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Using Technology to find numbers

N941WR

Legacy Member
A recent thread regarding verifying Vx, Vy, and Vbg had me thinking, can we use the technology in our EFIS's to help calculate these numbers?

For example Vbg should be easy to figure out. Get to altitude, dial in a lower altitude and observe where the "banana" is. The "Banana" is a mark that indicates where on the chart the airplane will be when it arrives at the programmed altitude.

The pilot then starts downhill and watch the "banana". If it moves further out, your glide range is improving, if it moves in, your glide range is decreasing. Whatever speed provides the furthest range is your Vbg.

Vy is similarly easy to discover using the autopilot by programming in different climb speeds and letting it fly the plane while you observe the vertical speed after each airspeed setting stabilizes. (I would suggest starting about 20 mph/kts below a "known" Vy and working 20 mph/kts past that number.)

How can you use the technology to figure out Vx and are there easy ways to find all three than what I have described?
 
I think that is brilliant for Vbg. I will try it next flight. Have had much trouble nailing down Vbg because it appears to be a fairly flat graph and noise in my test data makes determination difficult.
 
I can't remember if the Dynon SkyView displays a "Banana" when a higher altitude is enter but if it does, then that can be used to find Vx by simply going to full power and trying to get the "banana" as close to the aircraft symbol as possible.
 
That will probably give a reasonable approximation provided there is little/no variation in wind and you are at or close to a given altitude.

For my performance data, I relied on technology to know the wind direction and velocity and log flight data so I could concentrate on holding the airspeed within +/-1 knot. After landing, I extracted the data from the log and used it to determine the performance numbers I was interested in.

For the actual performance tests, I did sawtooth climbs/descents repeatedly at 60/75/90/105/120 KIAS at a number of different altitudes, noting the OAT and wind at each. That gave me Vy / Vbg at each altitude. I then fit a curve to each Vy/Vbg at different altitudes which let me extrapolate for any altitude I'm likely to be flying at up to Fl180.

Vx at a particular point was found by drawing a line from the origin tangent to the curve produced by finding climb rate at various speeds for each altitude.

I would imagine it comes down to how "right" you want the numbers to be and whether you want to know how much Vy changes with altitude.
 
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banana

will creep inward toward the airplane symbol as you get lower, so each speed for Vbg testing would have to be initiated from the same starting altitude to correct for this, unless you are referencing the position of the banana over a fixed point on the ground dead-ahead on course line, right?

Just guessing; I never got my banana airborne before the big fire, so no hands- on experience here yet.

Agree with letting the A/P fly a smooth course and airspeed for Vy data. "George" is a better stick than I am, in smooth air.

-Bill
 
Bill, for a constant decent rate and airspeed, the banana doesn't move, even as you get lower. Winds shouldn't impact finding the best speed, only the maximum glide distance.
 
Bill, for a constant decent rate and airspeed, the banana doesn't move, even as you get lower. Winds shouldn't impact finding the best speed, only the maximum glide distance.

It's a clever idea, and worth investigating Bill - but if the winds are changing as you climb/descend (which they almost always do), then the banana will move if you hold a constant airspeed - so you'll have to do the testing in a very narrow range of altitudes, and do it quickly!
 
I made a similar posting some time ago on the Dynon forum...

With autopilots and EFIS systems, it's technically possible to have automated flight testing.

E.g. Airspeed calibration, Vx, Vy, Vbg, magnetometers and so on.

The challenge I've always had is that mountains, variable winds, heavy traffic and airspace restrictions have always conspired to keep my eyes outside the cockpit and it has been difficult to maintain control precision for data gathering. Automating this is well within the capabilities of current EFIS-autopilot systems.

i'm sure there are some excellent algorithms that can take the data generated from some simple manoeuvres to generate the required information.

Throwing the gauntlet down to the EFIS vendors...
 
It's a clever idea, and worth investigating Bill - but if the winds are changing as you climb/descend (which they almost always do), then the banana will move if you hold a constant airspeed - so you'll have to do the testing in a very narrow range of altitudes, and do it quickly!

I was thinking of doing early in the morning or later in the evening, as a test. I'll let you know.
 
As noted earlier, I decided to try it on the next flight. I did a descent from 4500 down to 2000 while on approach to Hickory, NC (KHKY). There were puffballs and thermals and it was bumpy but even with that noise it was relatively easy to read and understand (and control) the position of the green banana.

Of course, all the parties posting about the banana position are correct from different points of reference. The airplane symbol obviously gets closer to the banana during the descent. But at the same time, at constant descent rate, the banana stays in a fixed position relative to ground indicators like terrain, airport or airspace markings.

So, even in the bumps, it was easier than I expected to "maximize" the banana position.

Looking forward to trying it on a glass smooth day (a rarity around here :()
 
Given the challenge of really getting accurate numbers Vx, Vy, Vglide, what would be a true downside of just adopting the CAFE performance numbers for the RV9/9a?
 
. Winds shouldn't impact finding the best speed, only the maximum glide distance.

Unfortunately, this is false.
The gps-based EFIS data will show you distance that can be covered over the ground, which in an emergency is what you want to know. Simple example: suppose in a no-wind situation you find best glide to be 70 kts. Now suppose you have a 70 kt headwind. Is best glide speed still 70? Of course not, you'd make no progress. 80 kts will be better, 90 knots even better,...somewhere around 100 kts will turn out to maximize the forward progress.
Best glide speed - the speed which maximizes distance traveled before hitting the ground - is always higher than the no wind best speed if you have a headwind, and less than no wind speed if you have a tailwind.

Edit: Also, don't forget best glide speed (for whatever the wind is that day) also varies with aircraft weight.
 
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Given the challenge of really getting accurate numbers Vx, Vy, Vglide, what would be a true downside of just adopting the CAFE performance numbers for the RV9/9a?

If your plane is the one from the CAFE report, then nothing. Otherwise, best case is you get slightly less performance from your aircraft than it is able to deliver. Worst case: you crash and die because you didn't quite make the field following an engine failure. More likely skews towards the best case though.

In all seriousness, getting reasonably accurate V-speeds shouldn't be a big deal.
It's not a challenge - not really - and this is exactly the kind of thing that should be done during phase one anyway.
 
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If your plane is the one from the CAFE report, then nothing. Otherwise,
best case is you get slightly less performance from your aircraft than it is able to deliver. Worst case: you crash and die because you didn't quite make the field following an engine failure. More likely skews towards the best case though.

In all seriousness, getting reasonably accurate V-speeds shouldn't be a big deal.
It's not a challenge - not really - and this is exactly the kind of thing that should be done during phase one anyway.

Hmmm, I have flown many airplanes which use the flight test data from the Manufacturer's flight testing and I didn't "crash and die" in them. You say getting "reasonably" accurate speeds should not be a big deal. Why is using the CAFE test data not "reasonably" accurate?
 
What is this "banana" of which you speak?

Dynon calls it the "Altitude Intercept Arc". I know it is the "banana bar". Shows up on the map view of the Skyview. Indicates the point on the map when you will reach your selected altitude when climbing or descending.
 
It depends. A lot!

In this reply I am only going to address best glide. Along with all the less obvious factors is the big one: what is the prop doing?

Here is an example. Look at the CAFE propless glide test for the C-152 and then compare to the Cessna POH which gave the numbers for a windmilling prop. Which one will you use when your engine fails? Did it stop the prop or let it turn? The POH gives about 69 mph but CAFE found about 76 mph. If the prop is stopped it will be less drag but still more than with either no prop or a zero-thrust prop (turning at just the right speed). The glide ratio changes a lot in agreement with those speeds. The ratio is 8.36 for the POH and 12.8 from the CAFE test. You can find all of this and more in the presentations I gave at AirVenture a few years ago and which are on my website.

The prop on the 152 is not much different in length than an RV's but it has less area because it "absorbs" less power.

CAFE said 106 mph is best glide for the 6A. Their numbers agree so closely with VAN's that you can work out that the best L/D for the 7A should be (zero thrust) 109 mph. I can supply anyone who is interested with the spreadsheet to show how this can be found. I can do the same for the 9A but haven't yet. A minor adjustment would give the results for the tail draggers, too. This is a little tedious but not difficult.

My point is that those numbers are very interesting but worth nothing when it gets too quiet up there.

There are some very easy ways to work out best angle and best rate with a good EFIS, too, but that is for another post.
 
Hmmm, I have flown many airplanes which use the flight test data from the Manufacturer's flight testing and I didn't "crash and die" in them. You say getting "reasonably" accurate speeds should not be a big deal. Why is using the CAFE test data not "reasonably" accurate?

Well, I did say it was the worst case. And the "in all seriousness" should have indicated that it was a tongue-in-cheek response.

Then again, I don't understand why some folks spend thousands of hours to build a plane but can't take an hour or two to find the actual V speeds during phase one. It really is pretty easy, and you have to fly those hours anyway.
 
Well, I did say it was the worst case. And the "in all seriousness" should have indicated that it was a tongue-in-cheek response.

Then again, I don't understand why some folks spend thousands of hours to build a plane but can't take an hour or two to find the actual V speeds during phase one. It really is pretty easy, and you have to fly those hours anyway.

I do understand you were not being completly serious. However, there does seem to be a strong opiniom that the CAFE testing doesn't apply. I am just curious why? I understand there are differences in Calibrated Airspeeds. But I am sure that many others like myself find that its difficult to get consistant repeated results in flight testing. Seems like my stall speeds are very consistant but not my Best Glide. I have seen as much as 5kts difference in results. I am just losing confidence in my testing method. I am planning on using the Skyview as described in the OP and see what I get.
 
I do understand you were not being completly serious. However, there does seem to be a strong opiniom that the CAFE testing doesn't apply. I am just curious why? I understand there are differences in Calibrated Airspeeds. But I am sure that many others like myself find that its difficult to get consistant repeated results in flight testing. Seems like my stall speeds are very consistant but not my Best Glide. I have seen as much as 5kts difference in results. I am just losing confidence in my testing method. I am planning on using the Skyview as described in the OP and see what I get.

The problem with your method is the wind. A 12 knot headwind will raise your measured-across-the-ground best glide speed by 5 knots. I suggest:
Load to gross weight. Put the airplane into a stabilized glide at 65 knots, and measure rate of descent. Repeat at 67, 69, 71, ..., up to 95 knots. Plot rate of descent (vertical axis) vs airspeed (horizontal axis). Draw a smooth line thru all the points. Take a straight edge, lay it along the horizontal axis, then rotate it about the origin until it just touches that curve. The airspeed at that point is the no-wind best glide speed.
 
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It really is pretty easy, and you have to fly those hours anyway.

Maybe its easier for some than others. In my case, the mountains make it very difficult because a calm day is very rare. I have run multiple Vy/glide tests with the stopwatch, plotted them on excel and come up with a jittery horizontal line that is about 20 knots wide. Useless.
 
What is this "banana" of which you speak?

The yellow arrow points to the "banana". It indicates the point you should be when you reach the altitude entered in the SkyView. In this example, 1800 feet.

Banana_zpsz9kxjssq.png
 
The problem with your method is the wind. A 12 knot headwind will raise your measured-across-the-ground best glide speed by 5 knots. I suggest:
Load to gross weight. Put the airplane into a stabilized glide at 65 knots, and measure rate of descent. Repeat at 67, 69, 71, ..., up to 95 knots. Plot rate of descent (vertical axis) vs airspeed (horizontal axis). Draw a smooth line thru all the points. Take a straight edge, lay it along the horizontal axis, then rotate it about the origin until it just touches that curve. The airspeed at that point is the no-wind best glide speed.
Bob, you are correct if you are looking for best rate but we are looking for best glide distance. Thus, shouldn't Vbg be the same number regardless of weight? What will change is the distance you will be able to glide at that weight and speed.

BTW, I did get up and do some testing and sure enough, using the "banana" does seem to work to find the various speeds. I climbed from 1100 to 10,000 in fairly stable air and used the autopilot to adjust my air speed in climb (not ROC). You could see the "banana" move closer or further away as I raised or lowered my airspeed off the target speed.

What I did find was that the CAFE numbers are good for my plane and that what I was using for Vy was too high. I also found that my cooling worked pretty good for such an extended climb at full power. I also found that descending from 11,000' at Vbg is kind of fun.
 
Like many I made the mistake of calculating Max Glide as Minimum Descent Rate. Which it is not. From what I have been reading, Max Glide should occur at a speed approximately 5kts above minimum descent rate. I am doing some reading in Aerodynamics for Naval Aviators right now trying to wrap my head around all this.
 
...Thus, shouldn't Vbg be the same number regardless of weight? What will change is the distance you will be able to glide at that weight and speed....

Bill, From the AFNA "An unbelievable feature of gliding performance is the effect of airplane gross weight. Since the maximum lift-drag ratio of a given airplane is an intrinsic property of the aerodynamic configuration, gross weight will not affect the gliding performance." Page 32.
 
best glide vs minimum sink

Like many I made the mistake of calculating Max Glide as Minimum Descent Rate. Which it is not. From what I have been reading, Max Glide should occur at a speed approximately 5kts above minimum descent rate. I am doing some reading in Aerodynamics for Naval Aviators right now trying to wrap my head around all this.

Yes, they are very different. In a no-thrust (not the same as zero thrust) situation, the speed for minimum sink will be approximately 76% of the speed for best glide. Said another way, best glide will be at a speed approximately 1.32 times the speed for minimum sink. The 5kts rule would only be true if the actual relationship were going to match it.

If best glide is 100 the minimum sink is 76. That's NOT 5 kts.

The angle of attack will be constant for best glide but the speed will vary with load, btw.
 
This best glide discussion has occurred before and is out there in the archives. Another easy method along with the banana is to make use of your garmin x96 which can be set up to display glide ratio on the page with alll the various parameters presented (press quit once from the moving map page to get there). When you start descending , your glide ratio is displayed. Simply trim up or down to see if it increases or decreases and note the airspeed where it is maximized. You now have best glide, with current winds accounted for. Easy peasy and ready for use in an emergency.

Erich
 
Yes, they are very different. In a no-thrust (not the same as zero thrust) situation, the speed for minimum sink will be approximately 76% of the speed for best glide. Said another way, best glide will be at a speed approximately 1.32 times the speed for minimum sink. The 5kts rule would only be true if the actual relationship were going to match it.

If best glide is 100 the minimum sink is 76. That's NOT 5 kts.

The angle of attack will be constant for best glide but the speed will vary with load, btw.

1.32. That seems fast. In the case of the RV9a CAFE testing they have min sinc of 81.7 MPH and best glide 95. Which is about 1.16.
 
CAFE numbers

1.32. That seems fast. In the case of the RV9a CAFE testing they have min sinc of 81.7 MPH and best glide 95. Which is about 1.16.

CAFE's test for those numbers was with an idling prop, not stopped, not zero thrust and not windmilling.

If they had done a zero thrust test like they did with the 6A, in my mathematical opinion, the best glide would have been about 84. At 1600# it's around 89. That's useless in real life, of course. Also, I used solo weight but they used approx. gross.

As for the relationship between minimum sink and best glide when thrust is not at issue, that simply results from how the curves intersect and sum at each speed. It's a pretty reliable "fact" and not specific to the 9A or even to RV's.

This link should get you a graph on a public folder in my OneDrive.
http://1drv.ms/1hMOhSY
Let me know if it does not.
 
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Thanks, Tony & Bill. I don't have skyview (stuck in the dark ages with a D100...) so didn't understand the terminology....

Greg
 
This best glide discussion has occurred before and is out there in the archives. Another easy method along with the banana is to make use of your garmin x96 which can be set up to display glide ratio on the page with alll the various parameters presented (press quit once from the moving map page to get there). When you start descending , your glide ratio is displayed. Simply trim up or down to see if it increases or decreases and note the airspeed where it is maximized. You now have best glide, with current winds accounted for. Easy peasy and ready for use in an emergency.

Erich

That is essentially what the banana is doing for you.

The cool part about the banana is in the event of an engine failure, you can dial in a lower altitude, if you have time, and quickly tell if you which options you will have.
 
CAFE's test for those numbers was with an idling prop, not stopped, not zero thrust and not windmilling.

If they had done a zero thrust test like they did with the 6A, in my mathematical opinion, the best glide would have been about 84. At 1600# it's around 89. That's useless in real life, of course. Also, I used solo weight but they used approx. gross.

As for the relationship between minimum sink and best glide when thrust is not at issue, that simply results from how the curves intersect and sum at each speed. It's a pretty reliable "fact" and not specific to the 9A or even to RV's.

This link should get you a graph on a public folder in my OneDrive.
http://1drv.ms/1hMOhSY
Let me know if it does not.

Got it. So n your opinion would it be best to derive best glide from min sink?
 
no

Got it. So n your opinion would it be best to derive best glide from min sink?

I am working on an article about that but the short answer is "NO". The commenter that pointed out that the Garmin handhelds show glide ratio is also correct and that is a very good way to go as it incorporates wind in the solution.

Otherwise, my Grand Rapids EFIS has a little indicator that shows where the airplane is going. I would fly the dot, so to speak. I think most good EFIS's have something that does the same. I would not spend any time dialing in anything because I would be in near-panic mode and looking for a safe place to land. I would not trust the banana, but that's just me.

That same dot is how you can find best angle of climb at a given moment. As for best rate, that is just feet per minute.

If you have an AOA you can find out what you should know before you need it. If you don't have an AOA my personal opinion is that you should. Airspeeds vary but AOA is very regular for critical speeds. Mine is pure analog, driven only by air pressure so when all my fancy electronics fail, that is how I will control my landing speeds.
 
A recent thread regarding verifying Vx, Vy, and Vbg had me thinking, can we use the technology in our EFIS's to help calculate these numbers?

For example Vbg should be easy to figure out. Get to altitude, dial in a lower altitude and observe where the "banana" is. The "Banana" is a mark that indicates where on the chart the airplane will be when it arrives at the programmed altitude.

The pilot then starts downhill and watch the "banana". If it moves further out, your glide range is improving, if it moves in, your glide range is decreasing. Whatever speed provides the furthest range is your Vbg.

Vy is similarly easy to discover using the autopilot by programming in different climb speeds and letting it fly the plane while you observe the vertical speed after each airspeed setting stabilizes. (I would suggest starting about 20 mph/kts below a "known" Vy and working 20 mph/kts past that number.)

How can you use the technology to figure out Vx and are there easy ways to find all three than what I have described?

As Bob Turner has pointed out, the glide speed to give maximum distance travelled over the ground is affected by wind.

Whether you can use the "banana" to find Vbg or not will depend on exactly how the EFIS manufacturer has programmed the "banana". Is it programmed using TAS or ground speed (GS)?

You could probably determine whether the "banana" uses TAS or GS as the speed reference by going up on a day with very strong winds at altitude. Establish a glide at a slow speed with the wind, and note how far ahead of the aircraft the "banana" is. Climb back up to the same altitude, and establish a glide at the same speed into the wind. Is the "banana" closer to the aircraft, or the same distance away? If it is closer into wind, then they used GS, so the "banana" accounts for wind. If the distance is the same, then it uses TAS, so it assumes still air.

If they used TAS as the reference, then it is the distance you could glide in still air - not great in the real world with winds, but perfect for flight testing. If they used GS as the reference, then the "banana" is useful in the real world, but it is not useful for flight testing, unless we account for the effect of the wind.

You could minimize the effect of the wind by doing the glides at 90? to the wind - i.e. if the wind is from 270?, do the glides heading 360? or 180?. There will still be a small effect from the wind, but it much less than if you were going 090? or 270?.

There is a range of speeds that give quite similar glide ratios, as shown by my flight test results, with prop windmilling (i.e. mixture pulled).

20091120204806777_1_original.jpg


My climb results also show a fairly wide speed range with similar rates of climb.
20081026191726701_1_original.gif
 
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