Some thoughts on cross wind takeoffs.

All_In

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Called my brother Dave, had to know why the difference.
He's not hanging on the prop!

I built a black box, RC plane datalogger. We will strap it to the mast and we will see real data in reports and google map of what he actually flew in the tests he will run for us.
This will be the first time we needed it and I will get to play with it in an aircraft. Tested it with a car as the witch doctors had me grounded.
 

Jean Claude

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Thank you. That is about what I thought it said. I am taking off way ahead of that, more like 25.

More like 25 ? Did you not write 5'?

A pilot's feelings are often very different from the actual measured results.

20 kts of wind, really? measured with a calibrated anemometer? or just with the usual gyro anemeter without probe of static pressure?

Sans titre.png

AUW 500 lbs really? Did you weigh it before the flight?

Static prop. thrust 290 lbs was my guess because I not knows the setting of your prop. Did you check on your machine?

On my side, I confronted my distance calculations with the average measurements, published in the technical report n°1859 of the Air Research Committee, and they coincide also well for the lightened gyro pre-launched in overspeed, as well as at full load pre-launched at the recommended minimum (Tables 5 and 6) https://reports.aerade.cranfield.ac...2/1430/arc-rm-1859.pdf?sequence=1&isAllowed=y
 
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All_In

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Not sure over 10,000+ logged FW, 260+ ultralights, sailplane, hang-gliders
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dabkb2

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More like 25 ? Did you not write 5'?

A pilot's feelings are often very different from the actual measured results.

20 kts of wind, really? measured with a calibrated anemometer? or just with the usual gyro anemeter without probe of static pressure?

View attachment 1153606

AUW 500 lbs really? Did you weigh it before the flight?

Static prop. thrust 290 lbs was my guess because I not knows the setting of your prop. Did you check on your machine?

On my side, I confronted my distance calculations with the average measurements, published in the technical report n°1859 of the Air Research Committee, and they coincide also well for the lightened gyro pre-launched in overspeed, as well as at full load pre-launched at the recommended minimum (Tables 5 and 6) https://reports.aerade.cranfield.ac...2/1430/arc-rm-1859.pdf?sequence=1&isAllowed=y
I should have written 25MPH. My only airspeed indicator is my shirt sleeve, if it is blowing Im good. The numbers that did not make sense to me is the 22 for hover and the 40 for lift off.
 

Jean Claude

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The numbers that did not make sense to me is the 22 mph for hover and the 40 mph or lift off.
In flight, the angle of the disc can reach about 30 degrees, for which the cL of the disk is maximum,
Moreover, the thrust of the propeller comprises then a component of lift which relieves as much the rotor.
Hence the possibility of a very slow flight, if the propeller thrust is sufficient to overcome the high drag.
Sans titre1.png
While just before the takeoff, the angle of attack of the disk is limited to about 20 degrees, because of the tail wheel. The cL of the disc is then weaker, and the component of lift of the propeller, is also weaker.
Hence a speed/air at the take off greater than the minimum flight speed

Sans titre2.png
 

Mike G

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Dave

When testing my GWS, I record:

Airspeed with a calibrated differential pressure sensor connected to a commercial pitot static probe. Calibration using 3 triangular GPS flights and DA correction to establish TAS. The standard ASI on my open cockpit ELA had an 85% error between CAS and IAS (CAS = 0.85 x IAS).

Rrpm with a calibrated sensor measuring 108 pulses per rev on the pre rotator gear wheel compared to the standard 4 pulses per rev.

G with a 3 D accelerometer,

and flapping with an on-board flapping angle measurement device.

I regularly send my results to JC and we discuss any discrepancies between my “measured” results and his calculations and try to understand where the differences come from. Most times it’s due to instrument inaccuracies and tolerances, false assumptions (like the engine/prop thrust) or pilot inputs. Occasionally JC finds something that allows him to improve the accuracy of his calculations.

Trying to compare JC’s calculated results with your “by the seat of the pants” measurements of “shirt sleeve flapping” airspeed, approximate AUW and take off distance is a total waste of time.

JC’s calculations are remarkably accurate provided you give him equally accurate input data (rubbish in-rubbish out).

Mike G
 

dabkb2

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In flight, the angle of the disc can reach about 30 degrees, for which the cL of the disk is maximum,
Moreover, the thrust of the propeller comprises then a component of lift which relieves as much the rotor.
Hence the possibility of a very slow flight, if the propeller thrust is sufficient to overcome the high drag.
View attachment 1153607
While just before the takeoff, the angle of attack of the disk is limited to about 20 degrees, because of the tail wheel. The cL of the disc is then weaker, and the component of lift of the propeller, is also weaker.
Hence a speed/air at the take off greater than the minimum flight speed

View attachment 1153608
I understand the difference of the angel of attack, i just did not think it would change it that much. Maybe someday I will get all the instrument and get some actual numbers to compare.
 

Abid

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In flight, the angle of the disc can reach about 30 degrees, for which the cL of the disk is maximum,
Moreover, the thrust of the propeller comprises then a component of lift which relieves as much the rotor.
Hence the possibility of a very slow flight, if the propeller thrust is sufficient to overcome the high drag.
View attachment 1153607

And that component of lift from the propeller that relieves the rotor loading can also be a source of problems given certain conditions.
Light gyro, light pilot, very powerful high static thrust (powerful engine like 915iS). When you do this at full blast from the engine like 915iS at very slow speeds, you need to be a bit careful. It enhances any bad inputs you put and acts as a catalyst for making pilot's bad actions worse possibly.
 

Abid

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Dave

When testing my GWS, I record:

Airspeed with a calibrated differential pressure sensor connected to a commercial pitot static probe. Calibration using 3 triangular GPS flights and DA correction to establish TAS. The standard ASI on my open cockpit ELA had an 85% error between CAS and IAS (CAS = 0.85 x IAS).

Rrpm with a calibrated sensor measuring 108 pulses per rev on the pre rotator gear wheel compared to the standard 4 pulses per rev.

G with a 3 D accelerometer,

and flapping with an on-board flapping angle measurement device.

I regularly send my results to JC and we discuss any discrepancies between my “measured” results and his calculations and try to understand where the differences come from. Most times it’s due to instrument inaccuracies and tolerances, false assumptions (like the engine/prop thrust) or pilot inputs. Occasionally JC finds something that allows him to improve the accuracy of his calculations.

Trying to compare JC’s calculated results with your “by the seat of the pants” measurements of “shirt sleeve flapping” airspeed, approximate AUW and take off distance is a total waste of time.

JC’s calculations are remarkably accurate provided you give him equally accurate input data (rubbish in-rubbish out).

Mike G

LOL. Yeah that is why I don't believe the Eclipse speed numbers when people tell me ELA Eclipse is going so fast. In upcoming ASTM compliance I think it will be like airplanes where maximum of 5% error through the speed range to VNe would be allowed. In certified world only 3% is allowed. In AR-1 starting at some serial number a couple of years ago we added 2 static ports, one on each side after test flying and looking at yarn stuck on different locations to select the correct place for static ports. Well, AR-1 slowed down about 10% :).
108 pulses ... wow. We use 10 pulses per rev for rotor RPM.
Would be cool to see what flapping angle measuring mechanism you end up using and how is it exactly affixed to the rotorhead in GWS system.
 

Burrengyro

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In flight, the angle of the disc can reach about 30 degrees, for which the cL of the disk is maximum,
Moreover, the thrust of the propeller comprises then a component of lift which relieves as much the rotor.
Hence the possibility of a very slow flight, if the propeller thrust is sufficient to overcome the high drag.
View attachment 1153607
While just before the takeoff, the angle of attack of the disk is limited to about 20 degrees, because of the tail wheel. The cL of the disc is then weaker, and the component of lift of the propeller, is also weaker.
Hence a speed/air at the take off greater than the minimum flight speed

View attachment 1153608
Greetings, JC, Dave, Mike, Abid and all,
Very interesting video and analysis! If we have a nosewheel lifing device so that the angle of the disk could potentially be 30 degrees, giving more exposure to a runway head wind during prerotation, the blades could spin up quicker to higher rrpm, then drop the "nose wheel lifter", short roll and we lift off with less takeoff roll, just like in Dave's video?
Mike, is the ELA07 IAS clock really off true air speed by -15%? Last weekend, me in my ELA07S and my friend in his Magni M16 were flying down wind with indicated ground speed of 109mph and IAS of around 70mph. The ground speed was observed using SkyDemon. The tail wind was about 20mph, IIRC. Alot depends on the wind. John H.
 

Vance

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Greetings, JC, Dave, Mike, Abid and all,
Mike, is the ELA07 IAS clock really off true air speed by -15%? Last weekend, me in my ELA07S and my friend in his Magni M16 were flying down wind with indicated ground speed of 109mph and IAS of around 70mph. The ground speed was observed using SkyDemon. The tail wind was about 20mph, IIRC. Alot depends on the wind. John H.
With a 20mph wind at ground level it is my observation that there can be a substantial difference between the wind near the ground and the wind even 100 feet (30 meters) above the ground because of ground feature turbulence.

It I want to measure the accuracy of my air speed indicator I find a very low wind day and fly a square pattern of about a mile at a constant altitude and average the indicated air speeds from each leg and compare it to the average ground speed from the legs and take into account density altitude.

It is my observation if an open aircraft doesn’t have a static system the relative wind can make very large errors in indicated air speed.
 

Jean Claude

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John,
I calculated the takeoff run without wind for an ELA 07 S 115HP at 450 kg and a static propeller thrust of 2200 N. I get a distance of 105 meters at S-L . Then I redid this distance calculation with a 20 kts wind and I get 37 meters.

Sans titre.png
So, if you want the take-off distance of this ELA at 450 kg for a headwind of speed W (m/s), I suggest you to use the following formula:
Dist = 105 m - (6.8*W)
For example for a wind W of 5 m/s, the takeoff run would be 105 m - (6.8*5) = 71 m
 
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Burrengyro

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John,
I calculated the takeoff run without wind for an ELA 07 S 115HP at 450 kg and a static propeller thrust of 2200 N. I get a distance of 105 meters at S-L . Then I redid this distance calculation with a 20 kts wind and I get 37 meters.

View attachment 1153614
So, if you want the take-off distance of this ELA at 450 kg for a headwind of speed W (m/s), I suggest you to use the following formula:
Dist = 105 m - (6.8*W)
For example for a wind W of 5 m/s, the takeoff run would be 105 m - (6.8*5) = 71 m
Hello Jean Claude,
Thank you very much for this information. I look forward to using this information as soon as the weather permits.
My experience with my ELA07S compares to within 20% of your estimates. I think the difference may be the condition of the runway surface. On our grass runway, I often advance the throttle to WOT within 25m of the start of the takeoff roll to get out of the grass as soon as possible. We are also just a few feet above sea level. John H.
 

Mike G

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John (Burrengyro)
The correction for the standard ELA pitot on my 07 was TAS = 0.8793 x IAS - 0.6279 (kph), not CAS, sorry my mistake. The correction for CAS on the day was +2.5 kph, so CAS nearer 90% of IAS, my 85% was from memory.

This is because the static port is behind the instrument panel and there is a depression in that area due to it being an open cockpit.

If you want to calibrate your ASI the attached is a good start, but beware, this gives you TAS not CAS. I use this method and correct the TAS for the DA to get CAS.

Fara
The Eclipse has a closed canopy and there is a pitot static tube fitted so most of this problem shouldn't exist in the Eclipse. Having said that when I fitted an Aircraft Spruce pitot static to my ELA, in the same position as the Eclipse, it under-read dramatically and I had to tune it to get reasonable accuracy.
One of the first things we'll do when I finally get to Florida (waiting for my 3rd vaccination) is calibrate the GWS delta P sensor for your pitot/static setup using the triangle/gps method attached.
We discussed the flapping angle measurement in another thread, I'll bring my flapping sensor with me so that we can confirm the flapping angles against the alarm settings. You'll see that it's not very useful for the pilot, it's main value is to be able to plot measured flapping angles against the GWS parameters to see how close we got to flapping.

Mike G
 

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Abid

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John (Burrengyro)
The correction for the standard ELA pitot on my 07 was TAS = 0.8793 x IAS - 0.6279 (kph), not CAS, sorry my mistake. The correction for CAS on the day was +2.5 kph, so CAS nearer 90% of IAS, my 85% was from memory.

This is because the static port is behind the instrument panel and there is a depression in that area due to it being an open cockpit.

If you want to calibrate your ASI the attached is a good start, but beware, this gives you TAS not CAS. I use this method and correct the TAS for the DA to get CAS.

Fara
The Eclipse has a closed canopy and there is a pitot static tube fitted so most of this problem shouldn't exist in the Eclipse. Having said that when I fitted an Aircraft Spruce pitot static to my ELA, in the same position as the Eclipse, it under-read dramatically and I had to tune it to get reasonable accuracy.
One of the first things we'll do when I finally get to Florida (waiting for my 3rd vaccination) is calibrate the GWS delta P sensor for your pitot/static setup using the triangle/gps method attached.
We discussed the flapping angle measurement in another thread, I'll bring my flapping sensor with me so that we can confirm the flapping angles against the alarm settings. You'll see that it's not very useful for the pilot, it's main value is to be able to plot measured flapping angles against the GWS parameters to see how close we got to flapping.

Mike G

Hi Mike
The pitot static combined L shaped system used in ELA and sold by Aircraft Spruce does not read accurately under the shadow of the nose.

error on AR-1 when two side static ports were not there was around 10% higher than 70 mph. It was fairly accurate till 60 mph.
I think I remember the flapping angle measuring thread.
 
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