We've been conducting some emergency turnback experiments to support an EAA/FAA effort to develop planning tools and techniques for dealing with power loss during takeoff.
Foremost, the point of this post isn't to advocate for any particular course of action if the engine quits on takeoff--it's just to share data and lessons learned
My RV-4 is equipped with a light-weight, two-blade Catto propeller and is quite the glider in IDLE power. Some of that "glide" performance is the result of residual thrust provided by the engine in IDLE. If the engine is OFF (mixture in cut-off), glide performance suffers (by about 20% at the same AOA/speed). This is shown in the first chart for glides at L/Dmax and ONSPEED with the engine in IDLE and OFF:
In the RV-4 with it's O-320/Catto prop combination, the propeller windmills at 875-900 RPM at L/Dmax with the engine OFF. Since the propeller is now driving a four-cylinder compressor, this windmilling prop adds drag and increases glide angle. Incidentally, with a fixed pitch prop and the engine OFF, lowest windmilling RPM is obtained with the throttle in IDLE (butterfly valve closed). Assuming oil pressure is available, the lowest RPM for a controllable prop occurs in coarse pitch (low RPM). If I slow the RV-4 to an ONSPEED condition (1.3 Vs), the prop stops, reducing drag at that AOA.
Since most of us aren't keen on pulling the mixture at low altitude for aircraft handling practice, it became necessary to develop a technique to replicate engine OFF performance in IDLE. Not too many options with the RV-4; but turns out deploying flaps 40 fairly well replicates flaps 0 engine OFF glide performance:
The most important thing to compare is glide angle. You can see in the second chart, wing's level glide angle is a wash between IDLE/Flaps 40 and OFF/Flaps 0. Glide angle in a stabilized 45 degree banked gliding turn is a bit steeper IDLE/Flaps 40 than OFF/Flaps 0, but the difference isn't significant due the nature of the turns--with roll in, roll out and the alignment maneuver the airplane never truly stabilizes in the banked gliding condition. Also, for the test point at altitude (IDLE/Flaps 40), alpha was too high (in other words, I flew it too slow; so I'll have another chance to excel soon and will correct the table with new data if there is a change).
This "high drag" technique for dealing with residual thrust can be applied for practice or flight test: https://youtu.be/aIvBnva1At8
Incidentally, in the RV-4 the "high drag" configuration adds 20% to the minimum altitude required for turnback with a 3-5 second simulated startle delay. Winds were calm in the video. We need "no wind" data to build an accurate MatLab model. Eventually, we'll be able to use the model to "what if" various scenarios by adjusting conditions. In addition to flaps, it's also necessary to fly slower to replicate glide angle. In the video you'll hear me reference the "slow tone." That is a condition slower than ONSPEED (i.e., less than 1.3 Vs), but faster than stall warning (set to an AOA that provides 5 knots warning in my airplane).
The bottom line is if you practice and determine a "minimum turnback altitude" using IDLE power, things will be different when the engine actually packs it in. Assuming the data from the -4 extrapolates, using Flaps 30 (RV-9/10/14) or Flaps 40 (RV-3/4/6/7/8) will likely provide a better sight picture for practice.
And last thing to consider is that I'm able to "max perform" the airplane because I can listen to my alpha/energy and I don't have a requirement to look inside the cockpit when I maneuver close to the ground. I've never flown a maneuver like this looking at an airspeed indicator, because my brain isn't fast enough to adjust stall speed based on the square root of my G load and I'm not that brave as the two-seat RV's don't have good buffet cues at low G. One thing to consider is that a 45 degree banked gliding turn is still a 1.3-1.4 G maneuver and increases indicated stall speed by about 15%. I would be disinclined to conduct this type of practice without an accurate AOA system that allows me to know precisely what my aerodynamic margin (difference between my actual AOA and stall) is. Be careful out there.
Fly Safe,
Vac
P.S. Skylor, finally caught up with you
Foremost, the point of this post isn't to advocate for any particular course of action if the engine quits on takeoff--it's just to share data and lessons learned
My RV-4 is equipped with a light-weight, two-blade Catto propeller and is quite the glider in IDLE power. Some of that "glide" performance is the result of residual thrust provided by the engine in IDLE. If the engine is OFF (mixture in cut-off), glide performance suffers (by about 20% at the same AOA/speed). This is shown in the first chart for glides at L/Dmax and ONSPEED with the engine in IDLE and OFF:
In the RV-4 with it's O-320/Catto prop combination, the propeller windmills at 875-900 RPM at L/Dmax with the engine OFF. Since the propeller is now driving a four-cylinder compressor, this windmilling prop adds drag and increases glide angle. Incidentally, with a fixed pitch prop and the engine OFF, lowest windmilling RPM is obtained with the throttle in IDLE (butterfly valve closed). Assuming oil pressure is available, the lowest RPM for a controllable prop occurs in coarse pitch (low RPM). If I slow the RV-4 to an ONSPEED condition (1.3 Vs), the prop stops, reducing drag at that AOA.
Since most of us aren't keen on pulling the mixture at low altitude for aircraft handling practice, it became necessary to develop a technique to replicate engine OFF performance in IDLE. Not too many options with the RV-4; but turns out deploying flaps 40 fairly well replicates flaps 0 engine OFF glide performance:
The most important thing to compare is glide angle. You can see in the second chart, wing's level glide angle is a wash between IDLE/Flaps 40 and OFF/Flaps 0. Glide angle in a stabilized 45 degree banked gliding turn is a bit steeper IDLE/Flaps 40 than OFF/Flaps 0, but the difference isn't significant due the nature of the turns--with roll in, roll out and the alignment maneuver the airplane never truly stabilizes in the banked gliding condition. Also, for the test point at altitude (IDLE/Flaps 40), alpha was too high (in other words, I flew it too slow; so I'll have another chance to excel soon and will correct the table with new data if there is a change).
This "high drag" technique for dealing with residual thrust can be applied for practice or flight test: https://youtu.be/aIvBnva1At8
Incidentally, in the RV-4 the "high drag" configuration adds 20% to the minimum altitude required for turnback with a 3-5 second simulated startle delay. Winds were calm in the video. We need "no wind" data to build an accurate MatLab model. Eventually, we'll be able to use the model to "what if" various scenarios by adjusting conditions. In addition to flaps, it's also necessary to fly slower to replicate glide angle. In the video you'll hear me reference the "slow tone." That is a condition slower than ONSPEED (i.e., less than 1.3 Vs), but faster than stall warning (set to an AOA that provides 5 knots warning in my airplane).
The bottom line is if you practice and determine a "minimum turnback altitude" using IDLE power, things will be different when the engine actually packs it in. Assuming the data from the -4 extrapolates, using Flaps 30 (RV-9/10/14) or Flaps 40 (RV-3/4/6/7/8) will likely provide a better sight picture for practice.
And last thing to consider is that I'm able to "max perform" the airplane because I can listen to my alpha/energy and I don't have a requirement to look inside the cockpit when I maneuver close to the ground. I've never flown a maneuver like this looking at an airspeed indicator, because my brain isn't fast enough to adjust stall speed based on the square root of my G load and I'm not that brave as the two-seat RV's don't have good buffet cues at low G. One thing to consider is that a 45 degree banked gliding turn is still a 1.3-1.4 G maneuver and increases indicated stall speed by about 15%. I would be disinclined to conduct this type of practice without an accurate AOA system that allows me to know precisely what my aerodynamic margin (difference between my actual AOA and stall) is. Be careful out there.
Fly Safe,
Vac
P.S. Skylor, finally caught up with you
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