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Glide ratio

RV-6

No sure it matter that much which RV.......
but I use 1 mile per 1000 foot AGL.
Just a rule of thumb.
FYI...if you are trying to squeak out every last foot, you might consider looking into a AOA.
 
No sure it matter that much which RV.......
but I use 1 mile per 1000 foot AGL.
Just a rule of thumb.
FYI...if you are trying to squeak out every last foot, you might consider looking into a AOA.
Not sure if this is the intent for the OP question, but one of the potential reasons it would be good to know is due to Foreflight's latest update that uses the glide ratio along with best glide speed to display the glide range in the form of a ring around your location on the moving map display for your airplane. It graphically displays the glide range at all times should the need arise. It is a nice new feature but it requires knowing those two performance components of your airplane in order to configure it.
 
Garmin

The Garmin units take a little different data to program them.
The G3X touch we have asks for sink rate and speed. We are set up at 750'/ minute at 87 Kts. This was the mean average for Gross to as lite as we could fly. Keep in mind that they do fly 4-5 Knt. different on both ends of the weight range. Our system is all set up in Kts. not Sm./Hr. It is best to test your aircraft and get the numbers for that bird.
Hope this helps, Yours, R.E.A. III #80888
 
I think this is a situation where one should determine the numbers for oneself in one's own aircraft. Different props make a lot of difference.
 
The only way to know what your glide ratio really is, is to turn off the fan. In a real engine out situation the prop will probably continue to turn unless the engine seizes up or you slow enough to stop the prop. Your glide ratio will be less with it turning and higher glide ratio with it stopped. So you really need two glide ratios. And don't forget the effects of wind.
Alternatively, choose a lower than real number for planning purposes. Remember that unless you are as cool as John Glenn, you really won't have a lot of time to be calculating whether you can make it to that airport at 10 to 1 from your present altitude.
 
The GRT EFIS units look at your real, current rate of descent and your actual ground speed (so wind is taken care of) and show how far you can go in your current direction. Tells you right away if you can make an airport or other landing site, or not.
 
The GRT EFIS units look at your real, current rate of descent and your actual ground speed (so wind is taken care of) and show how far you can go in your current direction. Tells you right away if you can make an airport or other landing site, or not.

Bob, has it here. Most of the units out there now do all the figuring for you and will adjust the glide ring for wind, best runway, into the wind, surface of the needed runway, length, they will even tell you if you mite want to pass on the closest runway in order to get to a better choice. As has been said above it is best to pick a number that is on the conservative side after you have tested your aircraft in order to get your program numbers. " Don't round up- round Down". Also try to get your numbers with the aircraft as close to gross as you can. This will give you a little extra bonus distance if you have to glide at a lower weight. And just what we have found is it is not a bad idea to test you glide ring by putting an airport on the edge of the ring and pull the power, then see if you can get there safely. If not adjust you figures. As always it is not a bad idea to stay current and practice your engine out simulations. Just 2 pennies for the pot. Yours, R.E.A. III 80888
 
. . . Also try to get your numbers with the aircraft as close to gross as you can. This will give you a little extra bonus distance if you have to glide at a lower weight. . . . Yours, R.E.A. III 80888

Hi Bob, I was curious about this a found this:

"Variations in aircraft weight do not affect the glide angle provided that the correct airspeed is flown. Since it is the lift over drag (L/D) ratio that determines the gliding range, weight will not affect it. The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio. If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both aircraft will cover the same distance but the lighter one will take a longer time to do so."
Reference: http://www.skybrary.aero/index.php/Glide_Performance

As a practical matter, it would appear either AoA or known glide speeds vs weight should be documented for ones serial number - right?
 
In "Engine Out Survival Tactics", Nate Jaros quotes a glide ratio of 8:1 for an RV-8. That makes that 1.3 (k)mi per 1,000' a pretty good rule of thumb.

He also reaffirms that glide ratio is not affected by weight. Good book!
 
-8A Glide Ratio

I've done several semi-accurate glide tests (didn't shut the motor down) but had prop full low rpm, throttle at idle, 90 kts. Wind will have a significant effect.

I came up with an optimistic 1.5 miles per 1000 ft.

Procedures:
1) Get over the "What the **** just happened?" factor as soon as possible.
2) Turn toward intended landing spot while slowing.
3) Prop full decrease.
4) Trim for 90 kts.
5) Figure out the winds and recalculate / reevaluate decision.
6) Troubleshoot.
7) Recalculate/Reevaluate
 
Here is what mine does

Here is the glide performance for my RV-8: Flaps up, Throttle idle, Prop low RPM.
Nice thing is that the top of the curve is pretty flat from 90 - 110 KTAS so if you are a little fast or slow you don't get penalized that much. I tell ForeFlight that I glide at 100 knots with a 1000 fpm descent rate and this is a little on the conservative side. When practicing I prefer to fly a little faster than min sink, this way if I slow down my situation gets better.
One thing to remember if you are trying to calculate a glide ratio is that the forward speed has to be in units of True airspeed and you have to correct the vertical speed to tapeline (i.e. take into account the change in pressure altitude caused by non standard temperature).
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Cheer
Nige
 
When sailplanes race, they often carry water so that they go down their glide slope faster. They can jettison the water if the lift gets weak or cool off the crowd at the finish line.
If you calculate your glide speed with the engine running, even at idle, you will get a false glide ratio and sink rate. A windmilling prop causes significant drag that a prop at idle power will not simulate. So if you get 11:1 glide ratio at idle, you are likely to get no better than 8 or 9:1 in a true engine out windmilling situation.
 
Don't forget bank angle. If you have to do that 180 to get to a good landing spot and have to use a significant bank angle, best glide speed will increase and rate of decent will increase.
 
When sailplanes race, they often carry water so that they go down their glide slope faster. They can jettison the water if the lift gets weak or cool off the crowd at the finish line.
If you calculate your glide speed with the engine running, even at idle, you will get a false glide ratio and sink rate. A windmilling prop causes significant drag that a prop at idle power will not simulate. So if you get 11:1 glide ratio at idle, you are likely to get no better than 8 or 9:1 in a true engine out windmilling situation.

So - novice pilot/early builder here. Taking a stab at your logic... For me, I would want to measure the actual performance in the specific airplane I am documenting since no two RV's are the same. However, I don't know anyone crazy enough to actually turn the engine off in flight just to measure true power out glide performance.

So, my current plan when I finally make it to Phase 1 is to take my RV8 up fairly high (4,000' - 5,000') and cut to idle / low RPM and measure the distance and time to descend 1,000'. I would do this several times, in several different directions (noting the wind speed and direction each time). Then when back on the ground, I will average the figures together and subtract 10%-15% to account for actual engine out with windmilling prop. That's the number I would use for the POH.

Does that sound like a solid plan?

Thanks,
 
Damien,

I'd have the prop in high rpm for the tests; more conservative. If you have an engine failure with low oil pressure the prop will go to low pitch (high rpm).
 
Mine is 1:10 with an IO-360 fixed pitch Sensenich prop with the engine off at 90mph. Prop doesn't stop windmilling unless I get into a stall (and loose a lot of altitude doing so) so I wouldn't try to get my prop to stop if the engine quit on me and just take the 1:10 and be happy with it.

Oliver
 
The GRT EFIS units look at your real, current rate of descent and your actual ground speed (so wind is taken care of) and show how far you can go in your current direction. Tells you right away if you can make an airport or other landing site, or not.

What function is that? Where do you turn it on and off?
 
What function is that? Where do you turn it on and off?

IIRC it is just there - I don't think you can turn it off. Put your plane into a descent and look at the Hx's map page. Out in front of you you should see a green arc. That's where you'll hit the ground, if you continue at the same horizontal and vertical speed. If you make smooth adjustments to your speed while gliding, you can watch that arc move further away or closer to you, until you find the best glide speed for that day's weight and wind.
Edit. BTW, I use that arc all the time, e.g., to see if I'm at the proper descent rate to arrive at an airport not too low or high.
Edit. Please see post 25 for correction to this post, thanks, Dynon
 
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Hi Bob, I was curious about this a found this:

"Variations in aircraft weight do not affect the glide angle provided that the correct airspeed is flown. Since it is the lift over drag (L/D) ratio that determines the gliding range, weight will not affect it. The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio. If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both aircraft will cover the same distance but the lighter one will take a longer time to do so."
Reference: http://www.skybrary.aero/index.php/Glide_Performance

As a practical matter, it would appear either AoA or known glide speeds vs weight should be documented for ones serial number - right?

Thanks for the wright-ups. I can tell you we do sink faster when at gross Vs solo. It may have to do with me. I try to hit my air speed and stay on it after I use what is left to get that last little bit of climb from any speed above that. I do not try to very my speed per my weight, I just try to stay on it and work the problem and landing sights. So far I have not had to dead stick this one in, but it has been known to happen. Yours, R.E.A. III #80888
 
Does that sound like a solid plan?
Yes. Pick a calm morning day and a remote area away from mountains so you won't have updrafts and downdrafts. What I did was begin my descents at 5,500', stabilize at the test airspeed, and start the timer when passing 5,000'. You can double up and determine your best rate of climb speed on the way back up.
 
The variation in best glide speed is the square root of weight. A good approximation is to use half the percentage difference. E.g., if best glide at gross is 80 kias and you are 10% below gross, then reduce the gross weight number by 5%, or 76 kias. Remember too this is for no wind. Into the wind you'll want to increase the speed by 1/3 - 1/2 of the wind speed. In a lot of common training scenarios, these two effects cancel out. e.g., you may be 10% below gross, calling for a 4 knot reduction. But on final, you may have a 10 knot headwind, calling for a 4 knot increase.
If you try a few power at idle landings on a day with a 20 knot headwind, it's pretty easy to get set up where an approach at published best glide will bring you in short; but increasing airspeed by 7 or 8 knots will let you make the runway.
 
IIRC it is just there - I don't think you can turn it off. Put your plane into a descent and look at the Hx's map page. Out in front of you you should see a green arc. That's where you'll hit the ground, if you continue at the same horizontal and vertical speed.

In most EFIS systems, this arc shows when you will intercept the altitude bug, not hit the ground.

The thing lots of EFIS systems have is a ring around the plane all the time that shows if the engine failed, where you could glide to. This needs performance data for the plane as this ring is drawn even when you are climbing. Most of these systems also offset the ring for wind (if it's known).
 
When using the glide rings on Foreflight or EFIS, keep a few of things in mind. 1. The glide distance is based on the wind where you are now, so if you use the ring to make a decision on an emergency field from 10,000 feet, the winds will most likely be less and change direction as you descend. If you're counting on that tailwind you had at 10,000 you may not make it.
2. Also, the ring, if properly calculated, means that's where you flight ends. No room to maneuver. In my RV-6, 500 feet is not much if the engine ain't turning.
3. Don't be afraid to shut down the engine at altitude to get your numbers.
4. During cruise flight use the rings to visually help judge where your glide path would take you. For example, if I can see an airport in front of my nose, I probably can't make it. I have a reference for each wingtip as well. Then you have a good visual reference for making a cowpasture that's of course not on the efis.
 
I use the 'kiss' method (as I don't have all the fancy stuff) as there's too many variables on any given day. My 8 descends at between 700-800 FPM @ 75 kts with the donk windmilling (360/CS) so I use a loss of 800' with a dist covered of 1 NM. in a minute, that gives me a little something in the 'bank'. Circuit height is 1000' AGL so I never get more than 1.5nm away from the drome. I don't plan on a Rwy Ldg just reaching the clear surface area of a drone. If I'm cruising at say typical 4000 AGL I figure I have about 5mins (+ a little more to convert Crz speed to more heart beats!)
I build a mental picture in my mind which changes constantly when I'm flying, knowing the nearest dromes at any given time.
 
The only way to know what your glide ratio really is, is to turn off the fan. In a real engine out situation the prop will probably continue to turn unless the engine seizes up or you slow enough to stop the prop. Your glide ratio will be less with it turning and higher glide ratio with it stopped. So you really need two glide ratios. And don't forget the effects of wind.
Alternatively, choose a lower than real number for planning purposes. Remember that unless you are as cool as John Glenn, you really won't have a lot of time to be calculating whether you can make it to that airport at 10 to 1 from your present altitude.

As a CFIG, I teach the TLARs method for landing; practice (a lot) and learn where, when you're looking over the cowl, you'll reach the ground. Then add a serious fudge factor.
 
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