My 9A Became a Glider Today

[339A]

Intentionally that is…. Well this is something I have wanted to do for a while now. It was an absolutely perfect Colorado day, wind calm, about 45 degrees out, who needs to go to work on a day like this? It’s a far better day for flying than working. Off to the airport I go with a plan in mind.

A friend sent me this video of an RV4 doing a power off descent and landing from about 8500 feet. This is something I’ve contemplated and decided it was time to do. For my purpose today I would fly over to Limon (LIC) and perform my engine out glide test. Upon arriving over the airport at 9500 feet, I slowed down to slow flight speed and practiced. The 9A is pretty easy to handle at 70 MPH, a few minutes of this and I was ready. Everything was good so I decided to pull the red knob and throttle all the way back. The engine RPM finally dropped off and the EIS was not a happy camper, as all manner of alarms came in. The engine continued to windmill and I found this to be annoying. Keeping a close eye on my airspeed I pulled back on the stick in an attempt to stop the prop (Catto three bladed fixed). It finally stopped and stayed that way. I then tried various airspeeds and recorded my vertical descent speeds. These tests were done with no flaps so I’m sure the numbers will change when flaps are added.

70 MPH equals 400 FPM
75 MPH equals 500 FPM
80 MPH equals 600 FPM
85 MPH equals 750 FPM
90 MPH equals 1000 FPM

About 2000 feet AGL I pushed the throttle and mixture back in, mags on and hit the starter. She fired right up and I climbed back to 9500 and did it all over again. I did this three times with the same results each time. My next step will be to actually dead stick the airplane. But I was quite happy with my efforts for the day. It’s amazing how much different things sound once the engine is off. At 70 MPH it was fairly quiet, but as the speed increases the wind noise goes up significantly. Something we just don’t normally hear with the engine running.

I’m glad I did this, and now I have some real experience/numbers if the engine does quit. Anyone who is comfortable flying their plane should give this a try and find out some useful numbers. The hardest part is going to be discerning at what point to turn for the field. In my attempts today I would have been quite high on final, but I plan to work on that and come up with a better technique for making the runway/field from about 1000 feet.

[the_other_dougreeves]

Excellent! This makes a nice polar for your new RV-9G. Interestingly, your best L/D came at your minimum sink - L/D of 15.4 at 70mph and 400 fpm. 400 fpm and L/D of 15 means you have a lot of time to figure out what's wrong and pick landing spots if the engine quits at 9500 without warning.

Since your measured max L/D was the slowest of your data points, it suggests that you may not have found the actual best L/D. Normally you will see sink continue to go down but L/D increase as speed gets close to stall. ANy chance you can give us some data points at 60 and 65?

And you're right, the wind noise changes a lot with speed. In a glider, you have multiple ways of avoiding the stall - nose below the horizon, enough wind noise and ASI. If you point the nose below the horizon, you will find it very, very hard to stall.

Now, the power pilot part of me thinks this can't have been good for your engine.... Did your CHTs drop off as you were doing the powered slow flight? And as for watching the airspeed drop off as you're getting the prop to stop, well, so what if it stalls? It's just like a normal stall recovery. The wing doesn't know that the engine is off and will recover just like after a power-off stall without adding power.

TODR

[339A]

Thanks for the input Doug. I was really amazed how much time I had. It felt like I could stay up there all day, although I know that's not true At 70 MPH everything seemed so slow.

I did slow down to 65 once, but didn't keep it there long enough to stabilize and record numbers. I will do that next time. I didn't want to get into the stall today with the engine off as I wasn't sure it would re-start. Being as this was my first time, the excitement of the prop not turning was all the fun I needed. I will get better info on the next tests. I'm sure it won't be so exciting the next time.

As for the CHT's I do rememeber getting the C-RATE alarm from the EIS (set at 80) but it was not on my list of things to pay attention to. I sometimes see this if I slow down too quickly, but not very often.

This is really a lot of fun once you get over the amazement of the engine not turning. My whole purpose in this was to get a "feel" for what would happen if the engine did quit and how much time I would have.

I am headed to Texas next week with the plane for a month long project. Looks like I'll be staying at your airport, ADS, courtesy of someone on this list.

Regards,

[scard]

Great fun isn't it. I did this testing too, in a very similar fashion except I didn't have the ... to stop the prop. I can say from looking at your numbers that the C/S prop that I have results in a MUCH greater sink rate.

[N523RV]

Scott,

Keep us posted of further testing. 2 years ago while I was out at LOE I talked my original flight instructor (now A10 pilot) into doing an engine off test with me. I just wanted the experience so it wouldn't be a complete shock if it happened for real. (Just like spin training..) We did not take the time to document our results, it was more for the experience. Certainly was an interesting sensation flying with the prop stopped.

Originally Posted by 339A

Intentionally that is…. Well this is something I have wanted to do for a while now. It was an absolutely perfect Colorado day, wind calm, about 45 degrees out, who needs to go to work on a day like this? It’s a far better day for flying than working. Off to the airport I go with a plan in mind.

A friend sent me this video of an RV4 doing a power off descent and landing from about 8500 feet. This is something I’ve contemplated and decided it was time to do. For my purpose today I would fly over to Limon (LIC) and perform my engine out glide test. Upon arriving over the airport at 9500 feet, I slowed down to slow flight speed and practiced. The 9A is pretty easy to handle at 70 MPH, a few minutes of this and I was ready. Everything was good so I decided to pull the red knob and throttle all the way back. The engine RPM finally dropped off and the EIS was not a happy camper, as all manner of alarms came in. The engine continued to windmill and I found this to be annoying. Keeping a close eye on my airspeed I pulled back on the stick in an attempt to stop the prop (Catto three bladed fixed). It finally stopped and stayed that way. I then tried various airspeeds and recorded my vertical descent speeds. These tests were done with no flaps so I’m sure the numbers will change when flaps are added.

70 MPH equals 400 FPM
75 MPH equals 500 FPM
80 MPH equals 600 FPM
85 MPH equals 750 FPM
90 MPH equals 1000 FPM

About 2000 feet AGL I pushed the throttle and mixture back in, mags on and hit the starter. She fired right up and I climbed back to 9500 and did it all over again. I did this three times with the same results each time. My next step will be to actually dead stick the airplane. But I was quite happy with my efforts for the day. It’s amazing how much different things sound once the engine is off. At 70 MPH it was fairly quiet, but as the speed increases the wind noise goes up significantly. Something we just don’t normally hear with the engine running.

I’m glad I did this, and now I have some real experience/numbers if the engine does quit. Anyone who is comfortable flying their plane should give this a try and find out some useful numbers. The hardest part is going to be discerning at what point to turn for the field. In my attempts today I would have been quite high on final, but I plan to work on that and come up with a better technique for making the runway/field from about 1000 feet.

[Geico266]

Pretty much kills the theory of shock cooling.

Great thread! Great info!

[hevansrv7a]

Originally Posted by the_other_dougreeves

... Interestingly, your best L/D came at your minimum sink - L/D of 15.4 at 70mph and 400 fpm. 400 fpm and L/D of 15 ...
Since your measured max L/D was the slowest of your data points, it suggests that you may not have found the actual best L/D. Normally you will see sink continue to go down but L/D increase as speed gets close to stall. ANy chance you can give us some data points at 60 and 65?
...
TODR

Maybe we are using different terms, but the best or max L/D speed will always have a greater sink rate than the minimum sink speed by almost exactly 14% and the CAS difference between those speeds will be: MinimumSink x 1.316 = Max L/D. The numbers do suggest that the minimum sink speed would have been even slower (perhaps as low as 54). It is important to remember that the best glide will not be at the least sink speed but 1.316x faster.

The best way I know to determine best glide (max L/D) speed is with the Garmin handheld displaying the glide ratio field and this can be done even with the power on to reduce the sink rate. The speed at which the Garmin says the ratio is highest is the IAS at which (for that weight) the reported ratio is highest. Power changes with AOA/IAS held constant will not change the result, only the ratio. The same speed will be best. Of course, a constant wind is needed so heading must be held. The actual ratio reported by the Garmin is not relevant, only that the highest ratio tells you the best speed. Once you know the best L/D speed you can compute the minimum sink speed (assuming the IAS error from CAS does not vary much). It's likely these speeds won't be at 5 mph intervals.

CAFE's 9A had a reported glide ratio of 12 at 95 mph and a minimum sink speed of 82, stalling at 49. I don't think their speeds conform sufficiently to the known mathematical "facts", but it's interesting to note how different they are from this report. It's useful to remember that IAS and CAS can be very different, especially at high angles of attack.
http://cafefoundation.org/v2/pdf_apr/rv-9a.pdf

[647jc]

The URL you posted for the CAFE document is not public and at least 'I' cannot access it, probably most others also cannot access it. Would it be possible to post a public link or perhaps make the document available via some other means? I don't know what if any copyright issues might be involved, according to their URL, the Cafe Foundation is a non-profit organization after all.

Thanks!

[170 driver]

I had a c150 that used to engage in engine out glide contest with a luscombe until the 1st annual after that, I discovered 2 cracked cylinders. This put an end to my engine out experiments. It is an expensive test to do.

[rvbuilder2002]

Originally Posted by scard

Great fun isn't it. I did this testing too, in a very similar fashion except I didn't have the ... to stop the prop. I can say from looking at your numbers that the C/S prop that I have results in a MUCH greater sink rate.

Actually, I am pretty sure that any prop (fixed pitch or constant speed) produces more drag if left turning versus being stopped.

It would be interesting if Scott got data with the prop windmilling and stopped.