Glide testing and V speeds

This entry is part 7 of 7 in the series 16 - Flight Testing

One of the tasks during Phase I flight testing is to determine the significant flight speeds.  Things like Vx (best angle of climb), Vy (best rate of climb) and Vso (stall speed in landing configuration).

Stall speed is easy… especially in the Velocity.  Just keep going slower and slower until the canard stalls.

The climb speeds were a bit of work though. Basically you pick a starting altitude (I chose 4,000′) and then get into a full power climb at a defined airspeed before hitting that starting altitude. Then hold that airspeed for one minute and see how much altitude you gained.

Repeat for at least four airspeeds. I chose 70, 80, 90 and 100kts. Then take those results and plot them on a graph. On most conventional aircraft you end up with a pretty bell curve. The peak of the curve is your best rate of climb. Then you draw a line from 0/0 to where it just meets the curve and the point it touches is your best angle of climb.

Normally, it looks like this:

VX-VY Traditional

It this example, Vy would be about 86kts and Vx would be 80kts.

But the Velocity is anything but normal.

Here’s the chart I ended up with:

VX-VY Velocity

Not exactly what you would call a bell curve, huh?

For Vy, 80kts gave me the best rate of climb at 1,386fpm. But there’s almost no forward visibility at 80kts since it feels like you’re looking straight up.  And it’s about 20kts above the canard stall.  And finally, 90kts is only 10 feet per minute slower. 100kts is only 4fpm slower than 90kts. So I like 90kts as Vy.

Vx is real tough.  I could run another flight test and see where 60kts comes in on the graph, but I don’t think I would like the deck angle or being that close to the stall speed at that angle. So I would be happy saying that Vx is 70kts.

Except that I don’t like the idea of flying at that angle. And because I have all the data being recorded, I was able to determine the deck angle and the distance covered during the climb. The deck angle really isn’t significant since what you’re really trying to do is get as high as possible over a given distance. What I discovered is that the distance covered for 1,000 feet of altitude gained is:

70kts: 1.069nm
80kts: 1.082nm
90kts: 1.233nm
100kts: 1.532nm

While 70kts is obviously a better climb angle, 80kts provides the same altitude gain with only an additional 80 feet of distance.

So for me, 80kts is now the official Vx.

And if I’m ever in a situation where I REALLY need to get higher in the shortest possible distance, I know that 70kts will get me over that mounding pile of zombies. I just need to watch the CHT’s and make sure I don’t get any slower.

Even though this was done during Phase I Flight Testing, I wanted to lay the groundwork for some recent testing.

Foreflight recently added a feature where it draws a line around the plane that shows how far you can glide (it factors in terrain and winds). In order to use this, you have to know the glide ratio. During Phase I, I figured the best glide speed (basically, minimum sink rate), but didn’t know what the glide ratio was.

I did some research and discovered what glider guys have known for years. The Polar Glide Chart.  Basically, this is the same chart that I used to determine the climb speeds. It’s just inverted. Now the glider pilots take this way deeper than I need to go, but the concept is the same.  Pick a starting altitude, fly a decent at a constant speed, then after 1 minute record the altitude lost. Go back up and repeat at another three airspeeds. I used 5,000′ as my starting altitude and 100, 90, 80 and 70KIAS for airspeeds. Like I did with the climb tests, I did all the descents in the same direction in roughly the same place, this way I could use the flight data recorder to also determine distance.

Here’s what a chart would look like for a traditional airplane:

Vld-Vbg Traditional

Here we can see the airspeed which gives you the minimum sink rate (or most time in the air) is about 62kts. The airspeed which would provide the greatest distance looks to be about 65kts. I can’t find any official designations for these airspeeds. I’ve found Vld which is supposedly the best “Lift to Drag ratio” so that sounds like minimum sink rate. And I’ve seen some references to Vbg which is supposed to be “Best Glide”.  So I’ll use those. If anyone knows the official terms, please let me know.

Now for the results of my recent testing.

Vld-Vbg Velocity

Not very different from the climb chart.

In this case, minimum sink would appear to increase the slower you fly. But like before, teetering on the edge of a stall isn’t a smart way to fly. And the vertical speed difference between flying at 70kts and 80kts is only 20 feet per minute. So 80kts seems like a good choice for (what I’m calling) Vld.

For distance, it looks like about 88kts would give the best glide distance. But according to the flight data, 90kts gives me a glide ratio of 13:1 while 80kts gives me a glide ratio of 13.4:1.

So in the interest of simplicity, I’m going to call 80kts Vbg.

Now in the real world, if the engine ever stops, the glide ratio will increase since there will not be the drag from the windmilling prop.  Not sure how much it will improve the ratio, but more is usually better, right?

 

First Flight! (for me)

This entry is part 6 of 7 in the series 16 - Flight Testing

Lots of things going on.  Because we sold our Chicago area house and bought a house in Panama City (literally on the same day), and said Panama City house needed a LOT of work before the planned move in date of 9/5, I haven’t had much time for airplane related activities. I did run down and get the roll trim installed but that’s about it.

Because I’ve only got 25 hours to fly off, I was planning on doing that in one trip.  And to handle the transportation, my good friend Rody (who was in the plane when I had the nose gear collapse on the 182-RG in Greensboro, NC) suggested that I just rent a car and drive down then drop it off there if I can fly off the hours in one trip. Thanks Rody!

So I got a car from Enterprise (only nationwide car rental company with an office in Sebastian) and drove down on Sunday, September 13th.  Then I got John Abraham to go up with me for a couple of quick take off and landings. Remember, I only have 5 hours in Velocities and that was the small trainer that’s fixed gear.

So we hopped in the plane and took off.

Holy crap, this this is FAST!  Rocketed down the runway, lifted off and before I knew it the we were at 130kts and climbing… rapidly. Then the “Low Oil Pressure” light started flashing.  A quick check of the gauges showed the oil pressure was reading right where it should. So we stayed in a tight pattern just in case.  After a minute, the light went out. It turned out that I had inadvertently set the alarm (which drives the “Low Oil Pressure” light) to indicate for low oil pressure and if a cylinder head temperature exceeded 400 degrees. Oops.

The two landings were far from impressive.  Long with lots of over-corrections.

Then I was up on my own.  I decided to fly south to Stuart (the south end of my test area) and then back north to Sebastian. This takeoff was even more… exhilarating.  With just me in the plane it accelerated like a sports car. In no time I was at 1,500 feet and climbing in excess of 1,500 feet per minute.

Lots of challenges here.  First is staying in front of the plane. I ran into this challenge when I moved from flying fixed gear Cessna 172’s to my 182-RG.  Lots more power and lots more speed. Eventually you start thinking far enough ahead that you’re playing “catch up” all the time.

The other challenge is migrating from “steam gauges” to a glass panel.

Here’s the panel I’ve been flying behind for the past 16 years.

6408S Panel 2 (low-res)

If you’re not a pilot, it may seem daunting, for me it’s been home for the last 16 years. Airspeed? Top left.  I haven’t really looked at anything other than the needle position for a long time. When I’m on base leg, the needle is about 3 o’clock. Over the runway for landing, 2 o’clock.  Vertical speed? directly over the yoke. Hard to miss being level or in a 500fpm climb. Altitude? Directly above the VSI.  It’s second nature.

Now…

IMG_20150923_092939328

So now when I need to know how fast, there’s no needle. Just to the left of center is a vertical tape with (in this case) 165 in the middle. That how fast I’m going (indicated, not actual). Altitude? To the right of center. I’m at 6,510 feet.  Vertical speed?  On the left side of the altitude tape are some hash marks that angle up and down. If I was climbing at 1,000fpm, the area from the middle to the “1” would be shaded.  For a non-pilot that hasn’t been looking at the old six-pack or steam gauges, this probably makes perfect sense.  But I’ve been flying behind those old gauges for so long that this will take some getting used to.

Dodging weather

I flew down for this trip.  Instead of 7 hours each way, it’s only 2.  Of course I have to deal with weather if I fly.

Approaching the home field on the return, I had to contend with the typical afternoon thunderstorms. At first it looked like I would be able to sneak in.

2015-06-04 IMG_20150604_153654532

But then the opening between the two buildups closed and I had to loop around to the north and come in from the Northwest.

Oh well, flying in Florida.

10.1.3 Aerodynamic Trim (Sparrow Strainer)

Over the years, whenever I was waiting for something to cure or dry, I would sometimes spend a minute or two on the sparrow strainer.  This is a small inverted airfoil that attaches to one of the elevators. Just like the vortilons, when it was time to finish them, I let Malcolm do it.

Before starting the finish work, he created a pair of flanges where it attaches to the elevator. Once the flanges were done, he began the sanding, filling, sanding filling process. The the gray primer, more filling and sanding and finally the white primer.

Here’s the end result.

SS 1 SS 2

It will be attached to the inboard end of the right elevator but like the vortilons, I’ll wait until I’m closer to flying before attaching it permanently.

Here it is dry fitted.

SS 3