Supercop0184

True airspeed not what book says

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This discussion reminded me of this video and I just had to post it, I am still learning but one thing I learn from this video is that giving wiggle room to the numbers might be a good idea.

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2 hours ago, Aviationinfo said:

How does one check one’s airspeed indicator for accuracy?

Drop by my hangar.

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2 hours ago, N201MKTurbo said:

I have never had my airspeed indicator checked for an IFR cert. They check your static system for leaks and your altimeter and encoder for accuracy and friction.

I stand corrected, it's just a static system check.  I remember guys here talking about using the rolled up surgical tubing trick here, I think I was conflating the two

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Does your car meet or exceed the estimated gas mileage as advertised by the dealer?  I suspect a little sizzle in the salesmanship, but I am happy with my E.  Need more speed? Just descend more.

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On the IO360 the timing was 25 deg btc back when the books were made. Lycoming got nervous about detonation and reduced the timing several degrees.  They may have done this on the 0360 also.  I asked the Lycoming rep once how they could still say these were 200 hp engines.  He said the stated numbers are only accurate to within 5 percent.  

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On 1/13/2019 at 7:41 AM, Gary0747 said:

On the IO360 the timing was 25 deg btc back when the books were made. Lycoming got nervous about detonation and reduced the timing several degrees.  They may have done this on the 0360 also.  I asked the Lycoming rep once how they could still say these were 200 hp engines.  He said the stated numbers are only accurate to within 5 percent.

I thought the timing change was to lower CHTs. Anyway, Lycoming has been shipping all IO-360 engines (except Bendix dual mag versions) with 20 deg BTDC timing for a long time -- I have a Lycoming Operator's Manual from 1982 that states 20 deg BTDC as standard. Interestingly the current 1E10 TCDS (1-10-18) lists timing as 25 deg with 20 deg footnoted as optional. What that means is that you can change the timing back to 25 deg (with the appropriate changes to the magnetos internal timing,  data plate, and a logbook entry). I confirmed this with the Lycoming factory rep. Does it make a big difference? I don't know. But the APS folks don't seem to think so. Here's a graph I got from the APS presentation when I attended a seminar in Ada, OK several years ago.

Timing.jpg.24640c61bdde60f5a96d8a0f7cb29ddd.jpg

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That curve would really take the wind out of the sails of all the folks promoting the HP gain from electronic ignition. 

I bet you would have to change the engine data plate before you could convince your IA to sign off 25 deg btc at annual time. 

When I got my 67F back in 1992 it had a 1000 hour engine with the 25 degree data plate. I think it was an early 80s overhaul. 

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3 hours ago, ArtVandelay said:

The problem with that graph is it doesn’t give you RPMs, I imagine the curve would shift based on RPMs,


Tom

It doesn't give anything else, no afr, no atmospheric conditions, nothing.   It is only marginally useful to make a point about timing.

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On 1/7/2019 at 6:17 PM, Vance Harral said:

While there's some truth to this, as a CFI, I've frequently traced complaints about "not making book performance" to pilots who aren't actually using the charts correctly.

As an example, flying at 10,000' indicated on your altimeter is not going to give you the true airspeed from the 10,000' line in your POH unless the atmosphere is at standard conditions - not even if you're a test pilot in a brand new, perfectly rigged airplane.  If you're crossing the Rocky Mountains enroute from Ft. Worth to Durango, the temperature at 10,000' indicated is frequently much warmer than standard.  It would not be unusual for the density altitude at 10,000' indicated to be around 12,000', where the published book numbers for true airspeed are going to be slower for a normally aspirated airplane.  Given that you don't mention OAT or density altitude anywhere in your original post, maybe a misunderstanding about the POH tables themselves accounts for some of the discrepancy?

Best information so far. Don’t forget Density Altitude, its the basis for your airplane performance.  

 

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Since engine timing came up... a summary of that topic...

 

The coolest things about advanced timing...

  • The spark occurs earlier in the cycle, further before the piston reaching the top...
  • There is more time allowed for completing the fuel burn...
  • A higher percentage of the fuel is converted to useable hp...
  • More fuel converted over a longer period of time causes a higher CHT and internal cylinder pressure...
  • Slowing down the RPM has a similar effect of allowing more time for the fuel burn...

The challenge of simplifying the discussion comes in a variety of forms...

  • Measuring time using Engine Timing in degrees BTDC is not in seconds...
  • The amount of time available (seconds) actually changes as the rpm is slowed down...
  • Improving fuel conversion to hp, dynamics use timing in seconds...
  • Improved conversion comes from more advanced timing... 25 in place of 20 degrees BTDC.
  • Risk of pre-ignition or ping, increases with things that allow the fuel burn to to proceed too quickly...
  • High CHTs is one of the things that promotes the fuel burn rate, as does mixture (at or close to peak EGT)

In the old days...

  • 25 dBTDC was a common method of getting better efficiency...
  • Engines didn’t have sophisticated monitors.
  • Pilots were unable to recognize how closely they were operating to the edge of pre-ignition.
  • Engine manufacturers changed the engine timing for everybody to ‘lower CHTs’
  • The change made sense, the cost was lower efficiency...

Today...

  • The educated consumer has an engine monitor.
  • Understands how and when to use it.
  • Increased engine timing is back and is useable.

Where it can go wrong...

  • Pilot doesn’t use the EM and allows CHTs to run hot.
  • Pilot decides running Over Square is an OWT and completely ignores  the extremes of MP and RPM...
  • T/O and climb at max power are at the highest risk of problems arising related to timing...
  • Cruise and descent under lower power are not as challenging...

Where the limitations to changing timing are...

  • Some planes are authorized to have 25° timing...
  • Some are authorized to have 20° Timing...
  • Some were 25° and changed to 20°...
  • Some have been put back 25° because it works for that owner/pilot...

For comparison... Continental uses 22dBTDC

The gentlemen at APS have done a great job at bringing instrumented Engine ops to a pretty high level.  Their presentation materials are easy to understand and follow.

Just be careful to follow their guidance completely... going outside the lines can be hazardous to the health of one’s engine.

Fortunately they have guidance to follow to keep you in the right place, and guidance to follow to know that you are in the right place...

  • Outside of the red box
  • Cool CHTs

How is that for a summary on engine timing..?

PP thoughts only, not a mechanic...

Best regards,

-a-

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ROP full throttle takeoff power, is not a large difference. But LOP is. Leaning past leak has the same effect as retarding timing. Now your cylinder presssure is already on the low side because of the 20 degrees timing. Then pulling it LOP it lowers even more. A slower burning charge that is lit later.  From messing with this, which involved setting timing, flying, resetting, doing it again, 15LOP speed loss at 25 degrees of timing is about 3-5 Kt.  At 20 degrees of timing, its much more. 7-10 KT.  look at the fuel flow, its decreasing in a linear fashion with the speed, the NMPG isnt better.

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On 1/19/2019 at 7:33 AM, jetdriven said:

From messing with this, which involved setting timing, flying, resetting, doing it again, 15LOP speed loss at 25 degrees of timing is about 3-5 Kt.  At 20 degrees of timing, its much more. 7-10 KT.

Appreciate real data - thanks Byron. Curious if you noted any CHT differences between 20 deg and 25 deg ROP and LOP.  Did you wind up leaving it at 25?

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With the current mags does anyone know the tolerance limit in btc setting before having to change mags for 25btc operation?  I vaguely remember something like 20 degrees plus or minus two degrees but don’t remember where I got this. 

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