Van's Air Force
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Gliding speeds
- Thread starter John F
- Start date
John,
Each airplane is unique, particularly with regard to pitot/static systems; so the only way to know for sure is to flight test your airplane. There are some good techniques described in AC 90-89A "flight testing experimental aircraft." Kevin Horton's site on the web is an invaluable source of good information regarding calibrating your airspeed system (as well as applying other flight test techniques and procedures to RV's) and a quick google for "bootstrap flight test" will produce some very helpful work by Dr. John Lowry.
For a baseline, take a look through the pilot's operating handbooks in that section on this site. This will give you a couple of examples of the work that other folks have done and might give you a point of departure from which to work.
You can drop a private line if you'd like to discuss it more.
All the best,
Vac
Each airplane is unique, particularly with regard to pitot/static systems; so the only way to know for sure is to flight test your airplane. There are some good techniques described in AC 90-89A "flight testing experimental aircraft." Kevin Horton's site on the web is an invaluable source of good information regarding calibrating your airspeed system (as well as applying other flight test techniques and procedures to RV's) and a quick google for "bootstrap flight test" will produce some very helpful work by Dr. John Lowry.
For a baseline, take a look through the pilot's operating handbooks in that section on this site. This will give you a couple of examples of the work that other folks have done and might give you a point of departure from which to work.
You can drop a private line if you'd like to discuss it more.
All the best,
Vac
There's only one way to find out!
It's called "phase I flight testing" as stated in the operating limitations for your airplane.
It's called "phase I flight testing" as stated in the operating limitations for your airplane.
hevansrv7a
Well Known Member
Another Suggestion
Not to blow my own horn (honk!) but I do recommend that you go over my 2010 AirVenture presentation (PowerPoint) that you can find on my website. The 2011 one is good, too, but they really need to be taken in the correct order.
There are some techniques there that are helpful, I think. If you find them interesting, please feel free to PM or email for help or just to talk it over.
Kevin Horton is the "real deal" - a professional test pilot, a very generous person. I don't know Mr. Lowry, but I think there are some flaws in the "bootstrap" approach you will find in his article(s). Those flaws are the result of not having a way to deal with prop drag or residual thrust. That is why my stuff is worth a look. I could use your help improving it, too!
I will be at AirVenture this year, but don't know if I'll be presenting or not. I am always happy to meet and discuss at either the HomeBuilder's Hangar or the Beer Tent.
Not to blow my own horn (honk!) but I do recommend that you go over my 2010 AirVenture presentation (PowerPoint) that you can find on my website. The 2011 one is good, too, but they really need to be taken in the correct order.
There are some techniques there that are helpful, I think. If you find them interesting, please feel free to PM or email for help or just to talk it over.
Kevin Horton is the "real deal" - a professional test pilot, a very generous person. I don't know Mr. Lowry, but I think there are some flaws in the "bootstrap" approach you will find in his article(s). Those flaws are the result of not having a way to deal with prop drag or residual thrust. That is why my stuff is worth a look. I could use your help improving it, too!
I will be at AirVenture this year, but don't know if I'll be presenting or not. I am always happy to meet and discuss at either the HomeBuilder's Hangar or the Beer Tent.
Best Glide
I went out today and did some test glides in the RV-4. With flaps up and prop at full fine I descended a thousand feet and timed it. I did two runs of each 180 degrees from each other and the greatest variance was 2 seconds, probably within my margin of error. The following was the result:
Airspeeds (MPH): 70, 75, 85, 93
Average Seconds: 56.5, 60, 57, 50
Rate of Descent (FPM): 1062, 1000, 1053, 1200
I tried to work this out based on Kevin's info to plot speed versus rate of descent, and then draw a straight line from 0,0 to where it hits the curve tangentially, but could not work this out. Looks like I need to do a couple more tests at 100 and 110.
Just for grins I also did a two way test with full flaps at 70 mph and it took an average of 61.5 seconds to descend 1000 feet for a rate of descent of 976 FPM. I did one test with the flaps retracted but the prop set to full coarse. BIG difference. Took 97 seconds to descend 1000 feet for a rate of descent of 619 FPM. On this test the rpm dropped off to idle rpm ~700.
Tach time for the flight was .9, and I was 5 minutes (10 - 15 miles) from the airport.
FWIW
I went out today and did some test glides in the RV-4. With flaps up and prop at full fine I descended a thousand feet and timed it. I did two runs of each 180 degrees from each other and the greatest variance was 2 seconds, probably within my margin of error. The following was the result:
Airspeeds (MPH): 70, 75, 85, 93
Average Seconds: 56.5, 60, 57, 50
Rate of Descent (FPM): 1062, 1000, 1053, 1200
I tried to work this out based on Kevin's info to plot speed versus rate of descent, and then draw a straight line from 0,0 to where it hits the curve tangentially, but could not work this out. Looks like I need to do a couple more tests at 100 and 110.
Just for grins I also did a two way test with full flaps at 70 mph and it took an average of 61.5 seconds to descend 1000 feet for a rate of descent of 976 FPM. I did one test with the flaps retracted but the prop set to full coarse. BIG difference. Took 97 seconds to descend 1000 feet for a rate of descent of 619 FPM. On this test the rpm dropped off to idle rpm ~700.
Tach time for the flight was .9, and I was 5 minutes (10 - 15 miles) from the airport.
FWIW
pierre smith
Well Known Member
Just do it.
More of you guys should do what Mark did...set up a speed to descend at with the prop in fine pitch and carefully record the VS rate of descent.
Then, repeat the test at the same speed BUT with the prop knob pulled all the way back to coarse pitch and maintain the same airspeed as before. You'll note a much shallower pitch attitude and a greatly reduced vertical speed.
I checked out a CFI on Saturday morning in my -10 and showed him this technique as well. He was amazed at the difference. Knowing how your airplane reacts to either condition and the effects, can greatly help you in an emergency forced landing situation and can make the difference between a painless end result...or ?
Best,
More of you guys should do what Mark did...set up a speed to descend at with the prop in fine pitch and carefully record the VS rate of descent.
Then, repeat the test at the same speed BUT with the prop knob pulled all the way back to coarse pitch and maintain the same airspeed as before. You'll note a much shallower pitch attitude and a greatly reduced vertical speed.
I checked out a CFI on Saturday morning in my -10 and showed him this technique as well. He was amazed at the difference. Knowing how your airplane reacts to either condition and the effects, can greatly help you in an emergency forced landing situation and can make the difference between a painless end result...or ?
Best,
smokyray
Well Known Member
Your mileage may vary...
John,
All excellent inputs above. I have been blessed to fly many different RV's on a regular basis. For what it's worth I recently flew my 215th different RV yesterday on a delivery, they all vary slightly as to best glide. My other experience is based on flying a single engine Fighter (F16) for 25 years with one successful (actual) flame-out landing.
My technique, for what it's worth comes from my 1989' RV4 builders manual (go figure). I dislike V speeds even though I use them for a living. AOA is more user friendly when the feces hits the proverbial fan. http://www.seqair.com/skunkworks/Instrumentation/AngleOfAttackInd/AngleOfAttackInd.html
1. Establish best glide speed 100 MPH. (top of yellow AOA)
2. Locate emergency field and determine if you can safely reach it.
3. Use a flaps up approach until landing assured.
My addendum to that is to practice beforehand. First, read your builders manual and logbooks where they tested the airplane as Mel suggested. You can garner good information there.
My 170HP RV4 with FP prop would glide 10 miles for every 5000' of altitude using an 87 Knot Glide speed. That shows true for most stock RV's. Of course C/S props reduce that to less than half with a flat pitch (loss of oil pressure)
Next time you are out boring holes in the sky, try pulling power and setting your prop coarse as Pierre suggested. You will quickly find through experimentation that your glide speed will be somewhere around 87 Knots, give or take a few for the wife and kids.
V/R
Smokey
PS: You'll find that there are not many new questions on this forum, just new people. This question has been around for awhile with many threads you can search. http://www.vansairforce.com/community/showthread.php?t=83170&highlight=glide+speeds Remember, if your engine quits in level flight at cruise speed, your first act is to trade airspeed for altitude, intercepting best glide speed and establishing it while using GPS or visual lookout to locate an emergency field.Since you have a constant speed prop, worst case is loss of oil pressure which makes your prop go to flat pitch, greatly reducing glide distance. Having done many actual engine out scenarios over an uncontrolled airport in my HR2, I can assure you it is a significant disadvantage if you are not ready for it.
John,
All excellent inputs above. I have been blessed to fly many different RV's on a regular basis. For what it's worth I recently flew my 215th different RV yesterday on a delivery, they all vary slightly as to best glide. My other experience is based on flying a single engine Fighter (F16) for 25 years with one successful (actual) flame-out landing.
My technique, for what it's worth comes from my 1989' RV4 builders manual (go figure). I dislike V speeds even though I use them for a living. AOA is more user friendly when the feces hits the proverbial fan. http://www.seqair.com/skunkworks/Instrumentation/AngleOfAttackInd/AngleOfAttackInd.html
1. Establish best glide speed 100 MPH. (top of yellow AOA)
2. Locate emergency field and determine if you can safely reach it.
3. Use a flaps up approach until landing assured.
My addendum to that is to practice beforehand. First, read your builders manual and logbooks where they tested the airplane as Mel suggested. You can garner good information there.
My 170HP RV4 with FP prop would glide 10 miles for every 5000' of altitude using an 87 Knot Glide speed. That shows true for most stock RV's. Of course C/S props reduce that to less than half with a flat pitch (loss of oil pressure)
Next time you are out boring holes in the sky, try pulling power and setting your prop coarse as Pierre suggested. You will quickly find through experimentation that your glide speed will be somewhere around 87 Knots, give or take a few for the wife and kids.
V/R
Smokey
PS: You'll find that there are not many new questions on this forum, just new people. This question has been around for awhile with many threads you can search. http://www.vansairforce.com/community/showthread.php?t=83170&highlight=glide+speeds Remember, if your engine quits in level flight at cruise speed, your first act is to trade airspeed for altitude, intercepting best glide speed and establishing it while using GPS or visual lookout to locate an emergency field.Since you have a constant speed prop, worst case is loss of oil pressure which makes your prop go to flat pitch, greatly reducing glide distance. Having done many actual engine out scenarios over an uncontrolled airport in my HR2, I can assure you it is a significant disadvantage if you are not ready for it.
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hevansrv7a
Well Known Member
Use GPS to find best glide
As has been covered on previous posts by several of us, the Garmin handheld GPS's have a glide ratio field that displays it in real time. It's a real ratio of ground speed to sink speed and is affected, of course, by wind.
First, I suggest you use it to determine what your airplane will do when the noise stops. If you also have an AOA, this is the best way, IMHO, to find out what it should read for best range (also for minimum sink if that is wanted). If you don't have an AOA, then the Garmin can tell you what you need to know when you really need to know it.
This is a little arguable, but no amount of testing with your engine at minimum power will give you the same result as a dead engine and I strongly suspect that the glide will be worse when dead (unless the prop stops, but read Kevin Horton's posts about that).
As has been covered on previous posts by several of us, the Garmin handheld GPS's have a glide ratio field that displays it in real time. It's a real ratio of ground speed to sink speed and is affected, of course, by wind.
First, I suggest you use it to determine what your airplane will do when the noise stops. If you also have an AOA, this is the best way, IMHO, to find out what it should read for best range (also for minimum sink if that is wanted). If you don't have an AOA, then the Garmin can tell you what you need to know when you really need to know it.
This is a little arguable, but no amount of testing with your engine at minimum power will give you the same result as a dead engine and I strongly suspect that the glide will be worse when dead (unless the prop stops, but read Kevin Horton's posts about that).
Tomas J
Active Member
Glide ratio for my RV-4
I've made some test of gliding my RV-4. You can download the result here...
https://dl.dropbox.com/u/3745995/WAF/gliding RV-4.kml
Open the file in Google Earth. Click on the track with right mouse button and choose "view elevation profile"...
You see three downhills. First is flown at idle and speed 85 kt, second at idle and 80 kt.
The third is at 80 kt with stopped engine by full leaning. Prop windmilling (fixed pitch metall, Sensenich 70CM759-0-81, IO320-160).
No flaps in any of the examples.
As you can see the third curve is steeper. That was what I have expected, but it's intresting to see how much.
You find the glide radio below. Not big difference between 80 and 85 kt at idle. But with the engine stoped it's a lot lower. That is something to think about if the engine really stop.
Glide ratio
Speed 85 kt, no flaps, engin at idle, prop vindmilling = 12,8:1
Speed 80 kt, no flaps, engin at idle, prop vindmilling = 13,3:1
Speed 80 kt, no flaps, engin stopped, prop vindmilling = 9,2:1
These values are specific to my aeroplane and my flight. Do not use them as anything other than an example. I take no responsibility for the declared values. You have to try out what applies to your own airplane.
I've made some test of gliding my RV-4. You can download the result here...
https://dl.dropbox.com/u/3745995/WAF/gliding RV-4.kml
Open the file in Google Earth. Click on the track with right mouse button and choose "view elevation profile"...
You see three downhills. First is flown at idle and speed 85 kt, second at idle and 80 kt.
The third is at 80 kt with stopped engine by full leaning. Prop windmilling (fixed pitch metall, Sensenich 70CM759-0-81, IO320-160).
No flaps in any of the examples.
As you can see the third curve is steeper. That was what I have expected, but it's intresting to see how much.
You find the glide radio below. Not big difference between 80 and 85 kt at idle. But with the engine stoped it's a lot lower. That is something to think about if the engine really stop.
Glide ratio
Speed 85 kt, no flaps, engin at idle, prop vindmilling = 12,8:1
Speed 80 kt, no flaps, engin at idle, prop vindmilling = 13,3:1
Speed 80 kt, no flaps, engin stopped, prop vindmilling = 9,2:1
These values are specific to my aeroplane and my flight. Do not use them as anything other than an example. I take no responsibility for the declared values. You have to try out what applies to your own airplane.
