Climb & Glide Performance

[Mike Koerner]

I collected some data on my plane’s climb and glide performance (attached). I have been meaning to do this for a long time. It’s a COVID dividend - an excuse for flying when I can’t actually go anywhere.

Keep in mind that:

1. This is for my plane, not yours. It’s a CT2k with an extra couple feet of wing span as compared to other CTs. Our planes all behave differently anyway; based on prop pitch, idle setting, engine wear, aircraft rigging, empty weight, etc.
2. The test data is subject to variability due to atmospheric conditions and pilot skills… or lack thereof - it’s was hard holding each airspeed and I noticed the ball off centered on several occasions. A bunch of planes in the practice area on Sunday was a bit of a distraction. That’s my excuse anyway.
3. I may have screwed up the data compilation (less likely).
4. There are some apparent discrepancies anyway. The power-off curves show the negative flap settings falling off faster than the zero-flap curves at high speeds. I don’t believe that.
5. These tests were conducted at a pressure altitudes between 2,000 and 3000’ with a light wing loading and forward cg (180 lb. payload and an average of 11 gallons of fuel).
6. I tested climb and glide performance at 5 knot intervals between 45 and 80 kts indicated airspeed. Once stabilized on a condition I tried to hold each for 30 seconds.
7. I used a flight recorder, which records GPS position and pressure altitude every second, to collect the data. Sailplanes use them all the time. Some cockpit instrumentation may also provide this data.

Light Loading Conclusions:

My maximum climb rate comes with negative flaps at about 75 kts. If I’m climbing at a lower speed I should transition to zero flaps below 65 kts.

My best angle of climb comes at very low speed. In fact, too low. I’ll use 50-55 knots to clear the trees to make sure I maintain sufficient control authority to avoid an upset in the event of a gust or turbulence. That’s with zero flaps. Despite the low speeds, positive flaps don’t increase climb angles. If I want my best angle at a higher speed, I should use negative flaps above 65 knots.

Minimum sink rate at idle occurs at very low speed as well. Again, too slow. I’ll use zero flaps and 50-55 kts if the engine quits and I need some extra time to try to restart it or to decide where to land. And again, no reason to use positive flaps in this situation.

Maximum L/D comes with negative flaps at around 65 kts. But with zero flaps at 50-55 kts I can glide almost as far and the zero-degree flap curve is flatter so it’s less sensitive to my distracted airspeed control. Also, the lower speed means I’ll have more time to look over my intended landing site carefully… unless that landing site is upwind, in which case I’ll have to speed up. Again, no positive flaps until I have the field made.

The turnback decision with an engine failure on takeoff is greatly simplified by the fact that my climb angle (at a 2,000-3,000’ pressure altitude) is about double my glide angle. As long as I have plenty of altitude to make the turn and clear obstructions, I should be able to make it back. Wind (assuming I take off into it) helps both the climb angle and the glide back. The next problem becomes overrunning in the reverse direction. In some cases, a truncated pattern may be appropriate.

Mike Koerner

[AGLyme]

Great work Mike... 

[Skunkworks85]

Mike,

 

on your climb angle, Are you actually climbing at 45kts with positive flaps or is this just the pitch angle of the aircraft?

 

 

[Tom Baker]

I suspect it is climb angle since the data collected is GPS position and pressure altitude.

[Mike Koerner]

AG, thanks.

 

Yes Skunk, I am climbing at 45 kts indicated airspeed. At this light wing loading the plane climbs very well at low speeds. even at 45 kts the climb rates have barely fallen off and the climb angles are still increasing.

But I am not advocating yanking back on the stick to clear obstacles. We need to maintain control authority. 1.3 Vso is probably a good minimum target.

I didn't measure the pitch angle - I probably should. It was certainly steep. I did some stall testing too but lost the data. The deck angles during stalls were absurd, especially power on. Again, at low wing loading.  

 

Actually Tom, I only used the GPS data to keep track of my target airspeed - I assigned a different heading for each airspeed. Using GPS for climb angle calculations would make the results susceptible to winds aloft.

My climb angle calculations are based on indicated airspeed and the change in pressure altitude over a period of time. There is an error here due to the fact that airspeed is along the climb slope rather than horizonal, but it's minor. At my steepest climb angle (12 degrees) it's only a little more than a 2% error, less at lower angles.

A more significant error may be in indicated airspeeds at high angles of attack. It's indicated, not calibrated.

 

Mike Koerner

[Mike Koerner]

I repeated my climb and glide performance tests; this time with the tanks topped off, 170 pounds of iron buckled into the seat next to me and 110 pounds of lead and water strapped in the back – 1320 pounds gross.

Once again, this data, for whatever it’s worth, applies to my CT2k.

I tried to improve the accuracy of the tests by extending the time on each condition and by avoiding turbulence – I went out in the morning instead of the afternoon and picked a day with very light winds. I also went further out in the Catalina Channel, so I was well away from the distractions of the practice area. And finally, I tried to concentrate on holding my speeds more carefully.

It didn’t work. There’s still a lot of scatter in the data. I don’t know why. Again, the data used for these plots is just the airspeed indicator I’m monitoring in flight and a recording of the pressure altitude at one second intervals.

Still, there are trends in the maximum gross weight data that are worthy of note:

1. The maximum rate of climb comes with flaps zero at 70 knots. There doesn’t seem to be an advantage to lower flap settings at any speed. At higher speeds, if for example more effective cooling is desired while still achieving the fastest possible climb at that higher speed, the transition between zero and -6 degree flaps occurs around 80 knots.
2. The maximum angle of climb comes at unreasonably low speeds. I’ll use 50 to 55 knots instead with flaps at zero.
3. The minimum sink rate is also occurs at unreasonably low speeds. Again, I’ll use 50 to 55 knots with zero flaps.
4. The maximum L/D or glide distance (with the engine at idle) occurs at 80 knots with -6 degree flaps. But there isn’t much distance lost at 70 knots, which is the transition speed for zero flaps. The advantage of lower speed is more time to consider my sins, unless I’m trying to go upwind, which requires higher speeds.

Comparing this to my Pilot Operating Handbook:

1. The handbook specifies the best rate of climb of 885 feet per minute at sea level and 78 knots with zero degree flaps. While my data agrees with the flap selection, my indicated airspeed was a bit slower; and assuming the handbook gives data at maximum gross weight and correcting for the 4,000 foot average density altitude during testing, I should have been able to achieve 760 feet per minute instead of my 600. This may be a result of:
   1. The manufacturer’s optimism
   2. My courser prop setting
   3. Engine wear from almost 2000 hours of operation
   4. The cumulative aerodynamic losses associated with minor dents and dings
2. For best angle of climb the handbook recommends 66 knots with the flaps at zero. Again, I chose the same flap setting but my speeds are slower. The handbook I refer to also applies to some CTSW models, despite my aircraft having an almost 3 foot longer wing. The greater wing area and consequent lower wing loading would suggest lower speeds at various operating conditions. Maybe they just didn’t bother to parse out these differences in the handbook.
3. The handbook does not address minimum sink
4. The handbook does not address glide ratio

Comparing this to the light-wing-loading data from a few weeks ago:

1. The plane climbs faster when it’s light, duh (1000 vs. 600 feet per minute). The maximum rate of climb speed is about the same at either weight (70 to 75 knots) but with negative flaps when light and zero flaps when heavy. The negative flap transition occurs at a higher speed when heavy (8o vs. 65 knots). This seems consistent with my soaring experience – lower flap settings are required with increased weight for optimum performance at any given speed.
2. The maximum climb angle is almost twice as steep when the plane is light (10 vs. 5.5 degrees at 55 knots). So, if you’re worried about the trees at the end of the runway, tell your passenger to take a hike… and to take her bags with her. The flap setting is the same at either weight (zero flaps).
3. The minimum sink is about the same regardless of weight (500 feet per minute at 55 knots) and the flap setting is the same (zero flaps). This seems wrong. The minimum sink rate should increase with weight.
4. The maximum L/D is about the same regardless of weight (11:1, about a 5-degree slope), but it occurs at a higher speed when heavy (80 vs. 65 knots) with negative flaps in either case.

This last characteristic, that increased weight provides the same L/D at higher speed, provides the justification for sailplanes to load up with water ballast on strong days.

The big surprise for me in all this testing was that positive flaps are not needed in any of these scenarios.

By the way, I corrected a calculation error in the previous light-wing-loading data. I had calculated climb angle and L/D based on the length of the slope flown rather than its horizonal projection, but the difference is so small it’s not worth presenting those plots again.

Here is the data taken at maximum gross weight:

 

 

 

[MEH]

That is an impressive amount of work, both collecting the data and the analysis.  Thanks! 

[207WF]

x2!

Did you take this analysis to  the impossible turn on takeoff conclusion?

WF

[Mike Koerner]

WF,

That's the next step in my program. Stand By.

Mike Koerner