Max rpm or?

[Warmi]

I pretty much already know what to expect at various rpm settings given certain conditions in terms of fuel burn. just wanted to know what sort of speed numbers are you getting at various rpm  settings at a given altitude etc .I am trying to optimize my Sting for cruise performance ( it already climbs like crazy ) so I am just curious if it can keep up with ctws which are known to be fast LSAs.

[Tom Baker]

1 hour ago, WmInce said:

Be advised folks, Andy's CT is a miniture rocket ship. He gets spectacular cruise performance out of it. He leaves me in the dust.

YMMV.

Good luck with the surgery, Andy!

 

It could be that his analog tach is off, and he is turning his engine at a RPM faster that he thinks. There was someone over on SPT that mentioned they had an errant tach.

[WmInce]

59 minutes ago, Tom Baker said:

It could be that his analog tach is off, and he is turning his engine at a RPM faster that he thinks. There was someone over on SPT that mentioned they had an errant tach.

Good point. That may be worth checking.

[FlyingMonkey]

I will scare up an optical tach and check it, but I have no reason to doubt it and I suspect it's as correct as any other analog tach.  I get 5050-5100rpm on a WOT takeoff roll, which is right where it should be.  I can hear/feel the chatter in the gearbox start exactly where the yellow arc starts on the tach.

 

Oh, and I did hear "can you slow up a little" from Bill on the way to Page quite a few times, even when my tach was showing 5000-5100rpm... 

[Runtoeat]

Andy,  I also use 5300 for cruise and it's interesting because this is just where I like to  cruise and isn't arrived at by looking at fuel burn etc.  As you indicate, 5300 just seems to be a sweet spot for my Rotax and gets me to where I'm going at what I feel is a reasonable speed.

[WmInce]

2 hours ago, Runtoeat said:

Andy,  I also use 5300 for cruise and it's interesting because this is just where I like to  cruise and isn't arrived at by looking at fuel burn etc.  As you indicate, 5300 just seems to be a sweet spot for my Rotax and gets me to where I'm going at what I feel is a reasonable speed.

Same here.

[FlyingMonkey]

Okay, I checked my tach against an optical prop tach, and it's not reading wrong.  Once the 2.43 gearbox multiplier was applied, here are my numbers:

TACH RPM                     ACTUAL RPM

2000                                2065

3000                                3037

4000                                3994

4500                                4468

5000                                4981

Keep in mind this is an analog tach, so I could not put the RPM exactly on the rpm I wanted; I had to make my best guess for where any rpm number is "exactly".  I could not get significantly higher than 5000rpm in a static runup, so that is where I stopped.  I guess I could check it in flight at higher rpm...

I flew home last evening in dead calm air at 2000ft, at 5200rpm showing 116kt KIAS, 122KTAS, and 119kt GPS ground speed.

[Warmi]

CTLS/CTSW are, I think, the only popular LSAs, equipped with 912 ULS, that can actually approach the advertised 115 Kt KIAS cruise speed in average flying conditions.

In my experience flying Remos, Evektor, Sling and ( the plane I have most experience with ) Sting  - none of these can get much above 105 Kt - I have seen once 112 in  the middle of Winter in my Sting but that's about it.

Yesterday , while flying at 4500 in 95 degree weather , my Sting was struggling to keep 100 kts KIAS at 5300 rpm - that's with a prop pitched for 5650 WOT etc ... 

 

BTW... Andy, I know you are using Garmin Pilot logbook  ... if you go to their Logbook page , in the reports tab at the bottom , what do you see for your average ground speed ? I know GS is highly variable but over 100 hours or so of flying it should reflect your average traveling speed pretty well.

On my Sting I got ( over last 120 hours of flying) :

Average Speed : 111 mph

Average Speed ( Cross Country): 118 mph

[WmInce]

1 hour ago, Warmi said:

BTW... Andy, I know you are using Garmin Pilot logbook  ... if you go to their Logbook page , in the reports tab at the bottom , what do you see for your average ground speed ? I know GS is highly variable but over 100 hours or so of flying it should reflect your average traveling speed pretty well.

On my Sting I got (over last 120 hours of flying) :

Average Speed : 111 mph

Average Speed ( Cross Country): 118 mph

It would be interesting to see those results, for "Andy's little rocket ship." That thing is really slippery . . . to say the least.

[FlyingMonkey]

I will report back on that, but it's probably not going to be that high.  At least half my time is meandering around the patch with Cubs and other slow airplanes, at 80-85kt.  But I will look at the report and post the number.

[S3flyer]

On ‎6‎/‎30‎/‎2018 at 7:48 AM, Warmi said:

CTLS/CTSW are, I think, the only popular LSAs, equipped with 912 ULS, that can actually approach the advertised 115 Kt KIAS cruise speed in average flying conditions.

In my experience flying Remos, Evektor, Sling and ( the plane I have most experience with ) Sting  - none of these can get much above 105 Kt - I have seen once 112 in  the middle of Winter in my Sting but that's about it.

Yesterday , while flying at 4500 in 95 degree weather , my Sting was struggling to keep 100 kts KIAS at 5300 rpm - that's with a prop pitched for 5650 WOT etc ... 

 

BTW... Andy, I know you are using Garmin Pilot logbook  ... if you go to their Logbook page , in the reports tab at the bottom , what do you see for your average ground speed ? I know GS is highly variable but over 100 hours or so of flying it should reflect your average traveling speed pretty well.

On my Sting I got ( over last 120 hours of flying) :

Average Speed : 111 mph

Average Speed ( Cross Country): 118 mph

Warmi --  FYI.  I was seeing a steady 110KIAS at 4500' MSL/6100' DA at 5170 RPM in my Sting S3 today in the Dallas area.  Probably 200lbs under gross.

[Roger Lee]

If that 5170 rpm is WOT then that prop is way over pitched and can actually cause damage. There are zero redeeming qualities on over pitching and you loose all descent flight characteristics.

[S3flyer]

5170 rpm was just the cruise setting I was using.  I have it set for 5650 rpm WOT at 6000' MSL.

[Big Les]

So I’m lucky, Canadian and European rules allow constant speed props.

So what should I cruise at,  RPM/ manifold pressure ?

When can I go WOT , at all RPMs ?

Thanks.

[Roger Lee]

With your prop I would cruise somewhere between 5200 - 5500 rpm. Look at your fuel usage and see when you increase the rpm if the speed vs fuel is worth the extra rpm. It is kind of a personal thing.

[Big Les]

So I can fly WOT in the 5200-5500 rpm range at any altitude?

I don’t want to stress or overload the engine with a low  (5200 rpm) and a high manifold pressure.

Thanks.

[Anticept]

Personally, I set the blade pitch for pattern altitude of my home airport. This is where the strain on the engine is hardest.

There's not really a way for you to test lower altitudes without actually flying lower.

I'm sure it would try to overspeed, not underspeed, if you flew from high to low. The higher you go, the less power you develop and the slower the prop turns. So you would compensate by flattening pitch. Flat pitch with lots of power means lots of RPMs.

EDIT: Disregard, you were talking constant speed prop.

[Roger Lee]

Flying at WOT at anything under is over pitching. Flying at 5200 WOT is really over pitching. It will cost you fuel and like you mentioned unnecessary stress on engine parts. You will also loose speed because the prop is less efficient and thee engine doesn't have the torque and HP to do a good job turning it at that low an rpm at WOT.

[Ed Cesnalis]

35 minutes ago, Anticept said:

Personally, I set the blade pitch for pattern altitude of my home airport. This is where the strain on the engine is hardest.

There's not really a way for you to test lower altitudes without actually flying lower.

I'm sure it would try to overspeed, not underspeed, if you flew from high to low. The higher you go, the less power you develop and the slower the prop turns. So you would compensate by flattening pitch. Flat pitch with lots of power means lots of RPMs.

 

With a constant speed prop why pitch for a mid level field?  Unless the prop's pitch range is restricted I would want 5,800 at very high altitude and a max pitched cruise setting all the way down to sea level.  Max climb and best speed and best economy at all altitudes.

[Anticept]

I missed the part where he has a constant speed prop.

It's better to run coarser for efficiency, but there's a point where returns diminish and it gets hard on the engine. There should be an operator's manual that came with the airplane (might be something in the rotax operator's manual too) that says what manifold pressures and RPM you should run at.

In the case of an IO-360 engine that I had on my old mooney, at WOT, you couldn't set the manifold over square by much (When RPM/100 = Manifold in inches, it is "running square", running over square means manifold > RPM/100). But if you lowered the throttle just by a little bit, you could run over square considerably. Running 24 inches by 2200 RPM was very fuel efficient.

[Roger Lee]

But you  don't want a set point where WOT is ever lower than 5500 rpm. Higher is good.

[Big Les]

Ok, got it.

So WOT, if running at 5500rpm is acceptable at all altitudes, but maybe not the fuel flow !

[Tom Baker]

35 minutes ago, Roger Lee said:

But you  don't want a set point where WOT is ever lower than 5500 rpm. Higher is good.

Roger, you evidently don't understand how a constant speed prop work. What you say works for a simple controlable pitch prop, but not constant speed. With constant speed a governor senses the RPM of the engine and varies the blade pitch angle to maintain that RPM. If you have 5200 selected and move the throttle to WOT the blade angle will increase to maintain 5200 RPM. The only way it will go any higher is if the propeller reaches its maximum pitch stop.

[Big Les]

So what you’re saying is if 5200rpm is set with a WOT, then it won’t labor, it will just reach its coarsest, then if that maximum pitch is reached  then the rpm will increase?

[Ed Cesnalis]

48 minutes ago, Tom Baker said:

Roger, you evidently don't understand how a constant speed prop work. What you say works for a simple controlable pitch prop, but not constant speed. With constant speed a governor senses the RPM of the engine and varies the blade pitch angle to maintain that RPM. If you have 5200 selected and move the throttle to WOT the blade angle will increase to maintain 5200 RPM. The only way it will go any higher is if the propeller reaches its maximum pitch stop.

In today's very light aircraft world one might say 'constant speed' when he means only the pitch is adjustable in flight.