glide distance and speed

[jswareiv]

I am getting ready to do my testing for glide distance and best rate of speed for trip planning altitude and engine out procedures. I know all planes are different, but if you have done it already, do you have any starting guidelines? I have a -7 with a 200HP and constant speed prop. Thanks.

 

[hevansrv7a]

My Heresy

The conventional wisdom is that you should do iterative testing for best L/D which is or is very close to "best glide".

However, you can easily find best sink speed (Vx?) by finding the speed at which it takes the least power to maintain level flight. Of course, that speed is only accurate for one combination of aircraft weight and density altitude. But, that's true of all the V speeds. This is where Angle of Attack can be useful, but that's another subject.

OK, so now you know it flies on least power at between 75 and 80 knots, for example. As was pointed out to me by Kevin Horton and others, the Vy or Best L/D is 1.316 times the least sink speed. The beauty of this is that it's true for all airplanes, or so I am told.

Of course, there are various POH samples available on this forum and elsewhere.

As always, if I have gotten it wrong I invite corrections.

 

[gmcjetpilot]

idle v windmill....fine pitch v course pitch

Here is a good Refrence:
http://www.auf.asn.au/groundschool/umodule2.html#vspeed

The intent of the following is to get you to think about how you are going to test, configuration, technique and what you are going for. Are you trying to show how great your plane guides in the most ideal config or what the glide will be in a real world emergency. Also weight and even more winds have a huge affect on how far you will guide or not. You are looking for MIN SINK, independent of wind. Max glide distance involves winds. Keep that in mind for a real world situation and speed.

Consider pulling your prop control out, low rpm, full course. This may extend your guide (a quasi feathering of the prop if you you will). Now will that work if your engine is really shut down (windmilling) verses idle (which is making a little power still)?

If you want more conservative numbers leave the prop to fine pitch.

The question that always comes up is do you have enough oil pressure to get the constant speed prop off the stop with windmill RPM. This has been debated for a long time. I lost track of the common wisdom, but it comes down to the prop you have and governor. There is one AFM I was told for a single engine plane that suggest you do go to course pitch with an "emergency guide". I suspect if its for real it can't hurt.


Multi engine plane props have counter weights that throws the props to high pitch (feather) and also have gas charges in the hub to help it go into feather. Your prop of course has none of this. However some claim they can get the pitch to change with the engine shut down and windmilling, some say it makes no difference. So if you want to use the KISS principle you can just leave the prop full fwd.


I suspect with idle power you are making a little thrust (or at least less drag) verses a true engine failure with a windmilling prop.

When windmilling a dead engine the air flow has to push the engine around which is energy and drag. Some say if you have an engine failure you should pull up into a near stall to stop the prop from windmilling? Well that is another debate and I say nice and theory but not practical, at least for a generic glide test. The stop the prop technique may have some validity, but obviously it's not practical at low altitude.

Glide test have always had there issues or controversy when it comes to technique. In the end does it really matter if your best glide is off 10 mph or (9 to 1) or (11 to 1) in the real world? Weight and WIND has a BIG affect on best glide speed and distance. If you are gliding into a wind go faster, with a tail wind slower. For the space shuttle it matters. Bottom line if you want it for emergency glide planing you want conservative.

Consider (at your own risk) getting some altitude over a airport (non-towered, low use, long runway) and shutting the mixture off, for a one time test. Compare that to flying with idle power (w and w/o prop course). That way you can determine the affect it has if any. If you do shut down the engine, cool the engine off a little level at low power before shutting down.

When I was doing multi engine piston plane CFI stuff, I shut down many engines for single engine flight Demos. The all started up again. Of course if it did not re-light, I had another engine and you have only one.


The Real world: I have heard RV builders claim glide ratios of (7.5 to 1) to (12.5 to 1) and speeds from 87 mph to 120 mph? In RV's my personal engine failure speed is, Vos or Oh S#@^%. I go right to 100-110 mph at first and than fly the plane. Why? it is easy to remember and works pretty well. Than I'll fine tune speed, but first I am trouble shooting and picking and aiming at a landing point.

If you get into a real power off landing and are trying to "milk a glide", you are in trouble. Never go below 80 mph (unless you are flaring). I realize there are variation between RV models, but they all have the same wing. The best thing about doing the glide test is getting a feel for the plane and it will tell you when it is sinking. Try slow speeds and fast speeds, try gross weight and solo min fuel.

(edited: Larry P., thanks for the clarification, I concur. Hevansrv7a, as far as Max L/D x 1.316 = best range at SL, a little fast for min sink. I think Vy is a good starting point for min sink. For best efficiency the Carson speed is Max L/D x 1.316 = or about 120kts or 138 mph. It is faster than best range. Again min sink is not Vbg, best glide. With Vbg, head wind fly faster, tail wind fly slower. Here is a good article.)
www.eaa1000.av.org/technicl/perfspds/perfspds.htm
Here is a good description if Vx nd Vy:
http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page10.html

 

[n5lp]

...Also weight and even more winds have a huge affect on how far you will guide or not. ...

To clarify, weight has no effect on maximum glide ratio, but it changes the speed where maximum glide occurs. Therefore more weight can let you go further into the wind than you would with less weight. For rule of thumb to go the furthest, slow down a bit going down wind or speed up a bit going upwind, speed up a bit if heavy.

This is all referenced to some mid-weight best glide speed. If you have best glide speed figured out for maximum weight, it will occur at a slower speed if you are light.

 

[az_gila]

CAFE tests - glide angles- wind effects

The CAFE tests on the RV-6A calculated a 11.4 max glide ratio at 106 mph (I think at gross, but couldn't confirm)

http://members.eaa.org/home/flight_reports/rv-6a.html? (EAA membership required)

Same report in PDF format....

http://cafefoundation.org/v2/pdf_apr/RV-6A%20Final%20APR.pdf

This works out to about 2.1 miles for 1000 ft. of altitude.

Round it down to 2 miles per thousand ft at about 100 mph... this gives a good estimate of engine-out range.

If it's into a 25 mph headwind, then a quick rule of thumb gives a 25% reduction at 100 mph, and reduces it to 1.5 miles per thousand ft. Flying a bit faster - try adding half of the headwind - will stretch the glide a bit...

These rule of thumb calculations are good for a quick decision if the worst case happens and the engine quits, and the "do I go for the airport 12 miles away, or go for the grass fields 3 miles away?" question must be answered, and quickly....

Working out the glide baseline and a target glide speed for your plane, is a good test and knowledge to have.... the wind effects can be quickly estimated on top of your baseline.

Miles per thousand feet AGL are probably the easiest to think of in a hurry, with elevations coming directly off the sectional chart. Add 1000 to 1500 ft for a pattern if possible to an unknown landing area.

gil in Tucson - preferring the 38:1 glide of my sailplane...

 

[gmcjetpilot]

Good way to think of it

If it's into a 25 mph headwind, then a quick rule of thumb gives a 25% reduction at 100 mph, and reduces it to 1.5 miles per thousand ft. Flying a bit faster - try adding half of the headwind - will stretch the glide a bit...

gil in Tucson - preferring the 38:1 glide of my sailplane...

Good one gill, that is the best way to think of it. I assume (with 9 to 1) about 1.5 nautical miles (1.7 sm) per 1000 feet, from there add/subtract for the winds and pattern. If ground speed is about 105 mph, that works out roughly to be 1.7 sm (1.5 nm), 1000 fpm vert speed. It's pretty convenient.

So at 8,500 feet - 1,500 ft (pattern) you get 7,000 feet or 7 minutes and 10.5 nm or about 12 sm. Absolute total glide distance about 14.5 sm (12.5 nm).

Probably way conservative for the higher ratio of the Cafe Foundation RV-6A but that had a fixed prop. Constant speed prop RV's come down faster at idle, since the fixed prop has less drag at idle with it's high pitch angle (verses c/s fine pitch). The drag is higher with constant speed props, engine out, not withstanding the conversation above about going course pitch if able.

 

[Kevin Horton]

A bit more info on test technique:

1. Fly only on days with smooth air at the test altitude. It should be smooth enough so you can keep the airspeed within 1 kt of the target. Expect to throw out a few test points until you have had some practice.
2. It is useful to have a second person along to record data and watch for other traffic. It is very hard to fly an accurate speed, watch for traffic, and record data.
3. Chose a test altitude block large enough so it will take at least 2 minutes to glide through the whole block. Use the same altitude block for each test.
4. Start the test a few hundred feet above the top of the altitude block, so you can be nicely stabilized on speed when you hit the top of the block.
5. Don't rely on the VSI. VSIs often have large errors. Instead, run a stop watch and record the altitude every 30 seconds, and record the time at the time at the bottom of the altitude block.
6. Changes in wind speed or direction with altitude can have a big effect on the results. To minimize the effect of wind, do the glides at 90 degrees to the predicted wind, being careful to hold a constant heading during the glide. Repeat each speed twice, with the second run at 180 degrees to the first one.
7. Do each run over the same altitude block. Record IAS, OAT, estimated fuel quantity (so you can later calculate the aircraft weight), and altitude every 30 seconds. Note any comments about the quality of each test point. This will help you later when you see that you have one test point whose results look out of line. If your notes say that the IAS varied on this point, or that the air was not smooth, you can legitimately ignore that test point.
8. Plot altitude vs time for each run. Draw the best fit straight line through the altitude vs time points, and measure the slope of the line to get the rate of descent for that run. Average the two results for each speed, and then plot rate of descent vs speed. Draw a line from 0 speed and 0 rate of descent, and the speed where that line is tangent to the plot of rate of descent vs speed is the speed for best glide.
9. If you want to figure out the glide ratio, you need to do some corrections to the raw data. Correct IAS to CAS (if you know the errors in your airspeed system). Correct CAS to TAS, using the altitude in the middle of the test block and the OAT for that altitude. Take the rate of descent at the best glide speed, and correct for non-standard temperature by multiplying it by (273.15 + OAT)/(273.15 + std temperature)[for OAT and std temperature in deg C] or (459.7 + OAT)/(459.7 + std temperature) [for OAT and std temperature in deg F]. Take the corrected rate of descent and the TAS, convert them to the same units, and you can calculate the glide gradient.
10. Do the test on at least two different days, so you can see if you have a repeatable result.
11. I recommend doing glide tests at a good range of speeds with the engine at idle. Then, pick a small number of speeds around the best glide speed, and repeat with the engine shut down to learn the differences in aircraft performance. Try both with prop windmilling, and with prop stopped. Obviously this would be done close to an airport with a suitably long runway, and after you have done a bunch of practice forced landing approaches to touchdown.

 

[az_gila]

Borrow from a glider pilot...

......

5. Don't rely on the VSI. VSIs often have large errors. Instead, run a stop watch and record the altitude every 30 seconds, and record the time at the time at the bottom of the altitude block.
......

One easy option is to borrow a digital barograph/flight recorder from a glider pilot friend.

Example here.... http://www.wingsandwheels.com/page11.htm

With a GPS antenna and a hook-up to your static, it will record GPS position, GPS time and altitude at a set rate... every 2 to 10 seconds would be good for this task.
Software exists to massage the data into vertical speed - glider pilots are very concerned with rate of climbs... ...

It would be more accurate than a second person, just use a tape recorder for time/airspeed/temp recording...

Of course, if you have a fancy recording EFIS, forget the above....

 

[hevansrv7a]

GPS Altitude

The Garmin x96 series can show digital VS. There is, of course, a slight lag, but it should be better than a baro VSI alone.

 

[az_gila]

Oops - Baro. Alt. not GPS Alt.

The Garmin x96 series can show digital VS. There is, of course, a slight lag, but it should be better than a baro VSI alone.

Oops..... I mixed up my sensors....
I wasn't clear on sensor input in the previous post about glider data loggers....

Since they are used for record and badge attempts, they are replacements for old-fashioned aneroid barographs.

I believe the FAI does not accept GPS altitude.... these units record Barometric Altitude, hence the static connection....

This is a much more accurate altitude measurement, taken every few seconds, and hence the rate of climb/descent will be more accurate...

gil in Tucson

PS .. remember gliders use variometers, not VSIs... way more sensitive and faster reacting....

 

[Kevin Horton]

I'm not familiar with the glider barographs, but they sound like they would be quite useful for this sort of testing. It is very, very helpful to have a good data record that can be studied later.

But, what ever means is used, it will be much more accurate to look at the rate of descent (or climb) over a long period, rather than rely on an instantenous indication that is calculated based on the last few seconds of performance. The instantaneous rate of climb or descnet will vary quite a bit, due to small pitch changes as the pilot chases the target airspeed, and due to small variations in the wind. The wind variations cause the airspeed to vary, and then the pilot has to change the pitch attitude to get back to the target speed, and this leads to short term variations in the rate of climb or descent.

And, if you are using recorded data, be careful to figure out before hand how you will sync up the data records to your hand written notes. I.e. how will you know which bit of the data represents a climb at which speed? The data logger should have a time record, I hope. If so, be careful to make sure your watch has the same time as the clock on the data recorder, and record the time for each run on your hand written notes. Then you can use the time to match your notes against the recorded data.

 

[n5lp]

I'm not familiar with the glider barographs, but they sound like they would be quite useful for this sort of testing. It is very, very helpful to have a good data record that can be studied later.

But, what ever means is used, it will be much more accurate to look at the rate of descent (or climb) over a long period, rather than rely on an instantenous indication that is calculated based on the last few seconds of performance. The instantaneous rate of climb or descnet will vary quite a bit, due to small pitch changes as the pilot chases the target airspeed, and due to small variations in the wind. The wind variations cause the airspeed to vary, and then the pilot has to change the pitch attitude to get back to the target speed, and this leads to short term variations in the rate of climb or descent...

The glider data recorders along with the flight analysis software (e.g. SeeYou) would be excellent for this. You can look at instantaneous performance or the average for any selected time interval. SeeYou is an astounding piece of software.

 

[az_gila]

GPS time....

I'm not familiar with the glider barographs, but they sound like they would be quite useful for this sort of testing. It is very, very helpful to have a good data record that can be studied later.

But, what ever means is used, it will be much more accurate to look at the rate of descent (or climb) over a long period, rather than rely on an instantaneous indication that is calculated based on the last few seconds of performance. The instantaneous rate of climb or descent will vary quite a bit, due to small pitch changes as the pilot chases the target airspeed, and due to small variations in the wind. The wind variations cause the airspeed to vary, and then the pilot has to change the pitch attitude to get back to the target speed, and this leads to short term variations in the rate of climb or descent.

And, if you are using recorded data, be careful to figure out before hand how you will sync up the data records to your hand written notes. I.e. how will you know which bit of the data represents a climb at which speed? The data logger should have a time record, I hope. If so, be careful to make sure your watch has the same time as the clock on the data recorder, and record the time for each run on your hand written notes. Then you can use the time to match your notes against the recorded data.

Kevin... in general, these data loggers would be used as "blind" recording devices. I was not suggesting reading any data off them while in flight.

The time they record is GPS time, should be accurate enough for our purposes.....

That's why I suggested a tape recorder for extra data, record the time, and it will be easy enough to "sync" the data later - e.g. "starting a 90 kt climb at 8:42, OAT 75F"

This is needed since the data logger records GPS ground speed, not airspeed...

Data smoothing is done by readily available software after the flight

This should be easy enough to do, and take a lot of effort off the test pilot. A lot of glider owners have these units, and can be asked for a loan, especially in the off-season.

I was going to loan mine to the last RV-9A that flew in Tucson, but he had a recording EFIS....

gil in Tucson

 

[Kevin Horton]

The time they record is GPS time, should be accurate enough for our purposes.....

Yes, as long as the aircraft clock is also set to GPS time.

That's why I suggested a tape recorder for extra data, record the time, and it will be easy enough to "sync" the data later - e.g. "starting a 90 kt climb at 8:42, OAT 75F"

Tape recorders are a nice concept, but they can prove problematic. Be sure to do a proper test of the tape recorder in flight before counting on using it for a test flight. I got a nasty surprise when I was at test pilot school and I decided to use a tape recorder on one flight instead of recording everything by hand. It turned out that I didn't have the tape recorder properly connected to the intercom, and it recorded nothing. 6,000 lb of JP4 converted into noise, and I had no data to show for it.

 

[jswareiv]

Thanks for all the help

I would like to thank everyone for all the great input. This gives me enough info to start the testing. What an education!

 

[az_gila]

Pens....

Yes, as long as the aircraft clock is also set to GPS time.


Tape recorders are a nice concept, but they can prove problematic. Be sure to do a proper test of the tape recorder in flight before counting on using it for a test flight. I got a nasty surprise when I was at test pilot school and I decided to use a tape recorder on one flight instead of recording everything by hand. It turned out that I didn't have the tape recorder properly connected to the intercom, and it recorded nothing. 6,000 lb of JP4 converted into noise, and I had no data to show for it.

Kevin ... good point on the tape recorder....

Perhaps a pen, paper and an old fashioned kneeboard would be better for the extra data....

gil in Tucson