IV-P Best Glide Speed Testing

[DavidMandel]

I conducted a series of flight tests in my IV-P N540LM to experimentally determine my plane's best glide speed. Thought I'd share my results because I found my best glide to be around 137 KIAS/9.5:1, which differs from my POH: "The best glide speed tested to date is 120 KIAS, 1570 FPM resulting in a 7.7:1 glide ratio." Appreciate any thoughts or comments on my test procedure and results.

Edit: There is a caveat to my plane’s glide performance: my prop governor is only able to maintain 1650 RPM while at full low RPM with an idling engine. Bob Pastusek and I discussed my results and it sounds like most folks get around 800 RPM at full low RPM, which results in a much better glide ratio of around 18:1. See replies below for details.

glide ratio.png (16.13 KiB) Viewed 859 times

I chose to conduct the tests from 9000' MSL descending to 6000' MSL since this is an intermediate altitude block I would be descending through while in a real emergency glide from altitude.

Description of flight tests:
1. At a fixed intersection and heading direct toward a waypoint, pull power to idle and pull prop to full low RPM.
2. Maintain 9000' MSL until IAS decays to desired airspeed, continuing to fly direct to waypoint.
3. Once IAS decays to desired airspeed, lower the nose and descend to 6000' MSL while maintaining desired airspeed.
4. At 6000' MSL, initiate climbing turn back to initial intersection and repeat at next airspeed.

I conducted this experiment at airspeeds of 110, 120, 130, 140, 150 KIAS. I then performed the entire suite of airspeed experiments a second time to average results over two tests.

The attached results are based on my G3X's logs. For each test, I noted the timestamp at which my airspeed first decayed to the desired airspeed, and also noted the timestamp at which my indicated altitude first crossed 6000' MSL. Subtracting the distance to the waypoint at the 6000' timestamp from the distance at the top of descent yields glide distance to descend 3000'. I adjusted this distance by factoring the average winds aloft through each glide. Dividing this distance by 3000' yields the glide ratio, which is plotted on the y-axis.

Insights and Summary
Something I wasn't expecting was the amount of left rudder required to maintain coordinated flight during idle-power descents. As I bring power to idle, there is a significant yaw to the right, and I had to maintain constant left rudder pressure to maintain coordinated flight during the glide. Might be worth using rudder trim if this happens in a real emergency.

I also conducted two idle-power, 360 degree descending turns at best glide to gauge how much altitude I lose while spiraling down over an airport for a power-off landing. I used 20 degrees of bank and 30 degrees of bank, which both resulted in about 2000' altitude loss per turn

Best glide speed and glide ratio: 137 KIAS, 9.5:1
FPM at best glide speed: 1430 fpm
Glide range: 1.5 NM per 1000' AGL
How much altitude is lost in one 360 degree turn at best glide: ~2000'

[Tom Nalevanko]

Great post, Perhaps I missed it, but Gross Weight is a variable. What was yours?
Thanks
Tom

[Chris Zavatson]

What was the propeller rpm? The prop is a huge drag source, the lower the rpm, the lower the drag. In some cases the lowest rpm limited by design, in other cases it is an adjustment issue.

power off glide.jpg (189.05 KiB) Viewed 817 times

[DavidMandel]

Great post, Perhaps I missed it, but Gross Weight is a variable. What was yours?
Thanks
Tom

Good point. Weight was around 3000 lbs.

[DavidMandel]

What was the propeller rpm? The prop is a huge drag source, the lower the rpm, the lower the drag. In some cases the lowest rpm limited by design, in other cases it is an adjustment issue.

power off glide.jpg

The results I posted are with full low RPM prop which is what I would use in a real power off glide. I tested one glide without touching the prop control (2300 RPM) to determine the effect of a "flat prop" against the airflow. This resulted in a glide ratio of about 7.8:1, compared with about 9.5:1 for full low RPM.

[Chris Zavatson]

I suspect the propeller rpm is still quite high even-though the control is pulled all the way out. I would have expected a glide ratio between 15 and 20 on the IV.

[Rock Barchfeld]

Hi Chris,

As a reference point, on the turbines with the prop feathered, we get about a 17:1 glide ratio at 120 kts and right about 650 fpm sink rate. Lots of variables in each scenario, but for a recip IV with a windmilling prop, cooling drag amongst others, David’s numbers don’t seem unreasonable. Not that I would recommend it, you could really split hairs and do testing with the engine shut down, prop low RPM and throttle closed, (I understand there is less “pumping” loss with the throttle closed).

Editorial note: While a neat intellectual exercise, missing best glide speed by +/- 5 knots or so would only make a significant difference in very few/extreme circumstances. Probably something more conducive to the successful out come of an engine failure would be to routinely conduct engine out approaches. I do them all the time, at different airports, weather conditions, even simulated IMC. I have a set of numbers for speed, bearing to and distance from the threshold that I massage for winds aloft , configuration, etc. Obviously, an uncontrolled field with no traffic is the only way to play around with this and if not comfortable, get with a LOBO instructor, the ones I’ve come across seem to know their stuff.

One man’s opinion, and as always, when considering anything opined by a guy named Rock, consider the source.

[Chris Zavatson]

Rock,
This topic came up several years ago. Bob Pastusek had posted his numbers. They looked much better. I suspect it is tied to differences in governed rpm. For a while Lancair was selling governors that had a very restricted rpm range.

posted by Bob back in June 2011

My IV-P has a glide ratio (at 120 KIAS) of between 5:1 and 20:1, depending on configuration. The difference is truly amazing/impressive/sobering… depending on your situation and mind set. With the gear and flaps down and the RPM at max (prop all the way in), the glide ratio approximates that of a brick…not really, but you’re coming down so fast relative to forward progress that it’s difficult to plan and execute a planned touchdown (at least for me). With the gear and flaps up, AND THE PROP CONTROL ALL THE WAY OUT, my glide ratio at gross weight is approximately 18:1, increasing to a bit better than 20:1 at 800# under gross. At this glide ratio, the performance and “sight picture” approximates that of some gliders........My MT 4-blade is not full feathering, but allows the engine to run at approximately 700 RPM when at idle and the RPM set to min..