Posts tagged ‘Airplanes’

Hot cylinder in an air cooled engine

air channel on cylinder baffling
air channel on cylinder baffling

Why do we always think that our problems are totally unique and no one has the same problem?

In the world of RV aircraft, engine cooling is a common topic of discussion – mostly because its a common problem. When you consider the 150hp engine in the RV-8 is the same as the engine in a Piper Cherokee but the RV-8 is cruising at 150-160kts while the Cherokee is down around 110kts, there must be something difference. The buggy is “drag”. Part of the drag reduction on the RV is the tight fit and flow of the engine compartment. This means its critical to pay attention to the airflow through the engine. This is controlled by the baffling.

So, when the sensor on my front left cylinder was reporting high, I needed to do something about it. The common wisdom is that anything above 400 F degrees is bad. When I would take off and climb for a more than 5 minutes I would see 405 headed for 410 and so I would decrease my climb rate.

I looked at my engine and read all that I could to find a solution. I made my best guess and then wrote up my idea so others could comment. I knew “my situation had to be different from everyone else” so all of the suggestions that I read over the past three years educational but “not relevant”. (Read that last sentence with significant sarcasm.)

Well, it turns out my problem was just like all the others and the solution was the same too. Thankfully, a very smart and patient pilot/builder we call DanH wrote up a nice description of he problem and his solution, complete with pictures.

I implemented his recommendation – but with a bit less skill – and my maximum temperature never went above 370 F degrees. Sweet !

Propeller design – science and experience

The following testing was performed in July, 2012. I just realized I never archived it in my blog so this is old news …

Performance tests of Aymar Demuth vs Catto Prop

Test Platform is my 150hp slow build RV-8. It’s not the cleanest or lightest RV-8 but it’s mine.

Tests using a modified version of the methodology described by Bob Axsom. I chose an 8,000′ reference altitude.

FYI: It turns out that simply substituting 8000 for 6000 from Bob’s method actually results in a normalized test altitude of 8300 ft.

Use the following test method:

  • Set altimeter to 29.92 climb to 6,000ft
  • Check outside air temperature if it is above 3C, subtract 3 and descend 100 feet X the remainder. If it is below 3C, subtract 3 and climb 100 feet X the remainder.
  • Set the throttle wide open, Prop max (if constant speed), mixture for best speed/power ~100 ROP.
  • Fly 360, trim for hands off level flight set the autopilot for the 360 ground track if you have one otherwise fly a GPS ground track (not heading) of 360. Set the altitude hold if you have one at the altitude determined earlier that will provide 6000 ft density altitude.
  • When the speed stabilizes make a GPS ground speed recording every 20 sec. until you have five in a row that are within one knot of each other.
  • Turn to 120 and repeat
  • Turn to 240 and repeat
  • Go home, average the five consistent recorded speeds on each leg.
  • Enter the Average GPS speeds and GPS ground track angles into the three leg worksheet option (saves about 1 gallon of fuel per test and is accurate) of the NTPS spreadsheet.
  • Then do it again (I actually do both test runs on the same flight) average the two results to give you confidence in the test validity)

When I first put my clean piece of paper on my knee board I write the date and what what I am testing at the top. When I get the ATIS I record the altimeter setting and the information identifier there as well just for reference because depending on what altitude I need to fly the test at, I need to have a method to tell Approach the altitude I will be flying at in an altitude reference that they understand (since my altimeter will be set to 29.92 and not the current altimeter reading).

On my 360 leg I have a reasonably level piece of land to fly over, east of Springdale and Rogers. If the speed stabilizes before rogers it is best – it is much easier to meet the 5 consecutive 20 second interval reading consistency requirement. If not the list of recordings can get quite long before 5 consecutive 20 second interval readings are within 1 kt of each other. When I turn to 120 I will initially have a relatively consistent surface but if I don’t get my 5 early I will be flying over mountains and the readings will get less stable. When I turn to 240 there is no efficient way to avoid flying over mountains. When I complete the 240 leg I boundary line my data sheet for test one, turn to 360 and repeat the process. That is the best test method I have been able to come up with to date.

Your observation of difference based on where you are flying is correct but thermals are only part of the inconsistency problem. Changing wind velocity, updrafts and down drafts also affect the GPS ground speed which the NTPS spreadsheet calls for. You are smart enough to understand that so I will not waste any time on explanation. A way to minimize the effect is by doing your testing early in the morning or (less favored) late evening on days when the wind is calm. I have mods to complete and races to go to so I just have to go when I can and try to get good data – its is not perfect.

Because the conditions are not static, even though you fly your tests back to back they will not be exactly the same on two consecutive runs but if you control every thing else correctly they should be within 2 knots of each other. I used to just make one run for time and money economy but now I just bite the bullet and make two.

It is important to establish you airplanes density altitude baseline for testing so that the test results are relevant to one another – once you select 8000 do every single test at 8,000 ft density altitude or the results are not directly comparable.

After I enter my leg speed average numbers into the NTPS spreadsheet and I get my speed and wind numbers I print the sheet or sheets for multiple runs and I staple them to the in flight raw data collection sheet. I write on every sheet the date, what was being tested and I sign them before filing for later reference.

- Bob Axom


All tests were WOT and leaned aprox 100 ROP.

The propellers being tested are the original 2-blade Aymar Demuth 68-71 all wood propeller with epoxy pain finish and the Catto composite wood core 2-blade with nickel leading edges.

After 6 runs with the Aymar Demuth propeller and 6 runs with the Catto propeller the average was 157.8 kts vs 161.4 kts. If I throw out the high and low from both groups, the numbers do not change much with 157.6 kts vs 161.4 kts.

The Catto was manufactured as a direct swap for the AD. This resulted in a significant gap at the backer plate as the AD propeller was much rounder than the Catto. Filling this gap may contribute to slightly better performance of the Catto but that was not part of the test.

A few interesting details …

  • Craig had not previously built a 2-blade propeller for a RV-8 with only a 150hp Lycoming. Based on conversations, he predicted a 2750 rpm at WOT and a 4kts speed increase. The WOT max RPM during all tests was 2750. Averaging across all tests, there is approximately 3.75 kts speed increase.
  • The weight is 11 lbs vs 12.5 lbs
  • Static is 2300 rpm vs 2025 rpm
  • WOT is 2725 rpm vs 2750 rpm
  • CHTs are slightly lower with the Catto – likely attributed to the more refined airfoil approaching the hub.
  • Climb is about the same.
  • Fuel burn at cruise RPM is about the same.
  • Craig estimated the 3.75 kts equate to about 12hp more going to thrust.
  • I don’t know propeller aerodynamics but I do know I’m going faster with the same engine.

While a little extra speed is nice, the primary reason for the propeller testing was the nickel leading edges. However, if you can get an Aymar Demuth propeller that matches your engine, its a very good performer for the price.

Addendum: During a regional cross country, the flight is between 9000ft and 11,000 ft DA because there was little to no wind advantage at a lower altitude. My flight was calculated at 142kts and average fuel burn over the 470nm 3.6 hr flight was exactly 7gph. (Fuel burn is around 10-11 gph for the climb and 3-5 gph for decent). Of course, when all the numbers are crunched, the performance increase equated to just 6 minutes.

It’s not an iPhone. It’s a ┬ÁMac !

A web forum member asked about “racetracks in the sky” – referring to flight tracks that are depicted in three dimensions. I've had APRS in my airplane since the very first flight.

I answered the forum question but then decided to create a little video to demonstrate how I get those rolercoaster pictures.

Here is why I consider an iPhone to be a more of a computer than a phone.

I fired up DisplayRecorder on the iPhone to record a demonstration video. I started by opening Mobile Safari on the iPhone. I then visited which processes and maintains aircraft APRS data. I showed a map view of a flight. I then clicked on the “Google Earth” link and the website downloaded a KML file. Safari automatically offered to launch the mobile Google Earth app. I then did a little navigating – pan, zoom, and rotate. Finally, I stopped the recording.

I had DisplayRecorder save the video to my camera roll. I then launched Mobile iMovie where I cut out a few mistakes I had made, added some captions and a few fade effects. I had a 1 minute and 15 second video.

Next, I launched Mobile GarageBand, picked a tempo and then did some quick math to determine how many bars of 4/4 music I needed to make 1 minute an 15 seconds. I grabbed a couple loops of Latin rhythm and sequenced an electric keyboard track. I finished by having GarageBand transfer the new music back to iMovie.

iMovie combined the music and the edited video and exported it directly to Vimeo. Vimeo transcoded it and sent me a notification.

I launched Mobile Vimeo and grabbed the web markup from its “share” feature.

I finished by launching WordPress for iOS, pasted in the Vimeo markup and authored this blog post.

At no time did I turn my office computers on. They entire process was complete on my iPhone.


Dawn Preflight

Shortly after Dawn

It was seven months ago that I flew round trip to Stowe, VT on a nearly perfect September day. I just found the three photos I took that day. Given the task at hand, I understand why I only took three photos but I wished I had more.

In an attempt to keep myself blogging – if for no other reason than to justify keeping the site alive, I've decided to post just about anything I want. In this case, it's a seven month old photo.

Updated POH uses waterproof paper

The new panel brought about new procedures and a few changes to the aircraft systems as well as its weight and balance. I took the opportunity to update and print a new POH (pilot operations handbook).

The first POH was printed of half sheets of paper and inserted into heavy plastic sleeves. It was bulky and the plastic was yellowing.

I wanted a light weight, easy to read, and yet still resilient solution. The answer is waterproof paper.

I decided some real test data was required / and I was looking forward to it :-)

Using a color laser printer, I started by printing one of the pages – front and back – which was a mix of text and a colored chart.

I took the page outside to see how legible it was in direct sun light. Silly me forgot that it was drizzle and freezing rain. I carelessly dropped the page and it blew across the lawn before landing in puddle. When I leaned over to pick it up, I lost my balance and pressed the page under water and into some debris. I brought the page back inside to let it dry off. Somehow it made its way to the dirty laundry pile and was tossed in the washing machine where is tumbled and was washed with laundry detergent for 59 minutes. It stayed hidden in the pile of wet clothes and was loaded into the dryer where it was heated and rolled for another 20 minutes.

The results of my ever so cavalier handling were some wrinkles (like all of my work clothes) and a few small specks of missing color.

While humorous, the truth is the page was still completely readable and good for service. I was more than surprised.

The new Pilot Operations Handbook (POH)

I still fly with a kneeboard. It is the most convenient solution for writing down clearances, frequency changes, squawk codes, and various data.

I’ve been using a small US Navy issued unit for most of the past decade.

What I am not as good about is using my checklists. They are part of the POH – pilot operations handbook.

With all the changes to the airplane it was time to update the POH. The original POH was a bit bulky and was stored in the side pocket next to the pilot seat.

The new POH is now part of the kneeboard. This means the checklists are right in front of me at all times.

There is one thing wrong with the new kneeboard – it really should be made of HDPE plastic for better safety. I’ll be looking for this material to replace the front and back covers of the POH.