True Airspeed

[N6018Q]

I've been following @Dream to fly's post "Is it Possible?". I didn't want to hijack his thread but a comment by @Ragsf15e, "You need to do the 4 way test at about 7,000’, wide open throttle, mixture 100 rich of peak, Ram air open, trimmed to level flight.  Give us GPS groundspeed from each cardinal direction.  An iPad with foreflight for groundspeed will do.  Your indicated airspeed should be exactly the same in each direction" sparked my curiosity. I've been wondering if my '66 E's speed is slow or at least average. Yesterday I went on a rectangular pattern test flight. Conditions were 23°C at 7,200' (BP was 29.99"), mixture set at 1300° EGT, WOT with ram air open, 2550 RPM and 23.8" MAP. Using Foreflight, first leg was 160 knots gs. Second leg was 155 knots gs. Third leg was 128 knots gs. Fourth leg was 134 knots gs.  How do I calculate true airspeed?

[0TreeLemur]

There is an EAA spreadsheet that allows you to do this using only three legs.   The fourth is redundant.   Search the internet with "true airspeed spreadsheet three legs" turned up this  https://www.eaa62.org/technotes/speed.htm

[hypertech]

Try this: http://www.csgnetwork.com/tasgpscalc.html

[N6018Q]

Thanks! Both calculators returned 143.6 ktas. How does that compare to others with E’s on here? I have zero speed mods and all the original antennas, etc.

[Ragsf15e]

19 minutes ago, N6018Q said:

Thanks! Both calculators returned 143.6 ktas. How does that compare to others with E’s on here? I have zero speed mods and all the original antennas, etc.

Good for you!  Great excuse to fly!

So the guys are right, if you use the 3 way calculation online, that’s even more accurate as it uses a bunch of vector math to take out the winds and leave you with true airspeed.  Many people just average the 4 way cardinal direction test and that will be very close.  Like within 0.5 knots.  Either way will give you a good idea, but mathematically, the 3 way  is more precise.

So you’re gonna find a lot of people tell you they hit 150+ knots with their E.  My F has been as fast as 149 Rich of peak at about 7000’ density altitude and as slow as 134 Lean of peak at 13,500’ density altitude.  So conditions and parameters matter a lot.  

The fuel flow matters a lot - do you have fuel flow and 4 egt indicators so you can lean very accurately?  Also, the density altitude you run the test at is going to change your answer dramatically.  You’ll find that our non-turbo airplanes do best between about 6000’-9000’ density altitude.  At those altitudes you can generally run wide open (full) throttle, rich of peak about 100 degrees if you want speed.  Same full throttle setting and 20 lean of peak gets me about 5-7 knots slower but much better economy.

So, your 143 knots is probably slower than some Es, but it was real hot, so high density altitude.  You’re probably within the normal range with no mods.

Just as an aside, weight and cg will also affect your speed.  Lighter is faster, aft cg is faster.  Those are small factors compared to density altitude and power settings.

Edited August 1, 2019 by Ragsf15e

[gsxrpilot]

Someone will be along shortly with an opinion on that speed.

But what we don't know is how much power you were making because we don't know where you were in relation to Peak EGT. The number 1300° EGT doesn't say anything. We'd need to know where that number is in relation to Peak EGT. Do you have the factory single EGT probe, or do you have an engine monitor with 4 EGT probes?

[N6018Q]

Density altitude was a little over 5000’ at the 2200’ elevation airport that I took off from so by extrapolation (which I’m not sure is linear), the DA at 7000’ was probably around 10,000’. I do have a JPI 830 and my fuel flow was around 10.5 gph. My peak egt (#2 cyl) is 1460 degrees. I was about 145 lbs under gross (45 gallons fuel and 500 lbs in the front seats) I know the 160 degrees ROP wasn’t very close to peak power but my #3 cht was at 375 and I didn’t want it to go any higher. 

[Ragsf15e]

Awesome, you’ve got everything you need to check against others.  You can use a simple online calculator for density altitude too since you had your altitude and temp.

In my opinion, your in the normal range for a stock airplane.  

It is interesting that your cylinder was that warm in cruise.  Usually the higher airspeed in cruise cools it.  What was it in the climb?  

Lean of peak at a lower altitudes will likely give you cooler cylinders and same speed.

[N6018Q]

1 hour ago, Ragsf15e said:

Awesome, you’ve got everything you need to check against others.  You can use a simple online calculator for density altitude too since you had your altitude and temp.

In my opinion, your in the normal range for a stock airplane.  

It is interesting that your cylinder was that warm in cruise.  Usually the higher airspeed in cruise cools it.  What was it in the climb?  

Lean of peak at a lower altitudes will likely give you cooler cylinders and same speed.

I actually climbed at a little lower power setting, about 2450 rpm, cowl flaps open, ram air closed and 120 mph so the cht was about the same. I do try to never let it get above 380°. I have a significant spread between #3 and the other cylinders. All the same probes, no ring type that normally causes the difference. I guess its possible that I have a bad probe but I haven't had an opportunity to get a mechanic to swap two of the probes to check accuracy. I have considered at least the Lasar closure just for the possible cooling but I've read mixed reviews on them.

I've experimented a little with LOP, according to Savvy data my GAMI spread is typically .1, but its easier (lazy) for me to stay ROP on the short sightseeing hops I normally do.

 

[Ragsf15e]

17 minutes ago, N6018Q said:

I actually climbed at a little lower power setting, about 2450 rpm, cowl flaps open, ram air closed and 120 mph so the cht was about the same. I do try to never let it get above 380°. I have a significant spread between #3 and the other cylinders. All the same probes, no ring type that normally causes the difference. I guess its possible that I have a bad probe but I haven't had an opportunity to get a mechanic to swap two of the probes to check accuracy. I have considered at least the Lasar closure just for the possible cooling but I've read mixed reviews on them.

I've experimented a little with LOP, according to Savvy data my GAMI spread is typically .1, but its easier (lazy) for me to stay ROP on the short sightseeing hops I normally do.

 

Sounds like you’re doing just fine learning about your airplane and keeping it safe.  My cht is also warmest on #3, #2 is about the same.  1/4 are much cooler.  The airflow is really weird in there.  

So speedwise, I’d say you’re fine.  If I was you, I’d check out your baffles/doghouse and make sure it’s sealed really good.  You should be able to takeoff & climb full rich, then lean just enough to keep the takeoff egt as you climb at 120mph without pulling power or cooking a cylinder.  I applaud you for keeping it 380ish or below, but I would want to fix your cooling airflow so I didn’t have to pull power in the climb.

Otherwise, sounds good! 

[kortopates]

I actually climbed at a little lower power setting, about 2450 rpm, cowl flaps open, ram air closed and 120 mph so the cht was about the same. I do try to never let it get above 380°. I have a significant spread between #3 and the other cylinders. All the same probes, no ring type that normally causes the difference. I guess its possible that I have a bad probe but I haven't had an opportunity to get a mechanic to swap two of the probes to check accuracy. I have considered at least the Lasar closure just for the possible cooling but I've read mixed reviews on them.
I've experimented a little with LOP, according to Savvy data my GAMI spread is typically .1, but its easier (lazy) for me to stay ROP on the short sightseeing hops I normally do.
 

CHT probes never fail in a way that results in inaccurate temp indications. They are screwed into a cylinder well where they are kept in pristine condition. It's the wiring that causes them to fail and then you'll know it.
EGT/TIT Probes which live in the exhaust stream do become inaccurate when their tip is burnt off.


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[PT20J]

11 hours ago, kortopates said:

EGT/TIT Probes which live in the exhaust stream do become inaccurate when their tip is burnt off.

Do you ever remove EGT probes to inspect, or just wait until they fail? Any idea how long JPI probes last? What’s the most common failure mode you see?

Skip

[kortopates]

14 hours ago, PT20J said:

Do you ever remove EGT probes to inspect, or just wait until they fail? Any idea how long JPI probes last? What’s the most common failure mode you see?

Skip

Would only remove an EGT probe to inspect when doing other maintenance or when suspicious of the probe. By far, the most common failure mode of probes are the wires breaking at the base or the probe from maintenance activities (especially for CHT probes) and chaffed wire harness which can be repaired without replacing the probe. CHT probes should last longer than the life of engine but tend to get damaged during maintenance. EGT probes are less susceptible to damage during maintenance but eventually do loose their tip with hotter exhaust in Turbo's and LOP operations.