Sport Pilot Talk

We were flying today at almost full gross , at around 3500 feet level , with almost full power ( at around 5400 rpm ) trying to figure out how fast will the Sting 4 cruise.
The best we could do was about 105 knots which is somewhat disappointing , considering that I have seen other Sting ( 4 and older models) easily cruise at around 115-120 knots.

The plane is equipped with a Duc Swirl propeller , pitched ( according to the log book ) at 24 , with takeoff speed of about 5400 rpm - just wondering what other variables are at play here that would prevent us from getting above 105 knots ....

http://imgur.com/RWv3XVp
Thanks

Is that 105 knots indicated, or true? If indicated, you need to factor in not just altitude, but also temperature and barometric pressure, to determine how fast you're really going. Oh, and GPS ground speed doesn't really tell you anything, unless you know winds aloft precisely at the time of flight.

I have a Sting S4 and get similar performance IAS. My Dynon does the math, though, and the TAS is always a bit higher.

Climb to 8500 MSL and check your TAS. I'll bet you will be a lot happier.

Normally aspirated aircraft get slower the higher they go. So the TAS calculation is important. (Edit: applies at full throttle only. Otherwise the optimum altitude for any given RPM is when full throttle is reached to maintain that RPM ~ thanks to FastEddieB for waking me up)

Finding the "best pitch" for a prop is a series of compromises. The best cruise speed will be achieved by the plane that uses the most (significantly) runway for takeoff and distance to climb. These numbers get worse with density altitude.

So you have to weigh the runway requirements against any speed gain you will be likely to see.

You did not mention the static RPM at full power on the ground. Not that I even know what it should be, but others might.

Jim Hardin wrote:Normally aspirated aircraft get slower the higher they go.

This is certainly true at full power, which is only available at sea level. See the curved "FULL THROTTLE" line in the chart below.

But of note and worth considering is that one can also say, "Normally aspirated aircraft get faster the higher they go - for any given power setting".

IOW, let's say you decide to cruise at 65% power. In that case, higher is faster - up to a certain "critical altitude" where 65% power can no longer be obtained.

This is best seen in a cruise performance chart:



You can see in a Cherokee that 65% power at SL is good for maybe 113 mph. Climb to 10,000' and that same power setting gives you about 124 mph. All things being equal, higher is better up to that certain "critical altitude". This applies not only to speed, but of course to range as well. Wind and weather and oxygen requirements all considered, of course.

Not arguing - just food for thought and trying to liven things up a little here!

Indeed, this was 105 indicated and .... the relationship between IAS/TAS and altitude is interesting but ultimately , what I am trying to figure out why two planes from the same manufacturer , using the same propeller , engine and power setting achieve quite different cruise performance.

Here is a short video I was referring to :
https://youtu.be/yyLt652XICA

It is part of drdehave's impressive collection of Sting related videos. He had some additional comments there :

Note in the video that 120 SNL (straight-n-level) knots airspeed (the light-sport airplane speed limit) at 2,500 MSL, is achieved at 5,200 RPM. Maximum sustained cruising RPM for this Rotax 912ULS engine is 5,500, which, if you want to ignore speed limits, would pull the airplane at about 130 knots. And if you were to bury the throttle, SNL, you could pull about 140 knots! All this, with a pitch setting that gives the takeoff climbout performance shown in the video!

Well, I can't even come close to that Not that the additional 10-15 knots will turn Sting into a Mooney ( nor would I want that ) but just curious where the difference is coming from.

There are several reasons your S4 did not match speed with Rich's StingSport:
Weight - The S4 probably weighs 60lbs more than a StingSport. In the particular video, Rich could've been 200lbs lighter than you, which would make 5-10 knots difference.
Design - the S4s wingspan is about 2 feet longer with a slightly different design. Supposedly doesn't affect cruise speed but it seems to with anecdotal reports.
Density Altitude/temperature - big factor in performance.

FastEddieB wrote:

Jim Hardin wrote:Normally aspirated aircraft get slower the higher they go.

This is certainly true at full power, which is only available at sea level. See the curved "FULL THROTTLE" line in the chart below.

But of note and worth considering is that one can also say, "Normally aspirated aircraft get faster the higher they go - for any given power setting".

IOW, let's say you decide to cruise at 65% power. In that case, higher is faster - up to a certain "critical altitude" where 65% power can no longer be obtained.

This is best seen in a cruise performance chart:



You can see in a Cherokee that 65% power at SL is good for maybe 113 mph. Climb to 10,000' and that same power setting gives you about 124 mph. All things being equal, higher is better up to that certain "critical altitude". This applies not only to speed, but of course to range as well. Wind and weather and oxygen requirements all considered, of course.

Not arguing - just food for thought and trying to liven things up a little here!

Thanks for posting that chart FastEddie.

Warmi wrote: Maximum sustained cruising RPM for this Rotax 912ULS engine is 5,500, which, if you want to ignore speed limits, would pull the airplane at about 130 knots. And if you were to bury the throttle, SNL, you could pull about 140 knots!

Another "pet peeve" of mine: there IS NO "speed limit" on light sport aircraft for pilots!!!

The only "speed limit" is that the manufacturer must certify that the aircraft is capable of no more than 120kt CAS in a standard atmosphere at sea level. Does anybody have an airspeed gauge in their aircraft that measures calibrated airspeed? How often are you flying in a standard atmosphere at sea level?

Even if the airplane exceeds that performance spec, it's a problem for the manufacturer, not for the pilot. There is no FAR requiring a Sport Pilot to comply with any performance spec, only to fly only an LSA.

In fact, if an airplane was manufactured to meet all LSA performance specs, but was capable of 400kt CAS at 5000ft, it would be perfectly legal for a Sport Pilot to fly, as long as it could do no more than 120kt CAS at sea level, the only specified speed performance limit for LSA.

Let me put out there what I think are common conventions when discussing airspeeds.

If discussing approach or landing, it's safe to assume we are usually talking about IAS - you approach at the same IAS at both Death Valley and Denver. If you say you fly 55k for a short field approach, nearly everyone knows you mean IAS.

But when talking cruise speeds, TAS is almost always what we're talking about. If I say a Cirrus cruises at 172k at 14 gph at some altitude, it's safe to assume we're talking TAS - IAS makes no sense since there are too many factors that need to be considered to make sense of what a given IAS might actually mean.

Given that convention, I assumed the 105k in the original post was TAS - that's the only speed for which we can make meaningful performance comparisons.

Just a thought...

MrMorden wrote: How often are you flying in a standard atmosphere at sea level?

I don't know about the standard atmosphere, but I do fly at sea level occasionally - in a seaplane!

drseti wrote:

MrMorden wrote: How often are you flying in a standard atmosphere at sea level?

I don't know about the standard atmosphere, but I do fly at sea level occasionally - in a seaplane!

As a Ph.d. aeronautical engineer I know once told me: "There is never a standard atmosphere around when you need one."

MrMorden wrote: As a Ph.d. aeronautical engineer I know once told me: "There is never a standard atmosphere around when you need one."

Gee, I wonder who that might be.

drseti wrote:

MrMorden wrote: As a Ph.d. aeronautical engineer I know once told me: "There is never a standard atmosphere around when you need one."

It's actually my ex brother-in-law, he was trained at Georgia Tech. You probably don't know him.

I guess it's a common Aero Engineer joke...