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RV-8 engine failure glide ratio

donaziza

Well Known Member
I'm kind of embarrassed I don't know this::(. Everyother airplane I ever flew, they gave you the best glide ratio and what speed to obtain it in the event of an engine failure. So I set out to figure it out myself in my 8. I always thought 1 nautical mile was 6000'. Turns ot it's actually 6076.115 feet.:eek:

Anyway there are you guys out there who are way smarter than me. I've got XM Sat Wx on my GPS and it shows the winds. So I went out experimenting, today, by trying out simulated engine failures flying 90* to the winds ( no headwind or tailwind), throttle closed, and CS prop full decrease RPM. ( A glide ratio of say 9:1 means for every 9000' you go forward, you lose 1000' in altitude, right?--( Told you I was dumb). Lets forget the 6076. 115 Ft--let's just say 1 NM = 6000'. So if I go 2 NM or 12000' horizintally, and lose 1000' in the process, that's a 12:1 glide ratio,right??:confused:

If my thinking on how this works is correct, I found the recommended 71KIAS glide speed is a recipe for disaster. About 84 KIAS seems to give the best results--BUT-- of course the slowest speed possible at the bottom is life saving. ( I know--speed depends on weight 1518 Lbs. fully loaded for me-- but I'm just looking for some education)

HELP:
 
Best LD

Don

To measure the best LD (Glide Ratio) forget about your GPS, all you need is the ASI & VSI.

There are a couple of points to watch.

Make sure there is no thermic activity or wave or ridge lift about as this can dramatically alter your results.

Then set your power as you have done, fly at different speeds and note the relevant VSI rates, do the maths and choose the the best glide speed to give the best distance.

A point of interest that will probably not make much differece in an RV is that the loading will make a difference. A lower all up weight will have a lower glide speed for the best distance compared with a higher weight. (That is why gliders carry water ballast).

I would like to know what you work it out to be.

Happy testing
Rob
 
My RV-3 fastback glides at 10.7 L/D @ 85-90mph with prop stopped. Weight will not make any measureable difference in the L/D, more weight moves the best L/D speed higher, but does not lower the glide ratio. For nit-pickers there is a very,very small increase in L/D at the higher weight due to operating at a higher Reynolds number. Also, it is better to error on the side of to much speed than less for distance. In gliders they add more speed if you're flying into the wind. Check out "speed to fly" as it applies to gliders for more than you will ever want to know.
 
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The glide performance is affected by the prop, so it is not possible for Van's to predict the exact glide performance of your aircraft.

Forget about using the VSI. VSIs are often inaccurate, and even an accurate VSI will not show a constant value as you descend at a constant IAS, due to the affect of small air movements, wind changes, etc. Use the altimeter and a stop watch. Pick an altitude band of 2000 or 3000 ft, and start timing as you descend through the top of the band, and record the time when you hit the bottom of the band.

Small changes in wind will have an affect, even if you think you are at 90 degrees to the wind, as the wind direction will not be the same at all altitudes. Pick a heading that is roughly 90 degrees to the wind, and do each speed once on that heading, and repeat each speed on the reciprocal heading (i.e. 180 degrees out). Average the results from the two runs at each speed. For the absolute best data quality, do the tests either early shortly after sunrise or shortly before sunset, when you will find the best air conditions.

The results will vary depending on whether the engine is at idle, or shutdown with prop windmilling, or prop stopped. If you lay awake at night worrying about the dreaded throttle cable breakage that leaves your engine stuck at idle, then do the testing with the engine at idle. If the condition that worries you is an engine failure, then do the testing with the mixture pulled to ICO. Obviously you would want to be above a suitable airfield, with adequate altitude to allow a forced landing in the very unlikely event that you cannot get the engine restarted.

I recommend you practice the test technique with engine at idle until you can get repeatable results. Then try a couple of engine shut downs and restarts at altitude, above a suitable airfield, to confirm you have a good restart procedure (it should be as simple as slowly pushing the mixture forward, assuming the prop is windmilling). Then do the actual testing with mixture ICO, throttle idle and prop control full aft.

The results on my RV-8 showed a best glide speed of about 104 kt IAS with mixture ICO and prop windmilling at 1600 lb weight. The best glide ratio was about 8.75:1 at that speed. I'm claiming 8.5:1 and a best glide speed of 110 kt at 1800 lb weight in my POH. The glide performance was much better than that with prop stopped, but it was so hard to get the prop to stop that I would not try that in a real emergency unless I was above 15,000 ft AGL.

More details including plots of my results are on my web page.
 
RV 8 engine failure glide ratio

First off with all due respect to rv8or, I've tried checking my VSI against my ASI. From 71 Kts to 105 Kts, I've always gotten 800 to 1000 fpm rate of descent. Maybe mine is not very accurate.

With regard to Mr. Horton, Kevin, I was pretty sure you'd be one of the guys to answer this as I've noticed you're very sharp on math. Unfortunately with me, you're talking to a math dummy. When you say your glide ratio is 8.5:1, that "does" mean 8500 feet forward for 1000 feet down, correct? Then, as you said, supposing it takes me 45 seconds to glide and lose the 1000 feet of altitude.
I don't know what to do with the 45 second info mathmatically to figure out my best glide speed.

When I did do my GPS DME testing yesterday, I did it as you said, fly 80* & then the 180* recip of 260*, both 90 degrees to the wind according to my XM Sat Wx and also smoke on the ground from a big fire. Did them all several times.

Regarding stopping the prop, I've heard of this both from you and also a friend of mine with an RV 6. I've been up to 15000' with mine, but only for a minute due to no oxygen. ( Would love to see how high I can get it someday)
 
Visualize it

To help visualize the relationship between glide ratio and glide speed and other factors, try using this spreadsheet:
http://home.cogeco.ca/~n17hh/Models/GlideRatio&AngleWithCruisePower.xls
Just plug in Kevin's numbers in the appropriate spots.


It was designed to use best glide speeds with a "transparent" prop, but the triangle will be equally valid with the drag of a windmilling prop. It won't project to other speeds correctly, though.
 
This has all been hashed through before, but I still maintain that the easy way, if available, is to set up your Garmin x96 to show glide ratio on the flight parameters page. Then fly whatever direction you want under whatever conditions exist and move your nose up and down until the displayed glide ratio is maximized. Clearly, the airspeed where it is maximized is different under different wind speeds, directions, etc. What will the conditions be when your life may actually depend on achieving best glide ratio? If you dont know, it would seem that the GPS display may be very handy to refer to. Of course, a few practice sessions will tell you the 'typical' range of appropriate airspeeds so you can quickly get it in the ballpark...

erich
 
RV 8 engine failure glide ratio

Mine's a Garmin 396. I looked in the book. I don't think I have a "Flight Parameters" glide ratio page.
 
Here is simple math that you can remember during an emergency.......

For every 1000 feet above the ground, you have one minute till touchdown.
 
...I've got XM Sat Wx on my GPS and it shows the winds...

I often see the statement on this site that XM Weather "shows the winds" so think I lot of people must really believe that.

To be clear, XM Weather does not show the winds. It shows the same wind forecasts that you get from your favorite pilot weather briefing site.

All of us know how reliable weather forecasts are! Well wind forecasts, in my experience, are at the bottom of the heap.
 
With regard to Mr. Horton, Kevin, I was pretty sure you'd be one of the guys to answer this as I've noticed you're very sharp on math. Unfortunately with me, you're talking to a math dummy. When you say your glide ratio is 8.5:1, that "does" mean 8500 feet forward for 1000 feet down, correct? Then, as you said, supposing it takes me 45 seconds to glide and lose the 1000 feet of altitude.
I don't know what to do with the 45 second info mathmatically to figure out my best glide speed.
Tell you what - send me the following data, and I'll crank out a spreadsheet to analyze it for you:

Data that is common to all tests:
Altimeter setting
Starting altitude for all descent points
Ending altitude for all descent points
average aircraft weight for all tests
ASI units (kt, mph, km/h, furlongs per fortnight, etc)
Altimeter units (ft, m, etc)

For each speed you tested:
IAS or CAS
OAT at the top of the altitude block
time to descend through altitude block on run 1
time to descend through altitude block on run 2 (with heading 180 degrees from run 1)

I'm in the midst of a big test program, working long hours for another week or so, so it might take me a few days to provide the promised spreadsheet that analyzes your data. But, if you provide the data, I will provide the spreadsheet, and I will make it available to everyone else.
 
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When doing these glide tests something to consideris that you are determining a geometric ratio. Neither IAS or indicated altitude give you 'real' feet or nm. If the goal is glide ratio you need to plot true airspeed versus tapeline rate of descent.

1000' of altitude on your altimiter is only 1000 feet if you are at ISA temperature. To convert from indicated altitude to tapeline altitude go to the middle of your test band, stabilize for a couple of minutes and read the OAT. Then do your timed descents/glides and record the time to descend through 1000' on the altimeter. Post flight you can convert to tapeline altitude by multiplying the change in altitude (in this example 1000') by the ratio of the actual temperature at your test altitude to the ISA standard temperature for that altitude. Note temperatures must be absolute (Kelvin). So lets say you are going to test between 5500' and 4500' the middle of the test band is 5000'. The ISA temp for 5000' is 5 deg Celcius or 278 K. For example lets say the actual temperature was 20 deg Celcius or 293 K. The actual altitude you would use in your rate of descent calculations would be 1000*293/278 = 1054'. Or about 5% greater than what you measured with your altimeter.

Similarly you need to conver your indicated airspeed to true. Plenty of online calculators or an E6B will do this for you, but continuing the example above if you were descending at 80 KIAS at 5000' and 20 deg C this would actually be 89 KTAS.

If this mythical descent took 90 seconds, or 1.5 minutes, your rate of descent would be 1054'/1.5 min = 703 fpm, not the 667 fpm you would have calculated if you did not do the tapeline correction.

In the same 1.5 minutes you would have travelled 88*(6076/60)*1.5 = 13,367' forwards. 13,367 forward/703 down = 19:1 glide ratio.

If you had used 1000' in 90 seconds at 80 knots you would have gotten 18.2:1. i.e been in error by about 5%.

To calculate a glide ratio to the second decimal place if you have a 5% error to start with is being precisely inaccurate. The figures used in this example are fictional and are not meant to represent any actual aircraft.
 
RV 8 engine failure glide ratio

To Kevin Horton, OK Kevin, I'll get these numbers for you, but I can't fly before Feb 23 anyway, so that fits in with your busy work schedule. These threads will probably have timed out here on Vans Air Force by then, so I'll just send you a private message.

To "bkilby", I've been all thru my 396 for the last 30 minutes. Darned if I can find it. I see you are only 30 flying minutes from me on the other side of ATL. Why don't you send me a private message, and maybe I can just fly by you with my 396.
 
Don

From memory, as best I can:

Skip through the various pages using either the page button or the quit button until you get what Garmin refers to as the Position Data Page. it typically has stuff like your latitude and longitude, date and time, speed and altitude, etc. But no map, just numbers.

Get in the cell where you would like guide ratio to appear.
Hit the menu button and choose the option that let's you change the data fields. Scroll through the long list of possible data fields til you find glide ratio and select it. You should be good to go.

There are tons of useful and interesting other data fields to consider as well, so have fun playing with those as well and setting it up however you like it.
The vnav feature is a good one too, but that's different topic.

Erich
 
What Erich said... I just looked at the garmin manual for the 496 and it says it's on the Position Data Page. It's on p.45 of the 496 online manual. Probably the same as the 396 manual.
 
8 to 1

This subject interested me and thanks to Erich I found the Glide ratio on the 396 and went out and flew it today, about 8 to 1 is the best it will do with the engine cut off and the prop pulled all the way back, indicated air speed of 80sm (that?s about 100sm true in my plane) that?s 1000 FPM on the VSI. Boy cutting off the engine turns it into a brick compared to the engine at idle! I did not do multiple tests in multiple directions or conditions, one thing I did notice was you can tell just buy the seat of your pants what the best speed is.
 
one thing I did notice was you can tell just buy the seat of your pants what the best speed is.

So true.

Another simple concept: Assuming VFR conditions and an engine out, find your desired landing area, point your nose in that direction, establish best glide. If your landing spot starts moving down your windscreen over time,you have it made; if it moves up, you aren't going to make it. If its position stays relatively stable, its going to be close. This is all intuitively obvious when you are flying, but for some reason a lot of folks can't verbalize this when asked how to judge this.

Erich
 
one thing I did notice was you can tell just buy the seat of your pants what the best speed is.

So true.

Another simple concept: Assuming VFR conditions and an engine out, find your desired landing area, point your nose in that direction, establish best glide. If your landing spot starts moving down your windscreen over time,you have it made; if it moves up, you aren't going to make it. If its position stays relatively stable, its going to be close. This is all intuitively obvious when you are flying, but for some reason a lot of folks can't verbalize this when asked how to judge this.

After all, in the end, do you really care whether you flew at best glide the whole time, or just whether you made it to the landing area?
Erich
 
RV 8 engine failure glide ratio

I'm sure everyone answering here realizes, but I originally ask the question: OK the engine just quit, the "Nearest" airport on the GPS is X miles away. Doing some quick mental math if you know your glide ratio--"Can I make it" or do I look for the nearest flat field. (Hope I'm not offending anyone)
 
I'm sure everyone answering here realizes, but I originally ask the question: OK the engine just quit, the "Nearest" airport on the GPS is X miles away. Doing some quick mental math if you know your glide ratio--"Can I make it" or do I look for the nearest flat field. (Hope I'm not offending anyone)

Your not being offensive at all and I guys maybe your question was not exactly answered, the answer is yes, 9 to 1 = 9000 feet across the ground for 1000 feet loss in altitude assuming there is no wind helping or hurting you. Or it means 9 feet forward for 1 foot down.
 
Glide Ratio

Don

you are correct most aircraft VSI's are not very accurate. I must admit I'm used to accurate glider VSI's and I don't I look at the aircraft VSI much except when towing gliders and they are poor.

Keven mentioned to time it across a height band and I agree as this is the best way.

But Kevin can you explain how flying across the wind direction makes a differece to the numbers.

Happy Flying
Rob
 
But Kevin can you explain how flying across the wind direction makes a differece to the numbers.
It would make no difference at all if the wind speed and direction were the same at all altitudes. But, in the real world the wind speed and direction do vary as the altitude changes. For a simple example, imagine that you are gliding directly into wind, and there is a sudden increase in wind speed of 10 kt. This has the effect of causing a sudden 10 kt increase in the IAS, and you must pull the nose up slightly to get back to the target IAS. The rate of descent is lower for some time until you regain the target IAS and lower the nose to establish the correct pitch attitude to hold a constant IAS. The time required to descend through the altitude band will be greater than it would have been if the wind was constant.

If we now repeat a descent at the same IAS, but on the reciprocal heading, we now have a tailwind, and there will be a sudden 10 kt loss of IAS when we hit that sudden change in wind speed. We'll need to lower the nose to regain the target speed, and this will cause the rate of descent to increase for a time. The time required to descend through the altitude band will be less than it would have been if the wind was constant.

If we average the rates of descent we calculated on the two runs, the average value will be pretty close to what it would have been in a world with constant wind.

Wind changes have an effect, even if they are gradual. Doing the runs at 90 degrees to the wind minimizes the effect. Doing two runs on reciprocal headings and averaging the results further minimizes the effect of wind changes on the results.
 
Don, another easy way to quickly look at it is that with a 9 to 1 ratio you will loose one mile in height for nine miles across the ground, if you’re at 11000agl you could possibly depending on wind reach an airport 18 miles away. Once you pick your target I like Erich’s way of then determining weather you will get there or not based on weather your target is moving up or down in your wind screen.
 
RV 8 engine failure glide ratio

Russ, I'm so glad you brought this up-- 9 miles forward, 1 mile down. That's why I kept asking "Is it feet"? I kept racking my brain thinking I had learned miles, but it was so long ago, 1964, I had forgotten. In the 1970's and 1980's when I occasionally rented planes, there was no such thing as GPS or "nearest airport", you just looked for the nearest flat spot. Now we've got this ghee whiz stuff, and don't get me wrong, it's fantastic equipment. I've done 2 trips out to California with it, that I never would have done the old way.

Which brings me to the point for writing this thread. I finally found "glide ratio" in my 396, but I also found "glide to target". So I called Garmin and asked what's the difference between the two. The guy said ( and you can't make this work in Demo mode unless you mechanically lower your altitude setting--you actually gotta do it), once you hit "nearest airport", it will give you the glide ratio you need to make it to that "target" on the "glide to target" part. Then the "glide ratio" part will show what you're actually doing. This then becomes a mechanical way of doing what Erich was talking about, ie if your actual glide ratio is at or above what your "glide to target" is calling for, your gonna make it. If not, you're not. Now how good all this works or doesn't work, I haven't had a chance to go out and practice yet--but it sounds really neat.

Trying to punch in "glide ratio' & "glide to target" "after" the engine failure would be an exercise in futility--you've got to have them there all the time. This means I had to give up "distance next" and "ETE next" on my 396, but I've got the luxury of having a 2ed GPS, ( an older KMD 150). If one has only one GPS, one has to decide what they like best on permanent display.
 
I saw that to, instead of giving up something just leave the map screen alone and put glide ratio and ratio to target side buy side on the screen that just shoes the numbers etc. in blocks, I don?t normally look at that screen anyway so nothing lost, if I had the need to see these numbers I would just take a look at that screen.

This brings up an other thing we have talked about on hear not to long ago, I like my 396 but it sucks because it only shoes public use airports, in my travels over the western states I see there are a lot of good runways that are not on the 396.

Your ratio applies to any measure you like, 9 to 1 is the same weather it?s miles, sm or km or feet or inches, it does not manner and it all works out to be the same distance, but for our use it?s miles, either sm or km that we need to know at a glance.
 
Trying to punch in "glide ratio' & "glide to target" "after" the engine failure would be an exercise in futility--you've got to have them there all the time. This means I had to give up "distance next" and "ETE next" on my 396, but I've got the luxury of having a 2ed GPS, ( an older KMD 150). If one has only one GPS, one has to decide what they like best on permanent display.

Don:

Like Russ was saying - have the glide numbers set up on the Position Data Page where there are many cells available to hold them, not on the map page, where there are only a few and you really need to be choosy. Assuming you generally keep the screen on the moving map page during flight, a single button push (either 'page' or 'quit', I forget which) will get you to where you can view the glide info.

Another tip: really nice to have the lower left box on the Position Data Page set up to indicate the NEAREST AIRPORT (check the menu here). This will then give you distance AND direction to the airport and makes for brainless, error-free position calls on the radio when approaching unfamiliar airports.

regards

erich
 
Maybe not, Russ

.

...This brings up an other thing we have talked about on hear not to long ago, I like my 396 but it sucks because it only shoes public use airports, in my travels over the western states I see there are a lot of good runways that are not on the 396.

.

My 496 shows private strips as well!

Try this Russ: Press "menu menu," then scroll down to "Points", and scroll to the right to go into that area. Press menu and an option box appears that lists airport criteria...one of them is "Private/grass strips" or something similar. Next is runway surface and length. For my -6A and now my-10, I put in 2500'. Doing this added 4 airports that are grass to the first page of the "Nearest" function:).

Regards,
 
My 496 shows private strips as well!

Try this Russ: Press "menu menu," then scroll down to "Points", and scroll to the right to go into that area. Press menu and an option box appears that lists airport criteria...one of them is "Private/grass strips" or something similar. Next is runway surface and length. For my -6A and now my-10, I put in 2500'. Doing this added 4 airports that are grass to the first page of the "Nearest" function:).

Regards,

I?ll take another look but I?m not optimistic, even Garmin says the 396 does not support privet airports.
 
The 495/496 has private airports.

Ok I tinkered around with the 396 again today and you can turn on private airports in the nearest page as well as select them on the map page but there a complete no show, there just not supported on the 396.
 
V-glide testing

Sunday was a glorious day for blue sky flying. Didn't have any real objective so I decided to do Vg testing.

RV-8, Solo, Fixed Pitch Whirlwind prop, Idle engine, 20-15 gallons of fuel, decend thru 5000' to thru 3000' at 100knots, 90 knots, 80 knots, 70 knots and let the Skyview record the data.

The results are different than I've read for other RV's, i.e. best Vg is much slower than I expected, probably the prop at idle???
Glide Ratio (forward/down)
100 knt - 8.9
90 knt - 10.1
80 knt - 10.5
70 knt - 11.0

Can anyone confirm similar results on your FP RV?
 
One thing everyone should keep in mind is that wind has a tremendous effect on best glide speed. Best glide with a 30 knot tailwind might be 70. With a 30 knot headwind it might be 120. The garmin does provide a great way to sort that out.
Recently I flew down the SC/GA coast. Wanted to stay within gliding distance of the beach since the wife was with me. At 3000 ft the winds were 45 knots from the west. At 10.5 they were 70 knots from the west. Slowing to 80 knots in that situation would not be the best move.
George
 
Bud,

What parameters did you use for your calculations?

George,

What does wind have to do with it? The airplane doesn't know if the wind is blowing. VBG is the Best power-off glide speed – the speed that provides maximum lift-to-drag ratio and thus the greatest gliding distance available. Wind will affect the ultimate distance, but not the airspeed at which you will attain that distance.

Don
 
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What does wind have to do with it? The airplane doesn't know if the wind is blowing. VBG is the Best power-off glide speed ? the speed that provides maximum lift-to-drag ratio and thus the greatest gliding distance available. Wind will affect the ultimate distance, but not the airspeed at which you will attain that distance.

Don

The wind matters if you re-define best glide speed to be the airspeed where you cover the most distance over the ground, per 1000' of altitude loss. In the real world that is what matters. A simple extreme example:
Suppose your no-wind best glide speed is 80 kts. The engine quits, and there's an airport 100 yards away, but it is upwind, and the wind is blowing at 82 knots! If you hold 'best glide' of 80 you'll never make it, but hold 120 knts TAS and you will.
In the real world a headwind of 15 knots calls for increasing your no-wind best glide by about 7 knots.
 
The wind matters if you re-define best glide speed to be the airspeed where you cover the most distance over the ground, per 1000' of altitude loss. In the real world that is what matters. A simple extreme example:
Suppose your no-wind best glide speed is 80 kts. The engine quits, and there's an airport 100 yards away, but it is upwind, and the wind is blowing at 82 knots! If you hold 'best glide' of 80 you'll never make it, but hold 120 knts TAS and you will.
In the real world a headwind of 15 knots calls for increasing your no-wind best glide by about 7 knots.

But Best Glide Speed was NOT redefined. I submit that if you are at the maximum distance and altitude to Just make a field at Best Glide Speed, no other speed will get you there. At greater than VBG you'll get to the ground sooner, and at less than that you'll reach the ground later, but in either case you won't get there. Your example makes sense only in the extreme and with sufficient excess altitude to trade for a positive ground speed.

Maybe we should transition to the downwind turn discussion;)

Don
 
Glider guys sorted this out a long time ago with McCready and his ring.

This animated .gif shows how polars and speed to fly alters with head/tail winds - just ignore the lift/sink bit.

polar7.gif


In this case, 'Speed to Fly' is using your glider most efficiently - distance travelled for minimum height loss.

The whole article here might be well worth reading.

http://www.5c1.net/Glider Performance Airspeeds.htm
 
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I really hate to get into these arguments but I also dislike seeing mis-leading information being propagated - or are we saying the same thing and I don't understand what you are trying to communicate?

I think most people define best glide speed as the speed which will enable you to cover the most distance over the ground, following a loss of power from some given altitude.

But what the value of that speed is, and how far you can actually go, varies, depending on weight*, prop, and wind. Look in any certified POH. It will say "...prop windmilling, gross weight, no wind" when stating the VBG.

The fact that my example was extreme (and therefore easy to visualize) does not make it any less relevant.

If you want to see the physics look up a "Power required vs true airspeed" plot. In a glide, the power required comes from gravity, so it is proportional to the vertical speed. For no-wind, draw a straight line from the origin so it just touches the curve, at the shallowest possible angle. That line is proportional to the angle of the glide, and the speed where it touches the curve is VBG for those conditions. For a headwind of say, 10 knots, move the starting point of the line right to 10 knots (so the horizontal axis is now ground speed) and repeat. For a tailwind, move the starting point to the left, and repeat.

BTW, for a tailwind you want slightly less than no-wind best glide, but never less than minimum sink speed. The extreme, easily visualized example here, is that if you have a hurricane on your tail, you want to stay aloft as long as possible (minimum sink) and let the wind blow you along.

*the optimum speed varies with weight, but as someone else noted, the no-wind distance traveled at optimum speed does not. But add a headwind, and the heavier plane wins over the lighter one, which is why, again already noted, gliders engaged in racing can carry water ballast (used if the first leg is into the wind).

Edit: I see Gil 'published' something similar to the power required curve while I was typing
 
I yield....it's kinda like conservation of angular momentum. You know it exists, empirically and mathematically, but I could never draw you a picture of it.
 
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I can't add much more to what has been posted except that wind also effects your optimum range best AS in powered flight also. The computer figures it out in my day job and speeds up into the wind and slows down with a tailwind to generate max range. In my non computer days we had a rule of thumb on how much to speed up going into the wind. My tired old brain can't remember it now.

George
 
...But add a headwind, and the heavier plane wins over the lighter one, which is why, again already noted, gliders engaged in racing can carry water ballast (used if the first leg is into the wind).

Edit: I see Gil 'published' something similar to the power required curve while I was typing

Well, it doesn't really take a headwind, just reasonably strong conditions to need water ballast for best cross country speed. Where I live, on most competition days gliders will carry enough water to take them to maximum gross weight (or maximum contest rules weight) until right before landing. Only if they are in danger of landing out will most people dump water.

In olden days before each glider was weighed at the start of each day, people were known to cheat by loading saltwater into their gliders, rather than fresh water, for more weight. It really is astounding to experience the huge performance increase caused by carrying more weight in a glider.

I enjoy telling looking at power pilots when I tell them about how the increased weight helps the performance so much. It isn't intuitive to most people but is something we need to understand to understand our aircraft.

The essential thing is that best glide ratio is essentially the same with increased weight but the best glide angle occurs at a higher speed with increased weight.
 
Sunday was a glorious day for blue sky flying. Didn't have any real objective so I decided to do Vg testing.

RV-8, Solo, Fixed Pitch Whirlwind prop, Idle engine, 20-15 gallons of fuel, decend thru 5000' to thru 3000' at 100knots, 90 knots, 80 knots, 70 knots and let the Skyview record the data.

The results are different than I've read for other RV's, i.e. best Vg is much slower than I expected, probably the prop at idle???

There is almost certainly thrust from a FP prop at low speed, with the engine running at idle. The amount of thrust will be higher at low airspeed. The thrust from the engine at idle will affect the measured glide ratio and the airspeed for best glide ratio.

Another possible factor is the effect of wind, unless you used specific test and data analysis techniques to remove the effect of wind from the calculations. The method of calculating glide ratio is also a possible explanation of these results. Some people use some very creative, but incorrect, methods to calculate glide ratio.

Which data from the Skyview did you use to calculate glide ratio for each of those airspeeds?
 
FLY THE AIRPLANE!

Having just been through this experience awhile back, but at 8-900' AGL, one thing I might add as a caution is that I would be very careful about trying to fiddle with your GPS in an engine out situation while you are also trying to figure out why the engine quit...!

FLY THE AIRPLANE!

Don't mean to insult anyone's intelligence, but I learned a lot about how one can get distracted and almost stall the plane...

So I just thought I'd throw that out there to keep in mind.

-Ryan in Madison, WI
(RV-6 Glider pilot, and former sailplane pilot)
 
I just calibrated my AFS 4500 AOA indicator on my EFIS. The AOA manual claims it can show best glide. Curious if anyone has ever tested this with one of these things?
 
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