Mike D

Mike D

Well Known Member
Any ideas why I am slow?? Am I worried for nothing?

RV-6A, O-320 E-series ? 150HP, Sensenich 70CM7S9-0-77, slick mags.
Seems I can only get 155 knots (true) turning 2450 rpm and burning around 9.5 gph @ 25? MAP. DALT is around 4K? (full fairings, but have 2-comms, a transponder and a VOR antennas sticking out)

Lycoming manual said 75% power = 2450 @ 10.0GPH (so I am probably leaned more than this)

Vans states a 150HP RV-6A should be doing 163 knots @ 75% power.

So it seems I am around 10 knots slow.

I often read others are doing something like 170 knots @ 8.5gph. I don?t know the specifics on these statements, but I have flown alongside other RV?s (-4?s, -6?s and -8?s w/ O-320?s and O-360?s) who have 5-10 knots (GPS ground speed) on me with 1 to 1.5 less GPH fuel burn and turning 2500 to 2700 on the RPM.

I am using the GRT EIS to calculate the proper lean. CHT spread is around 50-deg and EGT spread is around 25 deg.

I am not looking to race but do want good efficiency. So why am I so slow and burning so much fuel (for an RV :D)???
 
Try 8k

If you use your E6B, set it at 4000', find your 155 TAS and then move the wheel to 8000 you will get 165 TAS. When Van's says 75% he means WOT, full revs at 8000'.

Your speed and fuel burn compared to other RV's could be a number of things, but you are probably close to Van's numbers. If 2450 is your max RPM then your FP prop sounds over-pitched; you should be able to get 2700 rpm at full throttle at 8000' if you are trying for max TAS. The prop would be one of the first places I'd look.

You say it's a carburetor engine. You usually cannot lean a carb engine as well as a FI engine, no matter what the EIS says. You can do max power well, though and you don't need an EIS, just an ASI and a little patience.

I hope this is not confusing you, but it sounds like your airplane is performing better than you think and that it could do better. Try 8000' before going any further.
 
Not making 75%

Do you know what your cylinder compressions are?
Leaned to best power? 100 degrees rich of peak?
Is your manifold pressure guage reading right?
Plugs all clean and in good shape?

Doesn't sound like your making 75% power at your 75% power settings.
 
Thanks for the help!!

I hope I am worrying over nothing. I guess I worry more now that I have all these numbers form the EFIS/EIS.

Compressions are 76-78. Plugs are clean. And MAP sensor appears to be reading right.

2450 is not WOT. I have a 2600 rpm restriction on the prop.

I will take it up to 8K and see what the numbers read.
 
one more thought

I have a -9a with the same low-time, good condition engine & -77 prop, and only get 2100 rpm static, and have trouble seeing 2500 revs at altitude.
I expect a -6 would be faster than me, so one last place to look would be rigging, how straight the fairings are, engine airflow etc.
...I too am puzzled by some of the figures 'expected'.
 
I just calculated how fast a 77 inch pitch prop with no slip at 2450 rpm would be and it was 155 kts. So you are going as fast as that prop will let you at that rpm. Seems pretty good actually. So you either have to go to a higher rpm or more pitch to go faster.
 
How'd ya do that?

RV7AV8R said:
I just calculated how fast a 77 inch pitch prop with no slip at 2450 rpm would be and it was 155 kts.
Click to expand...

I'm eager to learn, so how did you do that calculation? I suspect that your answer is going to be complicated and that I'll barely understand it, so a pointer to a good website or book on props that covers in detail everything I won't understand in your answer would be ideal. :)

Thanks in advance.

--Stephen
 
THere is 77 inch forward movement for each turn of the prop times 2450 turns per minute = 188650 inches per minute. That equals 1,1319,000 inches per hour. Which is 943,250 ft per hour or 178.64 miles per hour. 1 Kt = 1mph times 1.15, so to convert 178 mph to kts divide by 1.15 and you get 155 kts.
Hope this helps.
Kts=pitch (inches) X rpm X60 min/hr /12 inches/ft /5280ft/mile /1.15 miles/kt
 
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Yep, that helped

RV7AV8R said:
Hope this helps.
Click to expand...

Thanks. As simple as your explanation was, it still helped. As an EE, unit analysis and conversion is second nature to me. Believe it or not, your answer ended up finally getting through my head just exactly what is meant when people referred to the pitch of a prop. I understood it includes angles, and twist, and length, and bite, and other concepts such as those, but it had somehow eluded me that all this was being boiled down to one number for ease of discussion and for comparison between props.

Okay, so pitch is just something like "the steady-state forward motion of a prop as it completes one revolution at maximum efficiency." Got it.

(Slaps head, mutters "Dope!" under his breath.)

I hope I'm not the only one to pick that up so slowly. As George Leonard said, when we start something new, we all must be willing to "wear the white belt."

Which I guess puts the OP's question in a whole new light. Does anybody have any ideas for why I'm so slow? :)

--Stephen
 
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It's hard to tell over the internet.....

...because the fairings may not fit tightly to the wheelpants or the fuselage.....the canopy skirts and their snug fit....under cowl flow straighteners and such. You really need to be next to an airplane to look at everything. How many round head screws instead of flush ones...on and on.

Best,
 
Why Am I Slow?

Mike, My RV-6A has a FP 80" Prince Prop w/ 180 HP O-360. I am dragging 3 antennas and two steps (& a nose gear). My normal cruise is 173-175 MPH, 2450 rpm @ 8000' burning around 8 G/HR.

I have a couple of buddies w/ the 150 HP Fixed Pitch setups, one RV-4 and one RV-6A. Your numbers are very similar to theirs! Compare yourself to other RV side by side models w/ 150 Hp engines instead of Van's numbers! I really don't think your all that slow.

You'll hear all the "hangar talk" about side by side RV's cruising in the 170-180 KT range but, the true story is reported on websites like "Flightaware". Log in and observe side by side RV cruise speeds. You'll find 95% of them are in the 140-160 Kt range!
 
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fatherson said:
I'm eager to learn, so how did you do that calculation? I suspect that your answer is going to be complicated and that I'll barely understand it, so a pointer to a good website or book on props that covers in detail everything I won't understand in your answer would be ideal. :)

Thanks in advance.

--Stephen
Click to expand...


Or you can use some of the calculators and tables that are online:

http://www.culverprops.com/viewpitchtable.htm

http://www.culverprops.com/pitchselection.htm

These are theoretical calcs for a generic prop so your mileage may vary.
 
Some detail issues

This does not change your issues, but it's worth knowing for most of us.

The nominal pitch of a prop is usually the geometric pitch at the 75% radius station. Two props of different design/manufacture with the same nominal pitch are unlikely to perform equally. The calculation given was a good approximation, but only that. It's useful in a forum discussion but not precise.

The maximum effective pitch of a prop on a given airplane under given flight conditions is not necessarily when/where the maximum efficiency is. Intuitively, you would expect it to be, but it just isn't so in the real world. Try this thought experiment:

A prop is a rotating wing. Wings have both a best L/D speed and a Minimum Power for Lift speed. Those two speeds differ on a fixed wing by a factor of 1.32, approximately. The best L/D speed takes you the farthest in a glide, but requires about 14% more horsepower. The Minimum Power for Lift speed gives you the most vertical thrust (lift) for a given horsepower. We usually call that speed the Minimum Sink speed, but it's the same thing.

Now think about that in terms of a prop. Effective pitch is how far (TAS) the airplane travels per rotation. Efficiency is how much HP it takes to go a given TAS which is actually a given drag x speed.
 
Most likely it's due to your inefficient prop. I tested an RV-8 with a Sensenich 70CM65900-77 and he could only make a GPS-derived 156.8 mph at 9520' dalt and 2620 rpm. Contrast this with an RV-6 with 150 hp and an Elippse three-blade prop plus extended wingtips going 191 mph at 9600' dalt at 2670 rpm.
 
It's due to a combination of genetics and environment. :)

Sorry, I'm home puppy sitting today and I just couldn't resist answering your question.
 
elippse said:
Most likely it's due to your inefficient prop. I tested an RV-8 with a Sensenich 70CM65900-77 and he could only make a GPS-derived 156.8 mph at 9520' dalt and 2620 rpm. Contrast this with an RV-6 with 150 hp and an Elippse three-blade prop plus extended wingtips going 191 mph at 9600' dalt at 2670 rpm.
Click to expand...

Hmmmm....the metal FP Sensenich has an excellent reputation for efficiency. Vans has tested this prop and field history has proven it to have fine cruise performance if pitched properly for the particular engine/airframe combination. According to reports I've seen over the years it will yield cruise very close to a constant speed prop.

The only disadvantage to the prop on an O-320 is the 2600 rpm restriction. 190+ mph is within the capabilities of my O-320 RV-6 and fixed Sensi (80" pitch).
 
Metal fixed is good!

elippse said:
Most likely it's due to your inefficient prop. I tested an RV-8 with a Sensenich 70CM65900-77 and he could only make a GPS-derived 156.8 mph at 9520' dalt and 2620 rpm. Contrast this with an RV-6 with 150 hp and an Elippse three-blade prop plus extended wingtips going 191 mph at 9600' dalt at 2670 rpm.
Click to expand...

I have to agree with Sam- That Sense. prop is an efficient bugger. The CS will beat the fixed metal From climb. But at cruse that 160HP can make out like a scalded cat when you are able to get full RPM/HP/WOT at the 2595.23 RPM.

And the 180HP fixed Sensi will out perform my CS in cruse. I have seen it up close pulling away while I?m dialing in all the knobs ?(also could be I have that nose gear up front keeping me down a few knots):cool:
 
On my 7 I started out with a Sensenich 85 inch pitch (0-360) and later installed a Hartzell BA. If there is a difference in top speed it is so little that I can not tell you which was faster.
 
I feel a sickness coming on.

Thanks for all the feedback and for teaching me more on how to work out prop efficiency.

I will have to recheck the rigging and do some runs at 8K?when it stops raining.

So it seems I may be getting the right speed for the given RPM?s, but now how do I get more RPM?s for less fuel? Is that possible?

Hey Bob Axsom... So how do you do it?? Oh No I may be starting down the road of modifying for more efficacy. I hear there is no cure for this.:D

Another thought, I assume that an engine?s rated power is for the octane listed on the data plate. In my case 150hp using 87 octane. If I am actually using 100LL, does this mean I should either be more efficient or have more HP than stated on the data plate?
 
twsurveyor said:
...Another thought, I assume that an engine?s rated power is for the octane listed on the data plate. In my case 150hp using 87 octane. If I am actually using 100LL, does this mean I should either be more efficient or have more HP than stated on the data plate?
Click to expand...
100LL actually has less energy per unit volume than 80 octane.

However, the difference is so little; I doubt you will notice it.
 
With apologies, in my previous posting I listed the RV-8's speed at 156.8 MPH; it should have been 156.8KTS, 180.5 mph. Sorry about that but it is still too slow.
 
Huh?

Mike D said:
So it seems I may be getting the right speed for the given RPM?s, but now how do I get more RPM?s for less fuel? Is that possible?
Click to expand...
That's like asking for better fuel economy with larger HP/Torque/Cubic inch engine-Just doesn?t work out with all else being equal.

For better economy, yep, It?s the racer?s theory- clean it up. I fly with Mike ?KB? Smith?s 7A in formation with his fixed pitch wood prop and identical engine. His airframe is much cleaner and a little lighter, He will pull away in cruse with his WOT while I?m WOT and running the prop at 2650. Best I can do...
 
Update

Well I have done a few runs at 8K?. Here is about how it went. (first post has specs on plane and engine)

Looks like 145kts indicated, 160 ground speed, and 164kts true air speed @ 8K? density altitude. I flew a box and it all averages out to around 165kts true.
This was with full throttle, burning 10gph, and around 100-deg ROP.

speed-11-s.jpg


I could lean it out more and lose about 5kts but only burn 8.7gph at 20-deg ROP.
I could not get more than 2550 RPM?s at this altitude. But I do have that 2600 rpm restriction so this is okay with me.

So, is this slow?? Too much fuel?? Or am I believing others hangar talk and really shouldn?t. What do you guys think???
 
You need to calibrate your GRT

Your GS should be the sum of your HW and your TAS. There is a 2 knot discrepancy. Not bad, but you can get it closer if you want to.

I understand your being willing to accept 2550 rpm, but you are down some HP from 2700.

Van's expects a solo-weight 6A with 150 HP engine at 8000' to go 185 mph. You are going about 189 mph on a little less HP. That's really good IMHO.

Ten gph at 0.5 SFC would be about 120 HP. (10 * 60 = 60 pounds. 0.5 pounds per HP per Hour = 120). That is 80% of 150. Since you are not using all your rpm's that is perhaps a little bit high, but not grossly so. Are you leaning by ear or by temp? For this kind of test I suggest you lean it to get the maximum speed (perhaps you did) and that will be, by definition, the best power mix. For an engine in good condition, that should be about 0.50 SFC, more or less. I would expect best power at around 75 degrees ROP, but that's not necessarily correct for any individual engine and instrumentation. For a carb engine, a 50 degree CHT spread sounds pretty good to me.
 
Thanks

Thanks H. Evan.

It is no secret I am trying to learn here. :) So, what is SFC and where did you get the number?

Edit: Oops, just started reading your article. It stands for "specific fuel consumption". Thanks Again!!
 
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Now you're cookin'...

...and you're going pretty fast!

Try 75 and 50 ROP for another knot or two.

Take a good look at the details...like the fit of all the intersection fairings upper and lowers on the gear. Look closely at the fit of the empennage fairing, that it is snug and so on with all the fairings.

Paul Lippse has pointed out the drag increases around the prop blade cutouts at the spinner. The closer that fit, the better.

Good luck,
 
Me too

I'm a little slower than I'd like but there are so many variables holding you back. Did anybody mention trim tabs? They are there to help compensate the slight variation in the overall completed flying package. Necessary but they don't speed you up. My 320 CS was in 3 other planes before my 6A. Originally a turbo twin Comanche. I'm getting 152KTs most of the time at 8000. It's all a compromise in the end.
 
elippse said:
Most likely it's due to your inefficient prop. I tested an RV-8 with a Sensenich 70CM65900-77 and he could only make a GPS-derived 156.8 mph at 9520' dalt and 2620 rpm. Contrast this with an RV-6 with 150 hp and an Elippse three-blade prop plus extended wingtips going 191 mph at 9600' dalt at 2670 rpm.
Click to expand...

With a 77" pitch as indicated above, and no loss at 2600 rpm, the best he could do is 189.58 smph. And his speed is 180.32 smph. What would expect from loss?
 
Do you share a hangar?

It could be that you hangar with a slow airplane. Slow can be contagious. Hangaring with something like a Starduster will cost you 10 knots right off the bat.

Easiest way to check this is to take a look at your hangar mate. If he seems a little slow, Bingo!, there's your answer.

:D
 
Ha Ha. Now that is funny. :D
Yeah, I do hangar with a Starduster. But Bandit is a very smart. You should not call her slow. :D
 
HAH!

kasTX said:
Easiest way to check this is to take a look at your hangar mate. If he seems a little slow, Bingo!, there's your answer.
:D
Click to expand...
:D:D There ya go- or better yet- pass that starduster and ask him if it felt like he was standing still! my Cessna bud's don't like me anymore:p
 
Starduster

I had to laugh at the Starduster crack. :p
I learned to fly in the Airscouts in 1972. Stardusters were all the rage then.
We were teen agers drooling on Stolp brochures.
I still love the look, I just don't care for biplanes. So I have lots of napkin designs of Starduster monoplanes with elliptical wings.
You know: A tube and fabric RV-8 :D
 
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