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calibrating airspeeds, revisited


207WF

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I had not done this in four years, and the accuracy of the airspeed indicators has changed. I flew a triangle course, 7nm each leg, twice around and recorded the data on each of the six legs after heading, altitude and airspeed had stabilized. I was at 3200 feet, OAT 15C with a density altitude of about 3700 feet. I flew at my normal cruise power setting, which is 5200 rpm (I have not checked how accurate the rpm is!). The wind aloft was light, indicated by a maximum groundspeed difference between any of the two legs of 9 knots. I assume that the average of my gps groundspeed over the six legs is my average true airspeed. Here are the data:

 

Avg gps groundspeed = 115.0 knots

avg flight design airspeed indication, round dial = 116.7

avg dynon airspeed indication, DA10 = 108.2

avg dynom computed TAS = 114.5

 

Using a whiz wheel, a correct average CAS would have been 109 knots. My flight design gauge has become more optimistic as it has aged, while the dynon has become less pessimistic and more accurate!

 

I also confirmed what I had expected: getting my ailerons lined up with the flaps at -6 has added three knots to my true airspeed. (This is unlikely to be due to rpm error, as I saw that change over a one day interval.)

 

WF

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That's a nice cruise speed at 5200 rpm, unless WOT is 5300. My LS would never keep up to that. I am set at 5500 WOT in 70 degree temps. I noticed yesterday in the colder air (23 far) at DA of around 1000 ft I could only get 5400 WOT. The sky view was in the yellow at 124 knts. I have done your airspeed calibrations many times flying in all four directions and getting the average ground speed. My sky view is 7 knts higher on TAS and always optimistic on IAS too at low DA's compared to actual ground speeds. I do have a 430 WAAS installed and those two antennas on the tail are supposedly costing me 3 knts combined. My actual airspeed and groundspeed drops off about 5/7 knts when we get back up in the 70's and above. It's quite dramatic the difference that cold air makes on horsepower. anybody else see these dramatic performance differences from warm to cold ar temps ?

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Cold verses hot air cab make those difference because of engine performance and prop performance. The engine runs leaner in cold dry air verses richer in hot humid type air. The prop gets a better bite in the cold dense air verses the hot less dense air. You should be getting a little better engine performance in the cooler air and prop performance, but your prop pitch is set just a tad too course. It should drop below that 5500 rpm WOT. Having it set to get around the 5600-5650 mark allows good performance all year and at different altitudes. Our seven CT's at my field all run at the 5600 rpm WOT mark and we tend to cruise around 5100-5200 when in a group.

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You would think the extra drag on the airframe, from the denser air would negate the extra performance from the engine and prop, but the airplane loves cold air. How many rpms do u see in maximum climb gradient? Does that change a little for you also in colder air? With flaps 15 and 62 knts with low temps and DA's we can barely make 4900 rpm on climb out.

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Not sure the physics works for me. More lift, but no more drag, It may not be enough of a difference to notice, but denser air has to mean more parasitic drag, and, more lift = more drag, doesn't it?

 

Drag has to do with speed. You will have a little more drag because of the increased speed, but not much more. If your speed is the same, so will be your drag. You are still lifting the same weight, so you are producing the same lift and induced drag will be the same.

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On my Tecnam P2008 I have set my Prop to 5650 max rpm strait and level also.

It does seem to be a good setting for all year here in minnesota. I would say I also get 3-5 knots better speed in the winter.

I definitely get better climb in the winter easily over 1000fpm. Summer is more like 700fpm.

My static RPM has always been 4900-4950 winter or summer. I thought you CT guys were getting 51-5200 static with a max strait and level of around 5600.

I always attributed it to the difference in the airframes.

 

Mark

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If density of air had nothing to do with drag, do you really think those new biz jets flying at 51,000 ft are up there for the view? The higher you go the thinner the air, and if you can maintain your thrust and lift you will go faster in the less dense air because it has less drag on your aircraft.

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I don't know how you guys fly around at 5200 + rpm's i tried setting mine up at 5500rpm wot all it did was scream its nuts off all the time very little if any performance gain and burnt loads more petrol for no speed/performance gain ?? mine is set for 5000rpm wot i cruise at 4500rpm at 120mph and burn around 14 lts an hour

 

Mike

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Hi Mike,

 

The engine should sound like it's turning more rpm it was designed to run up there, you just weren't use to hearing it at its proper rpm..

There are a few here that have -12. With those prop settings on your plane you're really over stressing your engine and causing more vibration and heat. The Rotax like many other engines were specifically set up to run within certain rpm parameters. Yours is way outside these settings and will take its toll over the life of the engine. Mid-range cruise usually generates more heat, cost you in fuel economy climb performance and cruise. Over the years I have tested, researched and observed many a 912 with different settings. Sure we can pull back to 4500 rpm here too and burn under 4 gph, but that isn't where it should be run. Fuel isn't a consideration if so why not fly around at 4000 rpm and save more and say 4500 is too much rpm.

When someone comes in here with a 5200 rpm WOT setting i'll adjust the prop to see about 5600 rpm and when they go fly they all come back and think someone put a turbo on their engine. With my CTSW verses their plane I out climb them, walk away from them at top speed and burn 1.5 gph less. These are all verifiable numbers time and again. Running your engine in these numbers is like running a manual shift car in 4th gear at 15-20 MPH. Can it be done sure, but the system is straining because it wasn't designed to run there. 4th gear was meant to run at the car's cruise range on faster roads.

I'm very serious Mike, this isn't good for your engine, these Rotax numbers are lower than I have seen from any MFG and in Rotax school they warn very stringently to stay away from these prop rpm settings.

 

p.s.

I have tested -6 verses -12 and it really doesn't get you much at all. It isn't the big winner like many think. I have flown planes side by side and it wasn't anything to write home about. I wouldn't even bother doing it any more now that I have actually done the test. The deciding factor on plane speed and performance isn't -12 or which prop you have. It pretty much all boils down to WOT rpm setting. (for ground adjustable props) I do all kinds of testing and research. I have 6 CT's at my field and get to use them for different things. (7 during the winter and looks to be 8 soon)

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Thanks for the reply Roger yes it may be a bit of im not used to the sound of it revving at that rpm speed but i did test it over a few months and apart from burning more fuel i didn't see any real improvement in climb or feel any extra power ... Ive had several 912 engined aircraft over the years they have all run in this speed range fixedwing and flexwing every ct and ctsw i have flown in which is around 6 all fly at these sort of rpms in the uk they come from the manufacturer set up like this so what do we do ? i will try it again this year from now till the winter and keep some fuel / speed recordings and see what happens i have a few 1000+ mile trips planed this year so should be a good test

 

Cheers Mike

 

Ps Mack what does your ct run at rpm wise in Ireland

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If density of air had nothing to do with drag, do you really think those new biz jets flying at 51,000 ft are up there for the view? The higher you go the thinner the air, and if you can maintain your thrust and lift you will go faster in the less dense air because it has less drag on your aircraft.

 

Yes, you will have more drag with more dense air. Induced drag will be less because of your increased speed, because you will have less angle of attack. Form and parasite drag will go up, but the thing is your power output and propeller efficiency will overcome the increased drag.

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I can't help but make a few questions and comments:

 

1. I call BS on any assertions of longer engine life with a higher rpm until I see some real test data. Not just anecdotal evidence. Many failures are due to cycles, both thermal and mechanical. Wear is related to load, cycles and amplitude. Higher rpm gives you more cycles. I find higher rpm giving longer life counterintuitive. Charles Lindbergh always advocated lower rpm and higher manifold pressure. We'll never know until Rotax Engineers release some of their test data.

 

2. Most rotational vibrations are related to unbalance. Unless you are sitting on a resonance, you can run at lower rpm. If there was a vibration problem at lower rpm that would affect TBO, then Rotax would placard an rpm restriction.

 

3. Why do many race cars and top fuel dragsters use carburetors if fuel injection were better?

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Glenn, I agree with #1, it reminds me of when people used to claim they got better mileage in their cars by driving faster on the highways.

#2 makes sense too.

#3 however has more to do with the delivery of massive amounts of power in a short period of time. Formula 1, and Indy Car use fuel injection, they are more concerned about quick changes in power. Niether of them really closely equate to what we are doing. Everything is a compromise, you look for what works best.

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I mention #3 as a rebuttal because carburetors are not so bad for specific instances. A NASCAR superspeedway car, a top fuel dragster and aircraft all run at higher percent loads than a car so they can get still get by with a carburetor. Although, NASCAR made the change last year.

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Hi Glen,

 

The data is out there, it isn't hidden or send an information request to Rotax or Kodiak. You can get the info from attending a Rotax Heavy Maint. class and talking to someone like Eric Tucker in a more in depth engine discussion above and beyond what is normally taught. I have been there with him in these conversations. The problem with your analogy is your only real concern cycles or rpm's when the issues are vibration and extra parts tress. You need to pick the lesser of the evils in this case. The 912 was designed to run at 5500 rpm its entire life and was never set up to run at 4500. Not one single Rotax publication says to run the engine in the mid 4000's. When I worked for Atlantic Richfield (aka ARCO Chemicals) we had engine's and compressor up to 50K HP and some pumps that handled methane in a liquid state. We had turbans and pumps that if not run within certain rpm that the duty of the pump or engine was cut in half. Wear isn't the only factor. You will not feel the vibration or stress, but it is there and we had to use instruments to set the rpm to be vibration free because you couldn't feel it or hear it. All of the problems that can cause excessive wear you have listed are of a concern and it usually bites people in the butt that don't follow the SB's, maint. schedules or have a lack of maint. Then when they have a major issue at 1000 -1500 hrs and it doesn't make it to the 2000 TBO and they blame the engine, but forget to look in the mirror so to speak. Charles Lindberg and the engines of those times aren't in the same class, type or time period. The same applies to your cars.

Why not run a 4 speed on the floor at 15 MPH in 4th gear? Less cycles, better fuel economy. I just ran a Nascar around the track for 15 laps 2 weeks ago. I was supposed to keep it in a certain rpm range for a reason and not let the rpms in the 3rd gear get too low.

 

The problem is you can't feel the problem and if you can then it's really bad or too late.

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I get that it's intuitive that more rpm=more wear.

 

But it's been pointed out that in normal operation, there should be virtually NO metal-to-metal contact - if there was, the engine would grind itself to a halt in very short order.

 

In fact, a large amount of wear occurs as the engine is started, before oil pressure can get the parts "floating" on a film of oil.

 

But you're right - data will tell the tale

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Cylinders for the 912 are coated with Gilnisil. A super hard nickle coating. Even after 2000 hrs. and you pull the cylinders the original hone marks are still there. It takes a diamond hone to even touch these guys. You should never have to replace the cylinders unless you fry it by running too lean. Valves have an armor coating and they can never be reworked. Cleaned from debris, yes, but they are too hard to re-work. Pistons usually don't need replacement, just the rings. There are many parts in the 912 that are far superior to an old design Conti. or Lycoming and people need to stop trying to compare maint. and or use Conti. or Lycoming maint practices on the 912. I agree that Rotax makes good money on parts, but they have made mega bucks on the people who fail to follow Rotax maint. and operation practices that have proven themselves out over 30 years.

I have friends with 3K-4K hours on their Rotax 912 and have never been touched, but they also have done the scheduled maint. and not listened to other non experienced Rotax people and done things that weren't in the book or neglected maint. schedules.

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1. Who is Eric Tucker? Is he an engineer that works in the design or testing department of Rotax in Austria? Many of these arguments I hear are all anecdotal. Has Eric Tucker run a 912 at 4500rpm for 2000 hours so he can say that there is more problems with one than another? That would imply that he ran a 912 at 5500rpm for 2000 hours for comparison. That is test data.

 

2. Your anecdote about running an engine in 4th gear at 15mph would imply an rpm just off idle. 4500rpm is not just off idle. At idle and just off-idle you have carburetion problems due to a lack of vacuum in the venturi. There is no carburetion problem at 4500rpm. Rotax may not say to run at 4500rpm, but it does not say that you must avoid this range due to harmful vibrations. There are engines with rpm ranges where continuous operation in that range will result in excessive cyclic fatigue. There was one in a Cessna that had the yellow range right on the tach. I can't remember if it was a 152 or not.

 

3. Most engines will not make TBO due to lack of regular use, not running at one rpm rather than another. Running an engine lean will not damage the cylinders - you should read Mike Busch's articles. There is a difference in neglected maintenance and operating the engine less than full throttle. How do you get more stress in an engine by running less power? There are more loads holding on to a piston flying back and forth at 5500rpm than there are at 4500rpm.

 

4. Charles Lindbergh operated geared, supercharged, multi-valve, water-cooled 12 cylinder engines with the same mantra of lower rpm, higher manifold pressure and had good results in engine life and fuel consumption during his flights in the Pacific during the last World War. That is anecdotal, but it was good enough for the Army Air Corps to have him go around and teach his style of engine operation to other squadrons.

 

FInally, 5. The major differences in older versus newer engines is electronic engine controls and materials. The concepts have not changed much since the very beginning. There are more demands on them to meet emissions, and fuel economy targets as well as durability and noise concerns, but the physics have not changed.

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The top people with Rotax engines and even the median service Rotax centers never have these huge varied debates as they have all gone to schools for their answer. These debates only crop up and are from one end of the spectrum to the other when there is a wide education gap.

 

 

Your comments tell me you don't have any Rotax schooling or a long history with Rotax engines. I don't mean that in a negative way, but it is all too common. The education and knowledge is out there for those who realize there is more to learn and that they don't know every nut and bolt on the engine and how it works. Don't ask other people for documentation or even there private ideas, go get it first hand from the experts. Your answers are out there in detail, but it looks like I'm not your creditable source so first hand schooling would be better.

 

If you had attended all the Rotax classes they would have in fact given you all your answers and then some and you would have a completely different knowledge base to work from. If you do decide to attend all three classes you will come out with a whole new perspective. You can't apply new 500 Fiat engine tools and knowledge to a 1960 Ford pick-up. Unfortunately that is what too many do. They think there is only this much to know------------ because they think that is all there is, but in reality there is this much --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------.

 

Eric Tucker is the instructor for Rotax and has worked for and with them for more than 30 years. He teaches the instructors to instruct around the world. He approves or dis-approves warranty claims.He is Rotax's problem solver. He investigates crashes that involve Rotax engines in conjunction with the NTSB. His creds with Rotax just keep on going. He owns Rotax Flying and Safety club. He was a racer many years prior to all of this with Rotax engines. He can tell you anything and everything about all Rotax 2 strokes (past and present), all 912's and 914's (past and present). maybe other that have attend all the classes will jump in here. The bottom end Service class isn't much and more for just a few basics. You'll need the next two classes to be worth anything. The Heavy maint. will will put even the seasoned A&P back in his place when he realizes he didn't know what he thought. I see it all the time in class.

 

Unless anyone can match these creds, has as much Rotax test experience, data problem solving world wide issues and time with all these engines it would be wise to realize you're actually way behind the curve and education on a specific engine and any experience with other engines rarely applies and a wise person would know this.

 

Always better to debate a point from strength and specific knowledge than from a weak inexperienced point.

Again this isn't meant as a negative towards anyone. it only means everyone had to start some where and take the first step and until they take the first step and then complete the journey personal opinions may be wrong. Some started this journey last week in class and some have been doing this for 20+ years and with repeated classes and many Rotax engines under their belt.

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Glenn says: "There are more loads holding on to a piston flying back and forth at 5500rpm than there are at 4500rpm."

 

You can't say that without comparing the prop pitch. Those that fly at low RPM tend to have overly coarse pitches and their upper end power isn't even available to them. One thing about running 5,500 is that even at lower RPM you know your are not lugging the engine.

 

It isn't hard to find the recommendation or best practice of running at 5,500 but it is hard to find exactly who is recommending it and exactly for what reasons.

 

Rotax needs to published a knowledge base that is accessible without attending their classes.

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