Thorp Air Command - T18.net

As most folks have probably been reading, Kitplanes is doing a series of articles on performance of specific aircraft. They are soliciting flight info in regards to Vx and Vy. This note is to try to encourage Thorp participation in those studies. I’m curious if anyone is doing it.
What say you?
Cubes

Guess I’ll have to subscribe to kit planes magazine. Happy early Christmas to me!

Kitplanes is the only unbiased magazine. They produce excellent content in the magazine monthly and have a well thought out and complete website that has more info. I highly recommend a subscription.
Cubes

Cubes,

You have motivated me to fly a flight test that I've been putting off for two years. I read your post yesterday and went out to fly the level performance, climb performance, and glide performance tests. These are the same tests that I've been teaching in my Flight Test Engineering course, but we've mostly been using Piper Warriors and Cessna 210s so that the students can ride along.

I ran through airspeeds from 60 knots to 150 knots in 10-knot increments. For the climbs, I started below 2500 ft MSL and climbed through 4000 ft MSL at 2500 RPM and 25" manifold pressure. I then reduced the throttle and descended at idle throttle at the same prop setting. The data comes from a flight data recorded mounted to the right wing tie down. It measures/estimates airspeed, attitude, angular rates, acceleration, and position at ~200 Hz.

Here's the Climb Polar:


The blue circles show the points on the polar, computed from an average of the stabilized section of the climb. Note that this plot shows the target airspeed rather than the measured airspeed - I'm still working out a calibration issue on that measurement. I was mostly between +/- 2 knots of the target airspeed during the maneuvers. The green line is a third-order regression that, ideally, represents the actual shape of the climb polar.

My maximum measured climb rate was about 1150 FPM at 80 knots IAS. The black starts at 0 airspeed and 0 climb rate and intersects the green line at what *should* be Vx, which here is looks like 75 KIAS.


Here's the Glide Polar:


It's not a great representation of glide, since the prop setting makes the propeller spin fast and create quite a bit of drag at idle. Even so, the slowest sink rate I measured was about 750 FPM at 80 KIAS. The intersection with the black line should show best glide speed, and that looks like it might be between 80 to 90 knots.

I still haven't processed the level flight data, but I tested the entire speed range from firewalled throttle to minimum controllable airspeed in increments of 1" of manifold pressure. After I convert the power setting to horsepower, I'm hoping it will give a nice power required vs airspeed curve that will give the minimum power and maximum range airspeeds.

I’m glad we have really smart people who love Thorps. Some day, I’ll understand what you wrote! Would you please touch base with the Kitplanes guys?
Thx
Cubes

I read it a second and third time and was able to convince myself that I understand it! Hahaha. That’s what I’m talking about!
Cubes

So Susan and I went out this am to see if our numbers came anywhere close to Dr Drift's. In an unscientific and uncalibrated instrument test, we came very close to matching Mujahid's results, especially in the climb test. I always had assumed Vy to be in the 120 range, but today it was closer to 90. Climb fell off quickly below 80 KIAS. I need to spend more time on the glide. Prop was cycled to coarse and drag noticeably reduced. Ground was coming up too fast to really get good numbers but 90 is in the ballpark. Our gross weight was 1550# and density altitude was about 2500 ft.

_________________
Bob Highley
N711SH
SN 835
KLAL

https://cafe.foundation/v2/pdf_cafe_apr/Thorp.pdf

An older article, but I think it contains very accurate data. Looks like best glide speed is 112.7 mph. That would be the equivalent of no prop installed (no prop drag or thrust). A number I find interesting is Carson’s speed (148.3 mph). For those that want to go fast but hold on to their wallet tightly! Who would that describe here??

Dr. Drift’s numbers are very similar to what I see in C-GABY on a standard day
160 hp O320-E2D turning a Warnke Air-Claw.

Dick

Bernie, I resemble that remark and I expect most of the rest of the horde on this site

Note: we have mixed units in the various post, 90 KIAS is 103.5 MPH, conversion factor 1.15, that's the engineer in me talking.

I have down loaded the flight test cards from the article but have not had a chance to fly them yet. Just installed a new autopilot that has an IAS feature and am looking forward to try it out, just need to get an annual condition inspection done, scheduled for next week.

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA

Cubes, Travis, and I have been talking about making performance charts for the Thorp, and it would help if we had data from a decent sample size of aircraft.

I have the portable flight data recorder that I would be happy to send to anyone who is willing to fly the climb, descent, and/or level flight test. You can use the EAA flight test card or mine, but ideally we would want to see:

- Steady climbs at 4 or more airspeeds
- Steady descents at 4 or more airspeeds
- Steady level flight at 4 or.more power settings

We are running this test on S-18s and T-18 with an I/O-360 and a CS prop. We would love to have the following variations:

- Airframe; S-18 / T-18
- Engine: 290 / 320 / 360
- Prop: CS / Fixed Pitch

For each test flight, we would need an estimate for the empty weight, pilot weight, and fuel onboard.

For sure it will take a bit of effort from anyone participating..Count on about 1 to 1.5 hours of flight time and a few minutes to mount the data recorder to your tie down loop.

Borrowing a phrase from Cubes: "What say you?"

PM, text, or call me if you're interested and I'll get the data logger packed and shipped to your address.

I love data!! Cant wait to see the results of the performance testing on my bird. Another benefit of having Travis and Mujahid ferry your plane, they collect performance data...

Travis and I had the pleasure of delivering a Thorp this weekend to new owners. With the owner's permission, we flew some performance tests in level, climb, and descent.

The attached image shows the power required versus airspeed plot for N9040P at 7,500 ft. I added a quick cubic regression through the data points (red dashed line), but it had some problems at high speeds - so don't take it too seriously.

Essentially:

The minimum power required is about 83 HP at 91 knots indicated. The best range airspeed is hard to estimate from the points. I'll work on the curve fit and update this later.

We ran the test be starting at the minimum speed, then applying maximum power and waiting for the aircraft to converge at the maximum speed. We then reduced the manifold pressure in increments of 1" of Hg and let the airspeed settle for each measurement. At the bottom of the power curve, further reductions in throttle led to loss of altitude, so we then switched to stabilizing at specific airspeeds and determining the required power to maintain altitude.

Ideally, we would have run this test at several altitudes and possible weight configurations, but we can try to estimate the sea level performance directly from the equation that I will (soon) use for the curve fit.

Just to interject a thought, FWIW... it would be interesting to be able to use the techniques that were developed and used by the Cafe Foundation while researching performance. That was quite a few years ago now. Among other things, a truly unique and innovative idea/technique allowed them to create a true "zero thrust" condition during glide tests, allowing quicker and more accurate data. A T-18 was one of the aircraft tested. They used a Barograph and then a highly accurate capacitive proximity sensor to detect fore and aft movement of the aircraft’s crankshaft. This sensor allowed the range and sensitivity of the zero thrust technique to be delineated. It was truly ground-breaking research. This interesting article explains a lot of what I'm referring to - https://cafe.foundation/v2/aboutcafe_orderfromchaos.php.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

What’s the data logger log? I am thinking some people may have equipment onboard that would record the information. I know the Garmin G5 can record several parameters when an SD card is installed. ForeFlight with an AHARS source may be able to supply the information as well.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson