155kt cruise?

[steve91t]

Hey guys, I'm asking this question for my dad who owns a very nice RV7 with 180 hp and a 3 bladed CS prop. I can't remember who makes the prop. Anyway, it has an amazing climb but cruise at 2450 or so is right at 155 kts true at 8.5 gph.

How is it that I'm seeing guys with less power cruising at 165 kts? I know the 3 bladed prop will have a better climb than a 2 bladed, but the cruise is going to be a little slower. How much slower? I think it weighs around 1100 full IFR, glass panel.

I just wanted to see how some of you guys are getting faster cruise speeds. I know Smokey's RVX can cruise at 160 KTS easily. Of course that's a light, slippery little plane.

Here's a quick video of the plane starting up last month.

http://www.youtube.com/watch?v=9-x-2UTXuGE

 

[Tandem46]

I've always got 170-172kts True above 8k on 8.5gph. That same 8.5gph down low only gives me 150-155kts true. I'm running IO-360 with CS prop. 1070lbs empty.

 

[craigvince]

Yeah, my first thought too....

At what altitude are you cruising at 155 kts?

I can pretty consistently get 170kts at 12k, which is where I prefer to ride, but down low you're not going to see that. By the way, I've had an RV-7A at 17,200 feet but it would only do about 140kts TAS.

 

[steve91t]

Sorry, this is 8000ft or so.

 

[Bavafa]

As it has been mentioned, it depends on a number of factors including to check to see if you have an accurate pitot/static setup to get a good IAS.

My typical TAS/burn rate is very much in line with the above posters.

 

[steve91t]

Well, everything seems to match the GPS. We were at 9000 last month showing 155kts true. GPS said we had 7 kts on the tail, showing 162 across the ground.

What else could it be? From some of the comments he's gotten, especially from mechanics, it's one of the best built RV's they've seen. It's rigged perfectly, and the engine runs flawlessly.

Is it the prop?

I mean, 15-18 kts is pretty substantial.

Steve

 

[steve91t]

Just got clarification, 65% power, 2450 RPM in cruise at 8000 ft. The 155kt cruise matches Vans numbers if he were using 55% power.

 

[NDrv8r]

do a gps box to verify the speed. Then you know you have a power problem if the actual speed is verified low, or an indication problem if the actual speed is higher.

right now I am getting around 165kt cruise at around 8gph, 8000 da.

flat out 185kt at 3000 da, about 16gph.

I am looking for some missing manifold pressure from my snorkel. A winter project perhaps.

 

[steve91t]

Engine is healthy. Just had an annual. If the engine is making proper MP, that means its developing the correct power, right? What is normal MP at 8000?

 

[steve91t]

Also, I know a 3 bladed prop isn't going to have the cruise of a 2 bladed. How much of a difference though? His is a light weight composite. Company is out of Florida.

 

[cjensen]

Just got clarification, 65% power, 2450 RPM in cruise at 8000 ft. The 155kt cruise matches Vans numbers if he were using 55% power.

Sounds about right with those numbers. I had a three blade fixed pitch, and always saw 160kts at 2450rpm, which was my preferred cruise setting. I had a light airplane too, but your numbers are comparable.

One other thing to check is to verify your OAT is calibrated and accurate. Mine was off in the beginning and I had to compensate for it's position (NACA duct) with the EFIS, but once I got that dialed in, I had an accurate reading that checked out with the GPS box run.

 

[leaker311]

I am with you

FWIW I am seeing the same numbers as you.

Hartzell BA CS (2 blades)
IO-360 mattituck std compression
RV-7
9k-10k
8gph
155-160 KTAS
I filled the emp tips and taped over the hinge gaps.

I am really wondering what magic tricks you 165 plus guys pulled out of your hats. I wanna know!!

High compression pistons maybe?
I know... its the marvel mystery oil!

Marco

 

[Brantel]

After paint I can get 175ktas @ 8000ft but that is WOT and leaned out.

180hp
FP Sensenich
RV7

 

[fehdxl]

Well, everything seems to match the GPS. We were at 9000 last month showing 155kts true. GPS said we had 7 kts on the tail, showing 162 across the ground.

Correct me if I'm wrong, but isn't the wind speed and direction derrived from the relationship of TAS, heading, ground track and ground speed. So of course its going to make sense...it's computed.

Now a 3- or 4-way gps ground speed test run factors out the wind so that all you have left is TAS by virtue of GS...which is what you are trying to verify.

Right?

Jim

 

[Chino Tom]

FWIW I am seeing the same numbers as you.

Hartzell BA CS (2 blades)
IO-360 mattituck std compression
RV-7
9k-10k
8gph
155-160 KTAS
I filled the emp tips and taped over the hinge gaps.

I am really wondering what magic tricks you 165 plus guys pulled out of your hats. I wanna know!!

High compression pistons maybe?
I know... its the marvel mystery oil!

Marco

Don't know what your power settings are, but I would guess if you are leaned
to 8 gph you are in the 60% power area. And if that is so, then your speed
is about right. Push the go knobs up to 2500 rpm and WOT with about 9 gph
and you would be around 65% power and I'll bet your TAS is in the 168 knot
range.

As others have mentioned, TAS is only as accurate as the ASI and outside
air temp gage. A three or four way gps run is the only accurate method
of determining TAS.

 

[steve91t]

So flying around at low alt with no wind, the gps matches the tas. Are you guys saying to fly the box pattern at 8000ft? Could it be accurate down low, then be way off at altitude?

Not saying that it isn't an indication problem, just saying after a year of flying the plane, everything seems to add up to proper indications.


If it was that off, on a long cross country, the ETA would be screwed up. And it's not. It's dead on.

Does this more or less rule out a low tas reading?

 

[Bavafa]

So flying around at low alt with no wind, the gps matches the tas. Are you guys saying to fly the box pattern at 8000ft? Could it be accurate down low, then be way off at altitude?

Yes, it is possible with a number of factor, including the inaccurate OAT or the fact that it is difficult to determine NO WIND condition. So the best and more accurate way is to do it at multiple altitude and compare results.

Another way to check your static accuracy is to check for altitude, setting and noting of your altimeter setting at any given airport and then do a relatively low pass to compare.

 

[Finley Atherton]

Steve,

Need to know how the engine is being leaned as this will have a huge effect on power.

From Lycoming's graphs for the I0-360 at your 8.5 g/h and 2,550 rpm (post #1):

At Best Economy settings (Peak or LOP) power is 112 hp or 62%
At Best Power settings (around 150 degrees ROP) power is 90 hp or 50%

This 22 hp difference for the same FF and rpm could explain the slower TAS.

Fin
9A

 

[steve91t]

Steve,

Need to know how the engine is being leaned as this will have a huge effect on power.

From Lycoming's graphs for the I0-360 at your 8.5 g/h and 2,550 rpm (post #1):

At Best Economy settings (Peak or LOP) power is 112 hp or 62%
At Best Power settings (around 150 degrees ROP) power is 90 hp or 50%

This 22 hp difference for the same FF and rpm could explain the slower TAS.

Fin
9A

I was incorrect in my first post. He actually runs it at 2450 revs. I know he runs it just rich of peak. Not sure exactly how much, but I know he said it's why Lycoming recommends.

I think he should just turbo it and be done with it

 

[steve91t]

Yes, it is possible with a number of factor, including the inaccurate OAT or the fact that it is difficult to determine NO WIND condition. So the best and more accurate way is to do it at multiple altitude and compare results.

Another way to check your static accuracy is to check for altitude, setting and noting of your altimeter setting at any given airport and then do a relatively low pass to compare.

I like the 2nd option. Sounds like more fun

 

[steve91t]

I'll definitely force my dad to run out to the airport on a calm afternoon and go fly to try out the box pattern idea at 8000 ft

I still don't think that's it. The ETA would always be way off if he had bad readings. Either that, or the GPS would show a tail wind all the time.

The builder of the plane thinks it's probably the prop. He said when he would take off in formation, he would leave them in the dust in the take off and climb, but then once in cruise, they would walk away from him.

Does MT sound right? Composite 3 bladed prop out of Florida?

Maybe the design of the prop is designed for climb performance. But at the same time, RPM is RPM, and I can't imagine there's "that" much of a difference.

 

[gereed75]

Don't have my charts or numbers in front of me

But my gut is saying that 8.5 indicates your running either LOP or less than WOT at that Alt. In my 180 hp 6, WOT at 8000' at 2450 I would be seeing more like 10.5 gph and 165 KTAS+.

He is running either less than WOT or LOP.

Finley, don't you have that opposite - Best power (150 ROP) at 112 hp and 62% with Economy (LOP) 90 hp and 50%????

At anyrate, 8000' and 8.5 gph at 150 KTAS or so looks about right. Does'nt much matter how you are burning it (ROP or LOP). Horsepower and thus speed will be relative to fuel burn. Firewall it ROP and you will see that 165 KTAS at least, but at 10 - 10.5 gph.

 

[DanH]

I'll definitely force my dad to run out to the airport on a calm afternoon and go fly to try out the box pattern idea at 8000 ft

It's a bit more than an idea. Go here for the tools:

http://www.kilohotel.com/rv8/phplinks/index.php?&PID=49&PHPSESSID=af2098b3b1a9eaddb2e093cf6eedfb88

I suspect a whole lot of folks will not take your issue seriously until you calibrate.

BTW, the winds need not be calm. You do need to hold an accurate altitude and heading, so best to do it before the thermals crank up.

 

[Finley Atherton]

Finley, don't you have that opposite - Best power (150 ROP) at 112 hp and 62% with Economy (LOP) 90 hp and 50%????

Remember my figures are for the SAME rpm and FF. At Best Economy settings you are getting better fuel efficiency so the same amount of fuel produces more power. Have a look at Lycoming's Part Throttle Fuel Consumption graph.

Fin
9A

 

[Finley Atherton]

Does'nt much matter how you are burning it (ROP or LOP). Horsepower and thus speed will be relative to fuel burn.

I think it does matter. Horsepower will be relative to fuel burn (FF) and how efficiently that burn is happening. The fuel will burn more efficiently and produce more power at Peak or LOP than at richer mixture settings.

Fin
9A

 

[gereed75]

Thanks Fin

thats interesting, and confirms the definitive answer to a question I've always had - Will you get better range burning the same gph at best power and less throttle or best efficiency (LOP) and WOT??

Sounds like LOP/WOT by far. 22 More horsepower on same fuel according to the chart! Suspected it was true (Best efficiency at close to ideal A/F ratio - duh) but did'nt think it would be that much.

Thanks again Fin.

 

[Mike S]

If my poor old brain is firing on all cylinders, I seem to remember that in WW2, Lindbergh proved that the best range was obtained by running the engine slowly, with WOT, and leaned out as far as possible.

 

[Finley Atherton]

I was incorrect in my first post. He actually runs it at 2450 revs. I know he runs it just rich of peak. Not sure exactly how much, but I know he said it's why Lycoming recommends.

So to be clear he is running at 2,450 rpm and 8.5 g/h? If so this would be about 118 hp at Best Economy or about 92 hp at Best power (150 ROP).
Any setting richer than peak will reduce the hp for the same FF (assumes constant rpm). Lets say he was running 50 degrees ROP. I am not sure if the relationship is linear but if we interpolate the above figures then 50 degrees ROP would give about 109 hp which is 9 hp less that that possible at Peak or LOP and could explain some of the slower speed.

Fin
9A

 

[RV10inOz]

That is true! Just for the record, best economy is usually Round 20-40lop and for a combination of economy and a bit more speed from 10-20lop. Best power is more like 75ROP, and at 8000 feet that is ok, but do not do WOT/2700/75ROP at 1500 feet for very long. It will clean out your heads nicely but if you do it all the time .

Kevin Hortons GPS box is critical. Static Port errors are amazing

I have been test flying Jamie Lee's RV7 lately and it generally does 155 with its cruise Catto 3 blade. I doubt that you will get near the fastest of the 2 bladers but that is the trade off.

Mt opinion for what it is worth, 3 blade props are a waste of money unless you have 310 HP or more to play with. You can argue all you like about this and that, but at the end of the day, facts are what they are. They are slower, and a fixed three blader that is pitched for cruise is not that lively off the deck as you only generate 150 HP out of your 180 HP engine.

I know an RV6 down here with a 160hp & fixed 2 blade, it runs rings around the 3 blade, well almost, but it is quicker and no slower off the deck on a smaller engine

PS: for csu planes, unless you are really needing to pull the throttle, it stays WOT all the time. You would not fly around with a filter full of dirt would you? Think about hat for a bit!

 

[gereed75]

Yep

Yep yep yep, agree Fin and Dave.

The OP's numbers and comments would still indicate he is at something less than max power (either via less than WOT or less FF than Best Power or some combo of both) and the TAS is in line with some less than max power condition.

You want 165+ KTAS?? - make more power!!

Mike, My work with trying to determine a best practical range showed that lower RPM helped. What exact RPM is optimal will have much to do with other factors (altitudes, props, chosen airspeed etc). I ran lots of runs and found down as low as 1800 RPM, WOT, LOP generated some incredible range numbers if you could stand the slow speeds (and had favorable winds). I like the 1.3 Vy speed at low RPM, WOT, LOP if I was in extremis - lost over open ocean or trying to get out past zero/zero weather to a far away alternate (with an adjustment for wind).

Fin, What do the charts say relative to RPM and efficiency??

 

[Finley Atherton]

Fin, What do the charts say relative to RPM and efficiency??

As an example lets take a FF of 7.5 g/h and Best Economy mixture settings for an I0-360 180 hp:

2,700 rpm gives 88 hp
2,000 rpm gives 108 hp

Another example. At Best Power mixture settings and a FF of 8.5 g/h:

2,700 rpm gives 82 hp
2,200 rpm gives 99 hp.

Fin
9A

 

[DanH]

Fin's observation is a bit more clear if you consider the necessary manifold pressure setting.

With constant RPM....

Lean the mixture to 50 LOP. Fuel flow decreases. To return fuel flow to 8.5 you increase MP. Same fuel flow and more manifold pressure means more HP.

Richen the mixture to 150 ROP, best power. Fuel flow rises. To reduce fuel flow back to 8.5 you must pull the throttle. 8.5 and less MP means less HP.

Exactly what change in manifold pressure would be required I do not know.

With constant RPM and constant MP LOP means less power and 150 ROP is more power, as you expect.

 

[Finley Atherton]

Fin's observation is a bit more clear if you consider the necessary manifold pressure setting.

Dan,
Thanks for the assist. You are clear and concise as always.

Just for the record, best economy is usually Round 20-40lop and for a combination of economy and a bit more speed from 10-20lop. Best power is more like 75ROP

Not trying to disagree with you (us Aussies have to stick together) but its difficult to know the exact figures as Lycoming puts out some contradictory information.

In the Operators Manual they say don't go leaner than 150 ROP at Best Power mixture settings. In their Lycomings Operations flyer they say lean to 100 ROP for Best Power cruise. Also in the Operations flyer is a representative diagram that shows max power for cruise at about 140 ROP and their Best Power band for cruise from about 100 ROP to about 170 ROP. So from this diagram at least, 75 ROP is outside the max power band.

The representative graph in the flyer gives the best Specific Fuel Consumption at about 20 LOP with almost no difference apparent between Peak and 20 LOP but I agree speed should be a bit higher nearer Peak (assuming throttle setting is the same).

Fin
9A

 

[fehdxl]

...on a long cross country, the ETA would be screwed up. And it's not. It's dead on.

Can you expand on what you mean by this?

Thx,

-Jim

 

[RV10inOz]

Ah Finley, you have started the path to discovery and true enlightenment!

This is going to sound cocky, but I am sure those who do know will agree. The Lycoming publications are often WRONG and as you have pointed out contradict each other

There is better info out there

 

[DanH]

This is going to sound cocky, but I am sure those who do know will agree. The Lycoming publications are often WRONG and as you have pointed out contradict each other
There is better info out there

Lycoming dyno chart....full throttle, 28.5 MAP and 2700 RPM. Mixture is pulled full range from bog rich to LOP.

Max power is a range from 200 ROP to 100 ROP, just as Fin found in the Lycoming publications.

Show us the better info, please.

 

[steve91t]

Wow, lots of great information. Thanks for all of the responses.

I will say this quickly, there is no way that my dad is setting the mixture so incorrectly that he's loosing 15 kts. Not possible. He is setting it correctly, I promise.


fehdxl: What I mean is that if the GPS says we'll be somewhere in 1 hour, and we are, then that's correct. We've also noted that the GPS and the TAS match. If they didn't, the EFIS would be telling us that we have a kickin tail wind ALL THE TIME.

It's not like I'm looking for a few knots here. According to some, we are missing 15+ kts.


If my dad flies 155 miles in 1 hour, he was doing 155 miles per hour. Now, let's say he's INDICATING 155 kts true and he flies 155 miles. If he gets there early, then his indications are reading slow.

Something just isn't adding up. And I honestly do not think it's the static system or the way he's leaning the engine.

I will say this though. We have not flown the box pattern like you guys have said to completely rule out the static system, but like I said, there's no way it's 15 kts off and we didn't know it. Also, leaning an engine so incorrectly that you loose 15 kts is not something that you wouldn't notice. I can tell you that isn't the issue.



It's got to be the prop then, right? That's the only thing that I've read on here that actually says will slow you down. But by how much remands a mystery. Some say not much at all, some say it's substantial.

Anyway, thanks again for all of the help. I know I've learned something.

Maybe we could switch to some prop talk since I think that's the problem.

Steve

 

[RV10inOz]

Dan, please read what I wrote...... I have been through this time and time again, as was pointed out previously they often contradict themselves from one publication to the next. And on many topics.

The "cherry picked" piece of information you have just posted is most likely perfectly representing the test conducted. So I am not sure what your point is. I am not suggusting this chart is fudged, and it does look exactly as I would expect. And this chart is at odds with what Lycoming often print. Sometimes they print great stuff, other times it astounds me that they still print it.

Lets look closely at what you and I are debating here. Remember Peak EGT is where everything should be referenced from. It is the point we can measure in flight and work away from in either direction.

Now first look at BSFC, and see where it begins to flat line. Using the lower EGT trace as it is easier to see (they should be all relative) then go up the the EGTs and you will see that the point at which you hit the EGT is at 10degF LOP and it continues to 40F LOP.

Now look at the HP line, from left to right coming from LOP to ROP, the HP max's out at 25F ROP and a smidgen more by 75/80 ROP. Moving further to the right the power does nothing through to 200ROP and beyond and it actually starts falling.

Gee look at that.....the graph just proves my earlier statement.

Just for the record, best economy is usually Round 20-40lop and for a combination of economy and a bit more speed from 10-20lop. Best power is more like 75ROP

Now lets remember that the chart you posted is a detonation test. Cylinders running really hot, and everthing full bore to test what they were looking for. I wonder what that does to the "actual numbers" as you get into a detonation test does that mean best power is achieved for longer going ROP due to the high CHT required for the test? In other words as the fuel is increased to richer and richer, did the HP measured stay higher for longer rather than taper off?

Here is a generic TCM chart, note the peak power at half way from 50 to 100 ROP, and scale depending it seems power drops away again more rapidly than the Lycoming chart.

And here is another one from Pratt & Whitney, same thing oddly enough.

These graphs are not new, in fact they all have been accepted fact for 50+ years.

Now my concern is that your detonation graph is possibly misleading for the typical real world application with CHT's in the mid 300's and not trying for detonation. I am not for one minute saying that Lycoming cheated, or fiddled the data. The only explanation off the top of my head for the very flat power curve at very ROP settings is that combustion was very strong in such a high temp test. Maybe it is just a scaling thing, but my eyes are hurting reading these graphs on my laptop.

So any better data out there you asked? No more so than that curve with a caveat that the flat HP line ROP could be affected by the nature of the test.

As for other sources, if you can find them in an old 2nd hand bookstore; The Aircraft Engine and its Operation by Pratt & Whitney, Basic Theory of Operation - Turbo Compound Engine by Curtis Wright.

 

[RV10inOz]

Some more reading which I had to go re-discover, this is lifted from an Avweb article so all attributions to them.

Heads up, crucial concept coming! Assume that we increase MP to increase power output to something substantially above 65% power. Now suppose we lean the mixture a bit, until the actual power drops to exactly 65% power again?

Here's exactly what that looks like:

What's Going On Here?

This is a real picture of a Cessna 414, previously trimmed up very carefully for straight and level flight at equal MP, RPM, and mixture settings. After that, the throttle is increased, and the mixture decreased, to produce what you see here, with no tendency to yaw/roll/turn. This situation is actually very sensitive to very small differences in power.

Remember,both engines are producing exactly the same power!

The left MP is 3" higher than the right, but the left fuel flow is 3.2 GPH lower! Note further, the EGT is only 10? F higher, but the CHT (as shown by the missing bars on the Graphic Engine Monitor display) are 1 to 3 bars lower, with each bar representing 25? F.

Ponder this: cooler CHTs, less fuel, same power. Sounds like magic, doesn't it? Why, if we could run that MP up high enough, and pull the mixture back far enough, we might invent perpetual motion! Unfortunately, this is another aviation case where a little is good, but "more" isn't. There are other forces at work.

Steve back to your speed tests,

I will say this quickly, there is no way that my dad is setting the mixture so incorrectly that he's loosing 15 kts. Not possible. He is setting it correctly, I promise.

Are you so sure? have you sat there and done the same? I can show you a 20 knot difference in my RV10.

More important is do the GPS box and compare the GPS calculated TAS, to the one shown on your EFIS or from manual calculation. If they are not within 2 knots, it might be time to play with your static port. I have gone from 9 knots too slow to 6 too high until I got sub 2 knots, just by small adjustments around the static port.

 

[David-aviator]

Hey guys, I'm asking this question for my dad who owns a very nice RV7 with 180 hp and a 3 bladed CS prop. I can't remember who makes the prop. Anyway, it has an amazing climb but cruise at 2550 or so is right at 155 kts true at 8.5 gph.

How is it that I'm seeing guys with less power cruising at 165 kts? I know the 3 bladed prop will have a better climb than a 2 bladed, but the cruise is going to be a little slower. How much slower? I think it weighs around 1100 full IFR, glass panel.

I just wanted to see how some of you guys are getting faster cruise speeds. I know Smokey's RVX can cruise at 160 KTS easily. Of course that's a light, slippery little plane.

http://www.youtube.com/watch?v=9-x-2UTXuGE

Vans reports 55% cruise with the 7A to be 177 mph. 155kts = 178mph. That figures, the airplane is a 7.

My spread sheet HP calculator says 8.5 gph = 55.56% power with 180 HP being 100%. (BSFC at .51)

I do not detect a problem with the performance of this airplane.

If your dad wants to go faster, tell him to increase power. Push it up to 75%, he will be going 198 mph or 172 KTAS.

If he decides to junk that MT prop anyhow, I will gladly go dumpster diving to retrieve it.

 

[Kevin Horton]

8.5 gph at just rich of peak EGT is likely somewhere around 63% power, which is pretty close to the reported 65% power. This calculation is based on an old Lycoming document that describes how to calculate power from fuel flow data.

As near as I can tell from Van's claimed performance, you are about 9 kt slower than would be expected, based on his 75% claims, corrected to 63% power, which gives 164 kt.

I changed from an older Hartzell two-bladed prop (7666 blades, as sold by Vans before the blended airfoil Hartzells came along) to a three bladed MT prop. I lost about 5 kt cruise speed with that prop change. So, if your airspeed indications were perfect, your engine was producing the expected power, and there was no excess drag on the aircraft, I'd expect your dad's aircraft to produce somewhere around 160 kt at that power setting with that prop.

The aircraft could easily have 5 kt of error in the static system and/or the EFIS ASI. My EFIS ASI had up to 2 kt of instrument error. And my static source position error was up to 2 kt also. The fuel flow indicator or tachometer might not be perfectly accurate either.

 

[David-aviator]

8.5 gph at just rich of peak EGT is likely somewhere around 63% power, which is pretty close to the reported 65% power. This calculation is based on an old Lycoming document that describes how to calculate power from fuel flow data.

As near as I can tell from Van's claimed performance, you are about 9 kt slower than would be expected, based on his 75% claims, corrected to 63% power, which gives 164 kt.

I changed from an older Hartzell two-bladed prop (7666 blades, as sold by Vans before the blended airfoil Hartzells came along) to a three bladed MT prop. I lost about 5 kt cruise speed with that prop change. So, if your airspeed indications were perfect, your engine was producing the expected power, and there was no excess drag on the aircraft, I'd expect your dad's aircraft to produce somewhere around 160 kt at that power setting with that prop.

The aircraft could easily have 5 kt of error in the static system and/or the EFIS ASI. My EFIS ASI had up to 2 kt of instrument error. And my static source position error was up to 2 kt also. The fuel flow indicator or tachometer might not be perfectly accurate either.

If 8.5 gph = 63% power, then at 100% burn will be just 13.49 gph.

That simply is not true for the 0360 at 180 HP, it burns more fuel than 13.49 gph at 100%.

I calculate a burn of 11.5 at 75%, and 15.3 gph at 100%. In flight performance confirms these figures to be reasonably accurate. I have seen 199 mph true (172kts) with WOT at 8500, burn right at 12, which is consistent with the prop turning about 2780, slightly more than 75% power.

63% power at 8.5 gph is a reasonable expectation with an 0320.

 

[Kevin Horton]

If 8.5 gph = 63% power, then at 100% burn will be just 13.49 gph.

That simply is not true for the 0360 at 180 HP, it burns more fuel than 13.49 gph at 100%.

This is a bit of an apples and oranges comparison. The 63% power at 8.5 gph is only valid if the fuel flow is just a tiny bit rich of peak EGT, as reported by the original poster. You wouldn't want to run nearly this lean at 100% power, and even if you tried the mixture would be leaner than best power mixture so the engine wouldn't produce 180 hp.

I calculate a burn of 11.5 at 75%, and 15.3 gph at 100%. In flight performance confirms these figures to be reasonably accurate. I have seen 199 mph true (172kts) with WOT at 8500, burn right at 12, which is consistent with the prop turning about 2780, slightly more than 75% power.

63% power at 8.5 gph is a reasonable expectation with an 0320.

The Lycoming data I referenced earlier suggests 75% power at 2700 rpm and mixture set for best power would give a fuel flow between 11.2 and 12.1 gph. 100% power would need a fuel flow between 14.1 and 15.4 gph. This is pretty consistent with your numbers.

 

[David-aviator]

Kevin,

Thanks for the quick reply, we appear to be on the same page.

So does Steve's father have a problem or not? It is common knowledge the MT prop is not quite as efficient on the top end, but I believe the deficit is just 2 or 3 knots, not 15.

I'd like to see what this airplane will do at 8500, WOT and 2600 rpm and leaned to 100 ROP. I'm betting it will come in at 198-200 mph true.

My experience with the MT 7 and the Subby H6 was IAS slowed up a couple knots when running at 2700 vrs 2600 rpm.

Like Steve said, it has great performance on take off and climb.

 

[steve91t]

I'll have him try everything recommend here.

On the specific flight that I was on, where I was flying, we were between 8 and 9000, WOT, leaned rich of peak, 2450 RPM. That was giving us 65% power according to the EFIS. Cruise speed was 155 kts true, 162 kts over the ground.

We did two flights that day. We flew from Roanoke, VA to Knoxville, TN and back. On the way there, it took us roughly an hour and a half. On the way back, just a couple of hours later, it took us an hour and 15 minutes. I flew both legs, and I can't remember exactly how much we leaned it. I do know we were WOT from the time we took off until our decent.

I took a quick video during one of those flights, but I can't make out much more than the MP. It looks like it's 22.4. This is at 8000ft.

http://youtu.be/H6pYg1ekrU4

 

[Kevin Horton]

So does Steve's father have a problem or not?

Well, it sounds like he isn't completely happy with the aircraft, so in his mind he has a problem. Certainly something doesn't quite add up with the reported performance. It could be one or many of the following (in no particular order):

• airframe with excessive drag,
• engine that is low on power,
• prop with poor efficiency,
• airspeed system error,
• OAT indicatotr instrument error,
• static source position error,
• tachometer indication error, and/or
• fuel flow system indication error.

I'd like to see what this airplane will do at 8500, WOT and 2600 rpm and leaned to 100 ROP. I'm betting it will come in at 198-200 mph true.

I agree that it would be useful to see the results from that test.

It would also be useful to see the results from a four-sided box pattern to check airspeed system error at the IAS that is normally seen in cruise. Fly four legs at roughly 90 degrees to each other, at the same altitude (within 50 ft of the target altitude). Adjust power as required to obtain exactly the target IAS on each leg. Wait a few seconds to allow the GPS to stablize, then record GPS ground speed and GPS track on each leg. Also report the altitude, altimeter setting and indicated OAT. It can be tough to get a good set of data using this test (a really, really good set of data has a standard deviation of 0.1 kt, as reported by the spreadsheet I created for Kitplanes - anything over 0.5 kt is probably not worth looking at). So, do this test three or four times in one flight, so hopefully at least one of the data sets will be a winner.

 

[DanH]

The "cherry picked" piece of information you have just posted is most likely perfectly representing the test conducted. So I am not sure what your point is. I am not suggusting this chart is fudged, and it does look exactly as I would expect. And this chart is at odds with what Lycoming often print. Sometimes they print great stuff, other times it astounds me that they still print it.

Ok, let's check the leading Lycoming competitor's clone.

 

[C. Brenden]

Need more power!

My experience is with a 6A but when we first started flying our prop (wood-Sterba) was over pitched and max RPM was unacceptable. After it was repitched and balanced we were suddenly making the power we needed. The engine was unchanged but the power, and also speed, increased to what was published by vans. We were able to cruise at 165 knots at 7500msl at aprox 2650 rpm. We later upgraded to the fixed metal prop vans sells and picked up another 3 knots tas. Our fuel burn was between 9.5 and 10 gph with a carburetor.
I think the rpm needs to get increased, increasing fuel burn, and the tas will be in the ball park of what is expected.

 

[steve91t]

I've got a quick question. Is MP effected by how the engine is leaned? Is an engine running at full rich at 8000ft WOT going to have the same MP as an engine leaned at 8000 ft WOT at the same RPM?

 

[Kevin Horton]

I've got a quick question. Is MP effected by how the engine is leaned? Is an engine running at full rich at 8000ft WOT going to have the same MP as an engine leaned at 8000 ft WOT at the same RPM?

For all practical purposes the answer is yes.

If you had a very sensitive, high resolution MP gauge you might be able to see a tiny change in MP after you leaned, as the airspeed would be slightly different due to the change in power and there would be a bit more or less ram pressure in the inlet. But I really doubt you would be able to see that effect with typical MP indication systems.