splitting the difference would yield CAS of around 43 knots.

I suppose with some more practice and tickling the controls and such, we could have gotten a few knots slower, but that's still, IMO, pretty far on the back side of the power curve (normal Vs0 is 50 knots)...at least 10% if going by IAS, closer to 15% if using CAS.

Beat *that* :).
 
Last edited:
and...

Vyse, as stated is the best rate of climb, single engine; it is the Blue Line

Vmc is minimum controllable speed and is typically noted as a Red Radial on the ASI of light twins according to FAR Part 23.
 
Vyse, as stated is the best rate of climb, single engine; it is the Blue Line

Vmc is minimum controllable speed and is typically noted as a Red Radial on the ASI of light twins according to FAR Part 23.

Yep, what I was getting at. Flying with someone the other day; max conserve. Was below blue line (both engines at about 28% torque); he was worried if we lost an engine we?d depart. Well no because above vmc; and even if we were below vmc still wouldn?t depart with one engine failed and the other near idle.
 
Back
Top