Toobuilder

Toobuilder

Well Known Member
This airplane reliably delivers 165 KTAS on my cross country trips. It has done this for as long as I've been flying it with the PMags and LOP. However, since the cooling mods I made recently, I have noticed some extra speed. On my first test flight it stabilized at 170, but I dismissed it as a fluke. However, I just returned from a weekend in Vegas and it delivered the same 169-170 KTAS on both legs. I'm running the same power setting, fuel flow, altitude, EGT, etc... The only real difference is the very much cooler cylinders (too cool now, actually - all under 300).

Not that I'm complaining about 170KTAS and 8.2gph, but it does not make sense. Any ideas out there?
 
That suggests more power... Fuel flow would have to increase, which it didn't.

I have even gone back to some old pictures to confirm the old 165 number... Winter or summer, it has always stayed the same. This TAS number is calculated by the GRT display and seems to correlate with the GPS ground speed. Is anyone else picking up TAS in the winter?
 
Mine is always faster(2-4kts) in the winter, even at the same power settings. I always figured it is due to the prop and wings getting a better bite.

Summertime, I usually run 178Ktas at 8.4gph. Yesterday that same FF was pulling me along at 181. I'll be doing some testing next month re-establishing a baseline for mods, then making runs for changes. Most of these will be all out speed, but I'll check cruise settings also.
 
If your mods altered the exit air in the cowling, that could decrease (or increase resulting in less speed) drag giving you more efficient flight.

Roberta
 
robertahegy said:
If your mods altered the exit air in the cowling, that could decrease (or increase resulting in less speed) drag giving you more efficient flight...
Click to expand...

With only changes to the baffling on a couple cylinders, the only possible difference is picking off more heat. Can't imagine that could alter the overall flow through the cowl much. That said, my newfound cooling ability suggests I have far more airflow moving through the cowl than I need, so I will choke that down in the future, but only then would I expect to see the drag reduction.

Oh well, I have a hop over to Phoenix this morning - lets hope for a repeat. Looks like I'm going to get a pretty good tailwind the whole way too, so my ETE should be pretty short.
 
I flew my wife over to OAJ to drive back with my daughter two days ago. At 7500 feet I saw my best true AS ever at 4 to 5 knots over the norm. It was however very cold at altitude. I also had a 60 knot tailwind for a good part of the flight. Seeing 240 knots plus on the ground speed was kind of nice. 46 minutes takeoff to landing for 168 NM trip!! Coming home however I paid the price as the choice was 4500 feet with 30 knots on the nose and moderate turbulence the entire way or 6500 with 55 knots on the nose and smooth.

George
 
It has certainly been cool with high pressure lately...

My best time on this particular 332 NM trip has been 1:59, and the Garmin is predicting 1:43. I guess we'll see in a few hours.
 
The baffling changes could well have improved the air flow and reduced drag - would have to know more to be sure. But, tumbling air inside the cowl is not good.

Separately, if somehow you had picked up a static leak - that might do it as well.

Dan
 
Mike - I'd suggest waiting till the summer to make any further conclusions about choking your system to increase temps. Fly a temp survey during WOT climb with higher OATs and determine your CHT margin under worst case engine and environmental conditions and base further modifications on this data.

In any case.. CONGRAT on the extra knots!
 
You might have a newly developed static system leak. The airflow going around the fuselage and canopy accelerates, so the pressure at the canopy skirt gaps is a bit lower than ambient, which affects the pressure in the cockpit. A static system leak would cause too low pressures in the static system, and make the airspeed read too high. I've seen this on my RV-8.
 
Low CHT's

rocketbob said:
There is no such phenomena of a cylinder running too cool in my opinion...
Click to expand...

From AvWeb (if you believe that stuff)...

http://www.avweb.com/news/savvyaviator/savvy_aviator_59_egt_cht_and_leaning_198162-1.html

Excerpt:

You caution against excessive CHTs, but is it possible for CHTs to be too cold?


Yes, it's possible to run CHTs so cold that the tetraethyl lead (TEL) in the 100LL is not properly scavenged and starts creating metallic lead deposits in the combustion chamber and lead-fouling the spark plugs. However, in most engines, it takes very cool CHTs (down in the mid-200s °F or lower) for an extended period of time (hours) for this to cause a problem. We usually see this problem in airplanes used for fish spotting, pipeline patrol, search and rescue, and other "loiter-mode" operations. Unless you fly at very low power settings (e.g., 50 percent) and/or at very high altitudes and very cold OATs (e.g., FL240 and -30°C), it's not usually a problem.
 
OAT accurate?

I wonder if your OAT is accurate. Go to some altitude and do some slow flight, then same altitude and go as fast as possible, see if your OAT changes with the different speeds, it shouldn't. Its possible to see it change if the probe is in the air stream.
Also check it against another thermometer, it might have changed since your recent work.

Tim
 
Kevin Horton said:
You might have a newly developed static system leak. The airflow going around the fuselage and canopy accelerates, so the pressure at the canopy skirt gaps is a bit lower than ambient, which affects the pressure in the cockpit. A static system leak would cause too low pressures in the static system, and make the airspeed read too high. I've seen this on my RV-8.
Click to expand...

I'd vote with Kevin, unless you have already checked and have seen the increase in ground speeds as well. I have had small static leaks that always show up as "increased performance" several times. It's always a little disapointing when they get fixed.... ;)
 
I'll hunt for static leaks and verify the temperature indication.

As an aside, I made it to Phoenix in record time as expected, but the winds, turbulence and unusual altitude (9500) didn't allow me to see if the performance was "normal" or not. We'll see on the way home.
 
Cold CHTs not good

Hey Fellas:
Same as with a water cooled engine, cold operating temps are less than optimal. I am warned by TCM that any temp less than 320F on their engines is a good way to get lead into the top ring land, requiring a top O/H before it would normally be due. Later, I rec'd a letter asking my operating RPM, and if it was generally less than 2500RPM, they wanted to know TT and oil consumption as the engine was operated. Dunno what that is all about?

But, I will suggest that if you are running low CHTs, you are also wasting fuel to get this excessive cooling - note what DanH has done got 'have his cake and eat it too'. Squeeze the exit to get the temps up where they belong (in cruise): about 380F, and watch your speed increase.

Carry on!
Mark
 
Well, the return flight from Phoenix today saw speeds more in line with my normal, though still a touch faster. With OAT at about 30 degrees, CHTs are very cool and a very tight spread. Instead of my normal 50 LOP, I tried to get some heat back by running closer to peak EGT, but it did not help much. Fuel flow and speed went up, but the cylinders stayed between 250 and 270. Even my 66% block off plate on the oil cooler failed to help, and the best I could muster there was 165 degrees.

I have another cross country to the Harris Ranch NYE party, so we'll see how it behaves on that one.
 
It is a bad time of the year to try it, maybe try it on the Wife's side of the plane, Open a vent and see if your altitude changes. If it does, you have a static leak in the cabin.
 
You must log in or register to reply here.