Streak 3 cruise speed

[Bluey]

Hi, went for a fly this morning and had a go at some fast flying in a streak 3. Seemed to be able to comfortably hold 70kt (true airspeed) with elbows over the bar and arms in somewhat of a crossed position. Took little effort as the weight of my arms did most of the work. My engine was revving at between 4900 and 5000 to hold level flight at this speed. Does this seem right to people? I was one up at the time. I noticed that the tall windscreen adds about 6kt at this speed (false reading). So with the tall windscreen in place callibrated airspeed is 76kts. How does the SST wing compare to the streak as far as the revs needed to hold the same airspeed. Some have said the SST is more efficient?

 

Bluey

 

 

[Guest Andys@coffs]

I suspect that you meant to say "uncalibrated airspeed" Windscreen extensions always add speed to the ASI as they cause an area of low pressure behind the screen which of course is where the static port is.

 

As to the last question, from a theory perspective they have to be more efficient in that all that drag inducing kingpost and briddle is now gone. Fortunately theres still heaps of drag in the beast to prevent warpspeed being an issue

 

Andy

 

 

[Bluey]

Yeah, sorry uncallibrated airspeed of 76kt with the tall windscreen. 70kt was the callibrated airspeed.

 

 

[Bluey]

Anyone have any idea of how much extra strain there is on the engine if it runs for long periods of time at near 5000 rpm?

 

 

[Guest Crezzi]

An indicted speed (KIAS) of 76 converts to a calibrated speed of (KCAS) of about 66 with the touring screen fitted according to the POH.

 

The 912 in most fixed-wing aircraft routinely cruise at around 5000rpm with no ill effects. The engine is massively underused most of the time in trike applications

 

Cheers

 

John

 

 

[Bluey]

The indicated airspeed was about 67kt (KIAS) while the airspeed displayed on the skydat digital display was 70kt (without tall windscreen). The skydat should be corrected for the effects of air temperature and density changes that result from altitude and so should be the true airspeed (KCAS). Please correct me if I am wrong.

 

Bluey.

 

 

[alf jessup]

Hi, went for a fly this morning and had a go at some fast flying in a streak 3. Seemed to be able to comfortably hold 70kt (true airspeed) with elbows over the bar and arms in somewhat of a crossed position. Took little effort as the weight of my arms did most of the work. My engine was revving at between 4900 and 5000 to hold level flight at this speed. Does this seem right to people? I was one up at the time. I noticed that the tall windscreen adds about 6kt at this speed (false reading). So with the tall windscreen in place callibrated airspeed is 76kts. How does the SST wing compare to the streak as far as the revs needed to hold the same airspeed. Some have said the SST is more efficient?Bluey

Bluey,

 

Well the SST does not cruise at 70kts as stated by airborne in their advertisment of the SST in magazines even in the front hole in hands off trim.

 

I would say 65kts absolute max in the front hole unless of course i bought a lemon.

 

Very easy to achieve 70 with little bar pressure, I find it glides better than the ST3 and maybe even climbs a little better.

 

One thing i dont like is it side slips (dutch rolls) far easier than the st3 especially if you change direction quickly (S turns), airspeed indicator will read zero in an instant as the airflow crosses the pod as you slip slide away and an awful uncomfortable feeling at the same time.

 

The other thing I found also that in the front hole it tends to fly slightly right wing low with a pax at the higher power setting for cruise with a pax onboard.

 

Other than that it i think handles turbulence far better as it has a more direct feel with the strutts instead off the wires slapping about in turbulence.

 

I have put mine back in the second hole from the front and it feels much better to fly.

 

Overall do i think it was worth the money to update the wing, Yes I do but that is only my view on it.

 

Cheers

 

Alf

 

 

[Guest Crezzi]

Indicated airspeed is what the ASI shows. Calibrated airspeed is the IAS corrected for position & instrument error (see the table in POH). When CAS is further corrected for temperature & density changes it is known as the true airspeed (TAS).

 

Hope that helps

 

John

 

 

[Bluey]

Thanks Crezzi, After I posted I did some reading and realised I was wrong. Someone once told me that the Digital asi on the skydat is corrected for altitude, pressure and temp and so should be closer to true airspeed (TAS) than the dial ASI. But after checking the skydat manual, I found no evidence of that. Also, The difference between the Dial ASI and the Skydat was 3kt. This is too large to be corrected as my altitude was only about 500ft. At that altitude and yesterdays balmy weather, the difference between indicated and true airspeed wouldn't exceed about 1.5kt at our speeds. The GPS seemed to support the lower value anyway on a two directional average (into win + downwind divide by 2). I was a bit perplexed by this difference at the time but now seems to make sense.

 

Bluey

 

 

[Kev]

Bluey, the Rotax 912 can run constantly up to 5500rpm with no undue stress. Max power ie 5800rpm is limited to 5min.

 

Safe Flying

 

Kev

 

 

[Bluey]

Thanks Kev.

 

Bluey

 

 

[Guest davidh10]

Interestingly, the SkyDat has its static port opening from the sensor inside the instrument box and the box is sealed onto the face plate with gasket goo, so static pressure is constant at whatever pressure it was sealed. As such it should not be affected by localised pressure changes in the Pod, such as may be caused by a tall windscreen.

 

The only exception to the seal would be leakage around the screws or via the grommet around the tube that goes to the transponder (if fitted) That would seem to be minuscule given the volume of the box.

 

I've posted a photo taken while I was replacing the screen back-light in mine. You can see the thin tube from the pitot curving around from the case to the sensor, and the static port just to the left of the pitot connection on the sensor. The second sensor (far left) has only one port that is connected via a tube to the transponder. That tube feeds tightly through a grommet in the box. As I don't have a transponder, I've removed the tube and replaced the grommet with a blanking plug. {The previous owner of the aircraft had a transponder that was moved to a new aircraft prior to me acquiring it}.

 

I believe that it is expected behaviour for the IAS on the SkyDat to be a bit different to that on the analogue IAS. It's only ever a few knots.

 

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[Guest Andys@coffs]

Ultimately the static port exists as a point of reference for the pitot port. Its purpose is to allow the display to display only pressure difference (due airspeed). If the static port is truely blocked from ambient pressure then the ASI becomes an altimeter...... The fact that this isnt occuring suggests that static does represent the ambient pressure, perhaps just more slowly in change than it otherwise would, which in turn means that an Airspeed at any point in time is likely to be more in error from that which otherwise would be displayed.... In reality static pressure can only change as fast as you can climb or dive, or if you feel the need to fly through weather fronts etc... So probably not a real issue, but I dont understand why the Skydat would not show the same error as the ASI with the touring windshied in place, pressumably at the point where the pressure transducer is the low pressure is not the same as where the ASI sits....

 

Andy

 

From an engine

 

 

[Bluey]

The static pressure to the skydat asi is most definitely the same as for the dial asi. It wouldn't work otherwise. The tall windscreen affects both asi's equally.

 

Bluey

 

 

[Guest davidh10]

Ultimately the static port exists as a point of reference for the pitot port...

I know what you say is absolutely correct.

It just seems strange that it depends upon what must amount to only capillary leakage around the screws which appear, to inspection, to have no gaps. I had expected some actual port or at least a pinhole. It's not like the pressure changes were talking about are numerically large, so flow through such small gaps must be slow, and yet it works.

 

 

[Bluey]

What you are saying Richard makes sense except that if it were correct then you would see the digital asi be slow to settle on a steady speed. This is not observed meaning that the rate of change of pressure in the static port is the same as for the dial asi. This wouldn't require a large static port anyway as the rate of climb is not fast enough for rapid pressure changes anyway.

 

Bluey

 

 

[Bluey]

I've spoken to airborne regarding the ASI's accuracy and the responce basically was that it is not very accurate at all as your speed approached Vne. It is reasonably close in trim but then deviates considerably away from trim. This is reflected in the calibrated Vne speed of 74kts for the streak 3 and SST wings. This is what your real speed would be at sea level if your ASI reads 85. The difference between KIAS and KCAS is generally a non linear relationship from what I have gathered. So from what I have determined, an indicated airspeed of 68kt is really around 63kt and 75kt is really around 68kt. This of course is what you get without a tall windscreen. So when airborne says streaks cruise at 65kt and sst cruise at 70kt what they mean is indicated airspeed only. This of course makes a bit of a mockery of the claims they make on their brochures when they imply that these speeds convert directly to true airspeeds.

 

Bluey