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Vx, Vy, Vg

Dorfie

Well Known Member
How exactly did you determine these numbers?
I would appreciate some of the numbers that were determined for the RV10.
Thanks.
Johan
 
I'm curious of this as well .. since there are a lot of differences between builds .. I'm assuming you just go figure it out by testing various speeds?
 
I'm assuming you just go figure it out by testing various speeds?
Exactly. It's a PITA, but it should really be done for each aircraft. Props, especially, make a big difference. Here's how I found the "best glide" on my plane, and the Vy and Vx are done the same way, only climbing. This is just the 80 KIAS run, and I also did them at 90, 100, and 110. My Garmin Virb allowed me to dictate the numbers into the Intercom and also see the numbers on the panel.

I did this particular test where there were mountains in the area, which was a bad thing; You never know if you are in an updraft or downdraft. Later, I reperformed the tests in a flat area on a windless day.

https://www.dropbox.com/s/dz98ah3d5x2k2h6/05-24-16 Best Glide_80Kts.mp4?dl=0

P.S. The "landing gear" warning was going off because I was below 90 Kts with my gear up. The "stall" warning was going off because I didn't have it set quite right (yet).
 
The first thing I'd suggest is that you find a local EAA Flight Advisor to chat with - he or she can help coach you through a real flight test program, provide good sources of information on precise flight testing methods and techniques, and probably help you to develop test cards. Box and Vy are numbers you legally must determine for YOUR particular airplane in order to sign it out of Phase 1, and it might take you a number of flights to refine them accurately - it all depends on teh conditions and your ability to fly accurately.

There are a number of good books on flight testing light aircraft - unfortunately, I'm not near my bookshelf at the moment to dig out titles. Check teh EAA store online. there are also FAA Advisory Circulars that will help.

But start with a Flight Advisor - if you're new to this (and it sounds like you are), they can give you a good start in a robust flight test program.

Paul
 
there are also FAA Advisory Circulars that will help.

Yup

Get your self a copy of Amateur Built Aircraft Flight Testing Handbook AC 90-89B HERE

It is not as in depth as some other references are but it is a good start towards getting familiar what is involved in a general flight test plan.
 
How exactly did you determine these numbers?
I would appreciate some of the numbers that were determined for the RV10.
Thanks.
Johan

Bootstrap to get close, then some additional testing at different weights and CGs to get closer, then development of mathematical models to account for different conditions (weight, density altitude, etc), then additional flight testing to validate the models.

The references listed previously are good starts.
 
How exactly did you determine these numbers?
I would appreciate some of the numbers that were determined for the RV10.
Thanks.
Johan

I got my license 35-years ago. Your subject line has Vg in it.

What is Vg? I am familiar with V speeds but not Vg.

Are you trying to determine a maximum gust speed? That would come from the designer
 
EAA has a Hints for Homebuilders video on finding Vy and Vx: http://bcove.me/8ch18ik8

Finding best glide is similar to finding Vy except you are recording distance covered for a specific amount of altitude loss (say 1000 ft) at a specific airspeed instead of the amount of time it takes to gain a specific amount of altitude (again say 1000 ft) at a specific airspeed.

This is one of those flight tests where the additional pilot program or an autopilot that has an IAS function comes in real handy.
 
EAA has a Hints for Homebuilders video on finding Vy and Vx: http://bcove.me/8ch18ik8This is one of those flight tests where the additional pilot program or an autopilot that has an IAS function comes in real handy.

It's not hard to hold an indicated airspeed to within one knot for 90 seconds of climb or descent given an electronic airspeed indicator and reasonable conditions. I doubt autopilots in our birds could do any better.
 
It's not hard to hold an indicated airspeed to within one knot for 90 seconds of climb or descent given an electronic airspeed indicator and reasonable conditions. I doubt autopilots in our birds could do any better.

Didn't say otherwise-- did it myself solo and my autopilot doesn't have an IAS mode. But you missed my point, which is having some help, whether human or mechanical, when trying to fly a precise profile, collect data, and look for traffic can be beneficial. IOW, it's not about having the autopilot fly a better profile, its about having the autopilot reduce the workload so the flight can be conducted safer and perhaps produce better results.
 
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Didn't say otherwise-- did it myself solo and my autopilot doesn't have an IAS mode. But you missed my point, which is having some help, whether human or mechanical, when trying to fly a precise profile, collect data, and look for traffic can be beneficial. IOW, it's not about having the autopilot fly a better profile, its about having the autopilot reduce the workload so the flight can be conducted safer and perhaps produce better results.

If you have a Skyview system - and I presume any of the other similar units - the task of collecting data has become trivial.

Just let the software do it and take the data home on a USB stick...:)
 
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For some ballpark numbers for your airplane you can look at the Pilot Operating Handbooks (POH's) that Doug has via a link on the home page. Another way to determine best glide is to record time to descend through a set altitude (1.000 or 2,000 ft for example) at various speeds and then calculate the ratio. I found it more accurate (and simpler) doing it this way rather than trying to determine a distance over the ground for each run. The references already mentioned are a good start. I also read Vaughan Askue's book which was of some limited help, particularly the performance chapter.

=dave=
N102FM
 
For some ballpark numbers for your airplane you can look at the Pilot Operating Handbooks (POH's) that Doug has via a link on the home page. Another way to determine best glide is to record time to descend through a set altitude (1.000 or 2,000 ft for example) at various speeds and then calculate the ratio. I found it more accurate (and simpler) doing it this way rather than trying to determine a distance over the ground for each run. The references already mentioned are a good start. I also read Vaughan Askue's book which was of some limited help, particularly the performance chapter.

=dave=
N102FM

After watching I had some major concerns over the EAA video referenced a few posts back.

I presume it came from a flat land area with moderate temperatures. :)

The video had no mention of temperature (hence Density Altitude) and of aircraft weight. These are both significant items that need to be taken into account and should used to "normalize" your performance numbers.

I am going to carefully examine the certified POH from my Tiger and try and try to extract some simplified mathematical corrections from it.

I'll start a new "Testing and Numbers" thread later tonight, so please post replies to the above on that thread.
 
Vx and Vy Debunked

Just to splash some gasoline on the fire, here's John Deakin's Vx and Vy Debunked. Deakin demonstrates that using a Vx or Vy climb (or Vx then Vy) really doesn't do much for you. He also brings up the "Carson Speed", the speed that uses the least fuel to a given altitude. It's Vy x 1.3. Personally, I think it's good to know all those numbers, although we seldom use them in real life flying. Besides, should my engine quit on takeoff, I'll be using my AoA indicator rather than airspeed.
 
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For best glide: if you measure distance over the ground you will have to account for any wind, to come up with the no-wind best glide speed. Remember the actual best glide speed in any situation depends on both wind and weight. I think it's more accurate to measure descent rates vs airspeed, and calculate glide ratios. Of course up or down drafts will affect this measurement.
 
I agree that differentiating between Vx and Vy can be seen as a mostly academic exercise in the RV family because of the already excellent rate of climb, which is also offset somewhat by cooling problems if the slower climb speed is maintained for any length of time.

The Carson speed is a cruise speed above max range that optimizes the excess fuel used vs the speed gained, or, as he put it, "the least wasteful way of wasting fuel." Here's a link to the paper for those interested:

http://cafe.foundation/v2/pdf_tech/MPG.engines/AIAA.1980.1847.B.H.Carson.pdf

Hurry up and start that new thread...

=dave=
N102FM
 
What the FAA says...

Just to splash some gasoline on the fire, here's John Deakin's Vx and Vy Debunked. Deakin demonstrates that using a Vx or Vy climb (or Vx then Vy) really doesn't do much for you. He also brings up the "Carson Speed", the speed that uses the least fuel to a given altitude. It's Vy x 1.3. Personally, I think it's good to know all those numbers, although we seldom use them in real life flying. Besides, should my engine quit on takeoff, I'll be using my AoA indicator rather than airspeed.

Maybe debunked, however the FAA requires us to know those speeds and sign off on a statement including those speeds at the end of Phase I testing -

Compliance with FAR 91.319(b) must be recorded in the aircraft records with the following, or a similarly worded, statement: “I certify that the prescribed flight test hours have been completed and the aircraft is controllable throughout its normal range of speeds and throughout all maneuvers to be executed, has no hazardous operating characteristics or design features, and is safe for operation. The following aircraft operating data has been demonstrated during the flight testing: speeds Vso ______, Vx ______, and Vy ______, and the weight ______ and CG location ______ at which they were obtained.”

Admittedly a bit wishy-washy on actual loading conditions (is it for Gross Weight and Aft or Forward CG positions?) and altitude conditions but a sign off is legally needed.
 
...the FAA requires us to know those speeds and sign off on a statement...
Well yeah, we all need to do that. Thankfully, the best glide curve on my plane is almost flat between 90 KIAS and 100 KIAS so I won't have to obsess about my airspeed while I'm looking for an emergency landing spot and trying to get the engine running again. :eek:

Funny story: I was taking a BFR a few years ago in a C-172 and the CFI had forgotten to do the "emergency landing" portion. So he decided to pull the power while I was in the pattern abeam the numbers. So I set up to go right in and land, and the guy basically screamed at me, "NOOO! You have to set up your best glide speed!!!" What I should have set up was my Minimum Sink Speed (which I think is 60 KIAS in a C-172) so I would have maximum time to troubleshoot the problem, but this guy was "by the book".

Y'all be careful out there, y'heah?
 
Originally Posted by BobTurner

For best glide: if you measure distance over the ground you will have to account for any wind, to come up with the no-wind best glide speed.

Thankfully, Foreflight Glide Advisor does that for me. :D

ForeFlight isn't as smart as a sailplane final glide calculator/display which will give you speed-to-fly...:)

It will give distance based on altitude AGL and corrected for wind, but it won't give a speed to fly.

Fly faster into the wind and slower with the wind for best "true glider" range. Luckily, if the winds aren't too strong and our RVs best glide is high enough it won't make too much practical effect.

Page 5-10 of this FAA Glider document shows it in a nice graphical format.

https://www.faa.gov/regulations_pol...s/aircraft/glider_handbook/media/gfh_ch05.pdf
 
Fly faster into the wind and slower with the wind for best "true glider" range. Luckily, if the winds aren't too strong and our RVs best glide is high enough it won't make too much practical effect.
rl]

In a lot of training situations there's an interesting cancellation: Less than gross weight, calling for a lower speed than the published (no wind, gross weight) 'best glide'; and a headwind on final, calling for a higher speed than published.
 
Everything said here is true but,to a certain extent, over-thinking things. What you really don't want to do is fly slower that your best glide - a few knots faster doesn't really make much difference. So my Vbg is technically 85kts but I always aim for 90kts during PFLs. The penalty is small but the benefits are:

- more margin from the stall if you have to manuoevre agressively
- helps account for any headwind
- avoids getting the wrong side of the drag curve

As for working out the speeds - simples:

- climb for a minute at 10kt interval speeds between 70 and 120kts
- plot the rate of climb against IAS
- the top of the curve gives best rate of climb - Vy
- tangent from the origin to the curve gives best angle of climb - Vx

Do the same for glide:

- bottom of curve gives minimum sink
- tangent from origin gives best glide - Vbg

And unless you are some sort of OCD test pilot, this gives numbers accurate enough for our practical purposes - no disrespect to TPs intended!
 
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ForeFlight isn't as smart as a sailplane final glide calculator/display which will give you speed-to-fly...:)
Thanks for that Glider link. There's a wealth of information in there, some of which I had forgotten... like the part about adjusting your best glide speed for headwind/tailwind. The chart on page 5-11 has a note "With flaps set to ?8 degrees, sink rate does not change significantly but airspeed increases." I can reflex the flaps on my plane -7 degrees, and this reminds me that I need to re-perform my best glide tests with the flaps reflexed rather than in trail.
 
Do the same for glide:

- bottom of curve gives minimum sink
- tangent from origin gives best glide - Vbg
!

And the tangent drawn, not from the origin but, say, 20 knots to the right of zero, will give you the best glide speed with a 20 knot headwind.
 
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