4-place Cruise Speed Survey

[whee]

I think it would be interesting and fun to collect a somewhat meaningful dataset of cruise speeds for 4-place Bearhawks. Both the original and the Model B included.

Please post the true airspeed and the fuel flow at the speed.

Other information is welcome, prop settings, MP, tire seize, etc.

[Battson]

On 31" bushwheels... We see 115 to 120 KTAS at lower level, 120 to 125 KTAS up high, burning between 40 and 42 L/hr.

Sometimes we go up to 5kts faster or slower, I can't understand what causes that. Let's not hijack this thread though.

With the 26" tires, we saw a more consistent 120 KTAS to 125 KTAS at lower level, 125 to 130 KTAS up high.

Up high is above 7000ft, normally 10,000ft.

[Battson]

I will add that our EFIS continually calculates TAS for us, and we have GS also to verify. Direct in-flight comparison to other Dynon and GPS-equipped aircraft has shown the TAS is accurate within 1kt or so. What fluctuates a lot are the actual readings, even the smallest climb / descent and especially turbulence throw the instantaneous numbers all over the place.

[Bcone1381]

41L is 10.8 gallons

[Bcone1381]

I hope I don't offend any one by converting data. I'm hoping it helps.

[robcaldwell]

I think engine sizes/types should also be stated in this discussion. Fixed pitch and CS as well.

[Isilverone]

A-Model , 0-540 4-place..I’m indication 125mph at 2500ft ASL, CS prop at 24/24 running ROP at 14GPH with 31” Alaskan Bush Wheels and VG’s.

[whee]

face-palm_1f926.png

[JimParker256]

OK, Bearhawk2015, I'll be "that guy"...

IAS is the raw reading taken from the airspeed instrument. Two "identical" airplanes can fly side-by-side at the exact same speed through the same air, and show significant differences in the indicated airspeeds. I've personally witnessed differences of 10 knots in indicated airspeeds (IAS) when flying in formation with another aircraft.

CAS is the IAS corrected for installation errors (indicator error, pitot misalignment, etc.). Applying the right "calibration corrections" to the indicated airspeeds would theoretically allow the two airplanes above to "correct" their IAS to CAS and thus have the same calibrated airspeeds (CAS). In the EAB world, we generally do NOT perform IAS-to-CAS calibrations, since that requires some expensive probes (pitot tubes that align themselves with the airflow, calibrated gauges, etc.) and somewhat more rigorous test flight schedules than generally performed. Thus we "ignore" the IAS-to-CAS differences...

But even if we ignore IAS-to-CAS differences between airplanes, and attempt to use IAS to compare the performance of two airplanes, we would also need to know the altitude, temperature,and barometric pressure to turn that IAS into a meaningful number that could be compared to another airplane's performance.

TAS is the CAS (or IAS, as generally the case for EAB) corrected for altitude, barometric pressure, and temperature.

* An airplane indicating 125 knots at 2000 ft (ISA conditions, 129 KTAS) which then climbs to 10,000 ft flies considerably faster than it was at 2,000 ft (ISA conditions, 146 KTAS). This is because at higher altitudes the air is less dense, so the pitot tube must fly faster to collect the same "pressure" that correlates to an indication of 125 knots.
* An airplane indicating 125 knots at 2000 ft in the winter (0ºC, 126 KTAS), then duplicating that same 125 KIAS in the summer will be flying faster in the summer (40ºF, 135 KTAS). Again, the hotter air is less dense.
* An airplane indicating 125 knots at 2000 ft in an area of very low baro pressure (28.92"Hg, 132 KTAS) then flying through a front into a high pressure region will be flying slower (30.92"Hg, 125 KTAS). Lower baro pressure means lower air density, so you have to fly faster to achieve the same IAS.

By definition, TAS is the "true" distance covered over the ground in a specified amount of time. Thus the only way to compare the performance of one airplane to another is to use TAS, and report the altitude along with it. One can easily compare two different reported TAS, by adjusting for altitude (remembering to also adjust the temperature for the standard adiabatic lapse rate of roughly 2º per 1,000 ft of altitude change) to have a true "apples to apples" comparison. Trying to do the same with IAS requires altitude, OAT, and baro pressure to obtain usable results. TAS and Altitude is just so much simpler...

[Gerhard Rieger]

Thanks Jim, being on this forum is truly a learning experience.

[zkelley2]

OK, Bearhawk2015, I'll be "that guy"...

IAS is the raw reading taken from the airspeed instrument. Two "identical" airplanes can fly side-by-side at the exact same speed through the same air, and show significant differences in the indicated airspeeds. I've personally witnessed differences of 10 knots in indicated airspeeds (IAS) when flying in formation with another aircraft.

CAS is the IAS corrected for installation errors (indicator error, pitot misalignment, etc.). Applying the right "calibration corrections" to the indicated airspeeds would theoretically allow the two airplanes above to "correct" their IAS to CAS and thus have the same calibrated airspeeds (CAS). In the EAB world, we generally do NOT perform IAS-to-CAS calibrations, since that requires some expensive probes (pitot tubes that align themselves with the airflow, calibrated gauges, etc.) and somewhat more rigorous test flight schedules than generally performed. Thus we "ignore" the IAS-to-CAS differences...

But even if we ignore IAS-to-CAS differences between airplanes, and attempt to use IAS to compare the performance of two airplanes, we would also need to know the altitude, temperature,and barometric pressure to turn that IAS into a meaningful number that could be compared to another airplane's performance.

TAS is the CAS (or IAS, as generally the case for EAB) corrected for altitude, barometric pressure, and temperature.

* An airplane indicating 125 knots at 2000 ft (ISA conditions, 129 KTAS) which then climbs to 10,000 ft flies considerably faster than it was at 2,000 ft (ISA conditions, 146 KTAS). This is because at higher altitudes the air is less dense, so the pitot tube must fly faster to collect the same "pressure" that correlates to an indication of 125 knots.
* An airplane indicating 125 knots at 2000 ft in the winter (0ºC, 126 KTAS), then duplicating that same 125 KIAS in the summer will be flying faster in the summer (40ºF, 135 KTAS). Again, the hotter air is less dense.
* An airplane indicating 125 knots at 2000 ft in an area of very low baro pressure (28.92"Hg, 132 KTAS) then flying through a front into a high pressure region will be flying slower (30.92"Hg, 125 KTAS). Lower baro pressure means lower air density, so you have to fly faster to achieve the same IAS.

By definition, TAS is the "true" distance covered over the ground in a specified amount of time. Thus the only way to compare the performance of one airplane to another is to use TAS, and report the altitude along with it. One can easily compare two different reported TAS, by adjusting for altitude (remembering to also adjust the temperature for the standard adiabatic lapse rate of roughly 2º per 1,000 ft of altitude change) to have a true "apples to apples" comparison. Trying to do the same with IAS requires altitude, OAT, and baro pressure to obtain usable results. TAS and Altitude is just so much simpler...

We have a name for those altitude, temperature and baro pressure variations. Density altitude. That's what should be listed.

Something like 125KTAS at 22"/2200RPM at 3000' Density, 10GPH.

But you're right. IAS is meaningless in performance discussions.

[zkelley2]

Density altitude is not that hard to just do in your head. We all had to know this for even our private exam right?

Let's say you're at 7000MSL, it's 10C and SLP is 30.02. ISA at 7k is 1C (2C per 1k lapse rate). 10C-1C = 9C, 29.92-30.02 = -.1
-.1*1000= -100, 9*120[a constant, you could round that to 100 for mental math purposes]= 1080. 7000+1080-100 = 7980.

Broken down to a rule of thumb, 10ft per .01inHg from standard pressure and 100ft per 1C from standard.

[JimParker256]

Still a LOT more work than "look at the gauge"... ;-)

[zkelley2]

I love being able to say this to someone, who I think is a good bit older than me....


Back in my day when we learned to fly, we had to do the math in our heads! And if we were lucky, they let us use a E6B!

[kestrel]

Additional considerations:

I often see people report that they burn more or less fuel per hour when they change the configuration of their airplane. I.e. going from 8..00's to 31" ABWs. They typically are running a fixed pitch prop and setting power by RPM only. The added drag of the larger tires slows the airplane down creating a need for more power to maintain the same RPM. They then come to a forum and report a limited loss of airspeed and in increase in fuel flow. ...not really a controlled experiment because they changed both the configuration of the aircraft and the power setting. Similar issues occur when comparing different density altitudes; they fail to maintain constant power.

power = force * distance / time = force * velocity

This means that as density altitude increases, velocity increases and, for a fixed power setting, force must decrease. This means that for a fixed power setting, your TAS will increase, but your IAS/CAS will decrease. You can either run a fixed power setting and take the increased economy, or you can increase power (if able) to maintain IAS/CAS and keep the same economy with an even greater TAS increase.

For a given leaning condition (mixture setting), fuel low is power. If you have a fuel flow meter (I don't) and follow a consistent leaning procedure, setting fuel flow is the same as setting power, regardless of density altitude, rpm, etc... (some allowance for things like small variations in Specific Fuel Consumption of engine, generally lower RPM/higher MAP is better)

If you aren't setting a consistent fuel flow and mixture, you aren't maintaining constant power. I look forward to the day that I have a fuel flow meter.

It is really cool that EFB apps have functions to predict flight times/fuel consumption and even optimize altitudes for you, but I haven't seen one yet that accounts for the effects of density altitude on TAS vs. fuel flow. I don't use Foreflight, maybe it does.

[whee]

I appreciate the responses from Jim, zkelley and kestrel. Everyone should now have a basic understanding why I was asking for TAS and Fuel flow.

[jaredyates]

Wouldn't it be cool if there was a place we could upload the data file from the efis and get an output that considers environmental conditions like pressure and temp, and resulting performance at a fuel flow. This would be a great job for the saavy folks.

[whee]

Wouldn't it be cool if there was a place we could upload the data file from the efis and get an output that considers environmental conditions like pressure and temp, and resulting performance at a fuel flow. This would be a great job for the saavy folks.

Your aeronautical knowledge is far greater than mine; does TAS not account for those factor is a good enough way?

My EFIS data includes TAS but for some reason is isn’t shown on SavvyAnalysis. I know they have that info since Mike B has talked about on one of his webinars. If I dump my data into an excel sheet I can plot it against fuel flow easily. Make for interesting nerding. I don’t have a OAT prob yet so my EFIS calculated TAS is not valid.

[Isilverone]

A-Model , 0-540 4-place.. yesterday I had 60 gal of fuel on board and luggage weighed in at 2300lbs.I’m indicating 125mph at 3500ft ASL, CS prop at 24/24 running ROP at 14GPH ( indicating 14 but it’s not accurate) with 31” Alaskan Bush Wheels and VG’s. TAS calculated to 133mph

[kestrel]

Wouldn't it be cool if there was a place we could upload the data file from the efis and get an output that considers environmental conditions like pressure and temp, and resulting performance at a fuel flow. This would be a great job for the saavy folks.

Its been on my wish list to make a box that taps into the EFIS (and/or pitot static), has an IMU and GPS, perhaps a few other sensors to help determine if wheels are on the ground, and collects data to determine aircraft climb performance, calibrate IAS, takeoff/landing data, etc... I'm astounded that I've never come across such a box. These days it would be very easy and affordable to make.

...I'm also astounded that airplanes are still built with pitot/static lines. The pressure transducers should be at the probe and communicated over a serial bus. No leaks!

[svyolo]

I believe there are remote Air Data modules. It should get rid of the pitot/static lines. I think it is an option for some brand of EFIS or two.
I would be surprised if there wasn't something out there that would record the data you want.

[jaredyates]

Your aeronautical knowledge is far greater than mine; does TAS not account for those factor is a good enough way?

It would be great to see real collected data, TAS or otherwise, alongside fuel flow, RPM, MAP, and the other variables. This would be much easier to compare than an occasional calculation or observation without the same context. Human brains are not well-equipped to average an hour of cruise flight in the same way that a data log could be analyzed.

[Weldingiron]

Whee,

What does ~370943FF TAS at 0.00002793AFH mean. I was hoping I could figure it out without having to ask but I'm drawing a hard blank. Thanks!