What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Vy and Vx testing using EFIS logging

Draker

Draker

Well Known Member
RV-7A, IO-360-M1B + Hartzell CS Prop

I've done about 4 hours of sawtooth climbs and idle-power glides in an attempt to determine the important V speeds.

I'll start with my test method.

For the climbs, I used this method: Target a particular density altitude (in this case, 4,500). Start about 1,000-1,500 feet below target altitude. EFIS does the density altitude calculation for me so I know what indicated altitude to start at. Set autopilot to IAS mode and establish test airspeed. Apply full power, highest RPM, and full rich mixture, and "help" the autopilot with a little control stick force. I found my AP servo could hold within ± 3kts, and with some "nudging" I could help the servos hold ± 1kts. Level off about 1,000 above target altitude. Let things cool off a bit and then do a glide test.

At home, I find a 30-60 second sample of data from the logs with uniform airspeed and with 4,500 as the midpoint altitude, subtract starting density altitude from ending density altitude, and divide by how long it took to fly through that block. I'm ignoring the VSI data, although I found the average indicated VS for each climb to be pretty close to the end-start/time calculation.

Same method for the glides. Pick a pressure altitude, start a bit above that, set AP to IAS mode, power to idle, prop pulled fully out. Then divide altitude lost by time, at my computer at home to find sink rates.

For the best angle calculations (Vx and best glide), I converted IAS to TAS (since I believe vertical speed is a true airspeed number) and used inverse tangent to find the angles.

Climb Results

Vy is tough to call. Configured at 1600 pounds weight, 4500ft density altitude, my rate of climb curve is essentially flat, about 1475f/m at all airspeeds between 73-93 KIAS. This might be expected with a CS prop. Tests were done full rich though. I should probably be leaning for best power at that density altitude.

Vx is basically "as slow as I'm willing to climb." The airplane will climb at pretty much any reasonable angle you command it to fly.

Neither Vy or Vx seem to be very relevant to actual flying, since CHT is ultimately the limiting factor, and prevents me from sustaining these climb airspeeds.

Glide Results

Best idle-power glide is also flat: 10:1 ratio at all glide airspeeds between 75 and 95 KIAS. Tests were at 1600 pounds and pressure altitude of 3500ft.

Best idle-power sink seems to be around 70 KIAS (788f/m), but I want to do a few more runs to confirm it.

So far, kind of disappointing to not be able to plot those nice, obvious airspeed-vertical speed curves that the flight testing docs show. I think I simply need to do more testing and gather more data. Or change something about my test method.
 
Last edited:
Your test methods are sound, and the results about what we expect for a short-wing RV. Yes, best rate of climb is virtually flat from about 95 - 125 knots, so cruise climb as you wish. If you want to know the best ROC, definitely do it leaned for best power!

I find best glide to be a narrower range with the Rv’s, with noticeable variation in sink rate on the plus or minus side of Best glide, which is in the mid90’s. You might try hand flying it, as our AP’s wander a few knots up and down, which definitely affect the results.

Good for you to do actual testing - keep posting your processes and results for others to see that it is fun and important to do!

Paul
 
Drake,
Love the testing and your write up. Methods seem sound. I have not done it yet but like your ideas.

Lots of power and short wing! Point it where you want to go and poor on the gas!

Toolman
 
Ironflight said:
Your test methods are sound, and the results about what we expect for a short-wing RV. Yes, best rate of climb is virtually flat from about 95 - 125 knots, so cruise climb as you wish. If you want to know the best ROC, definitely do it leaned for best power!

I find best glide to be a narrower range with the Rv’s, with noticeable variation in sink rate on the plus or minus side of Best glide, which is in the mid90’s. You might try hand flying it, as our AP’s wander a few knots up and down, which definitely affect the results.

Good for you to do actual testing - keep posting your processes and results for others to see that it is fun and important to do!

Paul
Click to expand...

He suggests Vy at 73 to 95 not 95 to125 ?
 
revenson said:
Have you considered the Lowry Bootstrap Approach?
Do tests at one density altitude and use his spreadsheet to get reasonable data at any other set of conditions.

https://www.avweb.com/features_old/...epart-two-constant-speed-propeller-airplanes/
Click to expand...

I like it! From my quick read of the article and part 1 it looks like I would need to at least confidently measure best-glide speed (in order to obtain some important parameters in the model). But this approach would help predict performance at other altitudes and weights.

EDIT: Looking closer at the spreadsheets, it looks like Lowry's constant-speed prop method claims to predict climb performances (Vx, Vy) at all altitudes and weights, without performing a single climb test. Simply using detailed measurements of the shape of the prop and fuselage, rated HP/RPM, and the C-sub-D0 and e parameters that come from doing a glide. That's astonishing.

For those looking for the spreadsheet, this Kitplanes article has working links.

Another method that looks interesting is described by this Kitplanes article called Using Level Accelerations to Determine Climb Performance. I would be interested in seeing if all of these different methods resulted in V-speeds that were similar enough to each other.
 
Last edited:
Draker said:
I like it! From my quick read of the article and part 1 it looks like I would need to at least confidently measure best-glide speed (in order to obtain some important parameters in the model). But this approach would help predict performance at other altitudes and weights.

For those looking for the spreadsheet, this Kitplanes article has working links.

Another method that looks interesting is described by this Kitplanes article called Using Level Accelerations to Determine Climb Performance. I would be interested in seeing if all of these different methods resulted in V-speeds that were similar enough to each other.
Click to expand...

Please share your results when your done
Thanks
 
Flight testing....

Draker said:
RV-7A, IO-360-M1B + Hartzell CS Prop

I've done about 4 hours of sawtooth climbs and idle-power glides in an attempt to determine the important V speeds.

I'll start with my test method......

So far, kind of disappointing to not be able to plot those nice, obvious airspeed-vertical speed curves that the flight testing docs show. I think I simply need to do more testing and gather more data. Or change something about my test method.
Click to expand...

It has been fun and enlightening to read about your flight testing! You are DOING IT RIGHT! You are actually going out and finding the data you need for YOUR airplane and you as a pilot! And plotting your curves! Nice job! Keep it up!

You have also brought back memories of my own flight testing. I was so excited to get out there and start I had my test cards made up months before SuzieQ was actually finished! And that was in the day you had to feel your way through what a test program would look like. There were few places to copy what others had done. When I was not building or relaxing, I would review and modify them. Even after flight testing started I was modifying them to match the data I was getting and what I needed to get. I went beyond the required number of test flight hours because I wasn't done collecting data!

It was SO COOL! I felt like a real test pilot.....because.....I WAS! I took the task very seriously because I wanted to know more about my brand-new airplane! I still look at my data cards and smile.....:)

I will soon be going back to testing as I modified the fuel system (filters) and will need the data on fuel flows and how that might affect the way the airplane flies. I am actually excited to do that again! Not completely back to Phase 1 but.......:)

Keep it up! And keep posting your progress. What you are posting is actually as important as reading about building as that will inspire those who have not completed their project to keep building...and start their own flight testing! And as an inspiration to do it right!
 
Does anyone who downloaded the Bootstrap2 excel spreadsheet back when the article was published have a clean copy of it you can upload here? The links appear broken.
 
I used the Lowry Bootstrap approach to airplane performance when I was doing the Phase One testing for the Pietenpol Air Camper that I built.

Once I had established best glide, I was able to populate the spreadsheet with that parameter, plus other information about the airplane (horse power, wing area, etc.). It then generates predictions for Vy at various altitudes and Vx.

After that, the goal is to confirm those numbers through lots of climb sequences. It was remarkably accurate.

For anyone interested, you can access the Bootstrap spreadsheet under the Files tab of the Pietenpol Facebook page. I modified it slightly to include mph in addition to knots. The Facebook group is a closed group, but if you tell the moderator you like Piets you'll be added right away :)

Cheers, Ken
 
You must log in or register to reply here.
Back
Top