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  #1  
Old 01-31-2016, 08:42 PM
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nigelspeedy nigelspeedy is offline
 
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Default Takeoff configuration effect on takeoff distance

The other half of my learning how to land my 8 was learning how the takeoff configuration and technique affected the takeoff distance. I did an experiment to find out by doing a series of takeoffs using flap 0, 10, 20 & 40 degrees. For the ground roll I also used a tail up attitude (approximately 0 deg pitch attitude), tail low (approximately 5 deg nose up pitch attitude) and tail down (tailwheel lightly held on the ground to hold the 3 point attitude of 11 deg nose up). So a matrix of 12 takeoffs overall, which I measured with a DGPS on runway 24 at California City, L71, in nil wind at 1540 - 1600 lbs. For each takeoff I adjusted pitch attitude after liftoff to achieve 1.2 Vs for the particular flap configuration at 50' AGL. This resulted in 72 KIAS for 0 deg flap to as low as 60 KIAS for 40 deg flap. Each takeoff started with mixture rich, prop full fine with the aircraft held stationary on the brakes until throttle was fully forward. There was only one attempt at each configuration so there is no statistical relevance. I've made no attempt to correct for runway slope, aircraft weight, wind, air density or power so these results are only representative of my aircraft on the test day, which is a polite way of saying they are incorrect everywhere else so don't use them for operational planning. I was really only after the relative changes not absolute distances.

The results are shown in the graphs below.

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So in general tail low or tail down results in a shorter takeoff distance than tail up. The advantage for tail down over tail low is minimal.
On the farm dad alway said "if a little is good, more is better and too much should be just about right", but it doesn't seem to be that way with flaps. No flap is always the longest takeoff distance but 10 deg does just as well as 20 or 40 for the most part.
Taking off with the tail up gives the best field of view. Tail low allows you to see the runway centerline so directional control is still easy. Tail down might be a bit harsh on other than smooth surfaces and the field of view is a not as good as tail low or tail up and there is no real performance gain.
When using 40 deg of flap you get off the ground slower but the extra drag means it takes the same ground distance as 10 or 20 deg flap to get to the lower speed. Using 40 deg flap and maintaining 1.2 Vs to 50 feet is great as long as the engine keeps on running but 20 deg nose up, 40 deg flap, 60 KIAS and an engine failure would be pretty sporting combination.

So based on this I've settled on the following:

Normal Takeoff from a sealed runway with no obstacles. Rolling takeoff, Flap 0 deg and tail up.

Short field takeoff: Hold on brakes until power applied, Flap 10 deg and tail low during the ground roll, 1.2 Vs till clear of obstacles then accelerate to normal climb speed.

Soft field takeoff: Rolling start, Flap 10 deg, tail low, level off to accelerate to normal climb speed after ground break.

Cheers
Nige
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  #2  
Old 01-31-2016, 10:51 PM
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AX-O AX-O is offline
 
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Default

Nice Speedy. Love data and actual test points. Thank you.
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  #3  
Old 02-01-2016, 06:44 AM
N733JJ N733JJ is offline
 
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Default Same results

Though the takeoff roll was shortest with 40 flap, I came up with essentially the same results in my 180HP RV-8. My AOA is calibrated for flaps up and flaps 20, no flaps 10 setting. I found the difference between 10 and 20 flap was minimal so I changed my short and soft field procedures to use flap 20 so I can take advantage of the AOA for more accuracy and therefore maximized performance.

Scott A. Jordan
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Old 02-01-2016, 08:11 AM
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Kevin Horton Kevin Horton is offline
 
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Default

Quote:
Originally Posted by nigelspeedy View Post
The other half of my learning how to land my 8 was learning how the takeoff configuration and technique affected the takeoff distance. I did an experiment to find out by doing a series of takeoffs using flap 0, 10, 20 & 40 degrees.
Thanks for the data. Nice work.

I experimented with a single GPS precise positioning setup a few years ago, but never got acceptable results. I'd be interested to learn more about your DGPS setup.

Quote:
Originally Posted by nigelspeedy View Post
Short field takeoff: Hold on brakes until power applied, Flap 10 deg and tail low during the ground roll, 1.2 Vs till clear of obstacles then accelerate to normal climb speed.
Be aware that applying too much power while stationary can result in sucking debris into the prop. I have a friend who destroyed the prop on a TB-21 Trinidad when taxiing on grass when the field was too wet. The aircraft started to bog down, so he applied high power to keep it moving. The prop sucked some tiny stones out of the ground and completely chewed up the back side of the prop. It looked like someone had attacked it with 20 grit sandpaper, and had to be scrapped.
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  #5  
Old 02-01-2016, 11:29 AM
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nigelspeedy nigelspeedy is offline
 
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Default DGPS Used

Scott,

I like your logic for using the 20 deg flap setting in your aircraft, like you say there isn't a significant performance difference between flaps 10 & 20.

Kevin,

Thanks for the timely reminder about stones and prop damage. I'll give mine a closer look over next time I am at the hanger and use full power on the brakes only when it is operational necessary.

The DGPS I used belongs to work and I just loaned it for a couple of days. It is a 'Novatel ProPak6'. We use 'Program: GrafNav/GrafNet Version: 8.50' to do the file merging between the base and rover to achieve the differential correction post flight. We don't broadcast the corrections real time.

We have 4 of these units, each is about the size of a house brick. They are pretty easy to use, on/off, record start/stop, power & antenna connection.

One unit is permanently fixed in the building as our base station. We determined the location of the base (BASE: 35 03 22.70460 N -118 09 01.53756 W 814.128m) by running it for 24 hours to record position and then applied the differential corrections from a local station. In essence running the base as a rover. Even with the differential corrections there was some dither over 24 hours so we took an average of these positions over that time and called it the surveyed point. Close enough for our main use which is education rather than certification.

The rovers are identical units to the base station just run off a battery rather than mains power. In my 8 I mounted the rover antenna using a RAM mount to the bar behind the pilot seat where it had a good view of the sky. The rover and battery were in a small bag on the rear seat secured by the harness and numerous zip ties to keep any cables from fouling the rear stick. The antennas we have only allow reception of the GPS L1 & L2 bands but with an appropriate (expensive) antenna you can also receive all the other constellations.

In the post processing you can adjust sample rate from 20Hz to once per 20 seconds, depending on your application and tolerance for file size. I just used 1 Hz in my test, 5 Hz would probably be better if you wanted more fidelity on the transition between ground and air. What I think is neat is that it is accurate enough in the vertical plane to detect the 6-8" change in height when the tail comes up during the takeoff roll. We teach that for the corrections to be valid the base and rover need to see the same constellation, which is a pretty solid assumption up to 50 nm, and pretty safe out to 100nm. The other thing that is pretty noticeable is the reduction in signal quality when maneuvering, so before any important test point it is best to be straight and level for 30 sec or so. During the base turn reception would drop from 12 to 9 or 10 SV used in the solution but would return as soon as you roll out on final.

Here is a portion of the processing summary from the flight.

Position Standard Deviation Percentages:
" 0.00 - 0.10 m: 99.2 %"
" 0.10 - 0.30 m: 0.6 %"
" 0.30 - 1.00 m: 0.2 %"
" 1.00 - 5.00 m: 0.0 %"
" 5.00 m + over: 0.0 %"

So for the most part 10 cm or less. Not sure what accuracy you experienced in the past.

Cheers
Nige
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  #6  
Old 02-01-2016, 12:43 PM
Bill Dicus Bill Dicus is offline
 
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Default Take off

Good data Nigel and much appreciated. OT, but I was surprised you could open the throttle fully while holding brakes. In my -8 I cannot hold the tail down at full throttle with full up elevator. If I don't back off on power it's goodbye prop! Elevator motion has been checked a number of times and is within specs, even toward the upper limit for up elevator travel. Could be CG location. Where is your empty CG? Thanks
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  #7  
Old 02-01-2016, 01:31 PM
Boomer506 Boomer506 is offline
 
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Default High/Hot/Heavy takeoff configuration

Very good summary and I like your conclusions. Especially the conclusion that when runway length is not really a factor, that zero flaps and tail high is the right configuration. As an old flight instructor, I have seen many students and rated pilots choose 10 degrees of flaps and a Vx (or slower!) climb when they are concerned about a high/hot/heavy takeoff. But when you really want best climb performance, getting to Vy as quickly as possible at zero flaps is the way to go. There are many mishap reports where the pilot is taking off (or rejecting a landing) with flaps deployed and attempting to climb at Vx (or slower!) and they wonder why they are not climbing. In the investigation, the pilot often indicates that they felt the engine was not developing rated power. Flaps up and Vy is your best friends when worried about climb performance.
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  #8  
Old 02-01-2016, 02:28 PM
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skylor skylor is offline
 
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Default Gross Weight Test

Quote:
Originally Posted by nigelspeedy View Post
...

So based on this I've settled on the following:

Normal Takeoff from a sealed runway with no obstacles. Rolling takeoff, Flap 0 deg and tail up.

Short field takeoff: Hold on brakes until power applied, Flap 10 deg and tail low during the ground roll, 1.2 Vs till clear of obstacles then accelerate to normal climb speed.

Soft field takeoff: Rolling start, Flap 10 deg, tail low, level off to accelerate to normal climb speed after ground break.

Cheers
Nige
It would be nice to see at least a couple of test points re-run at gross weight and/or at a high density altitude to compare the effects of no flaps vs flaps on takeoff/initial climb performance. I suspect at higher weight, the extra drag of the flaps may be detrimental...
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  #9  
Old 02-01-2016, 02:29 PM
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nigelspeedy nigelspeedy is offline
 
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Default Bill TO Throttle

Bill,
Good questions. In truth I can't get the throttle full open on the brakes. What I did was to hold the stick full aft with the brakes on firmly. Then advance the throttle smoothly as far as possible, until the aircraft started to roll forward, then simultaneously release the brakes and add the remaining 1" or so of throttle. Mine starts to move by the time I get to about 2500 RPM. Good pick up I'll be more specific next time.
Cheers
Nige
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  #10  
Old 02-01-2016, 05:47 PM
kamikaze kamikaze is offline
 
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If I may ask ... as a future builder ... these numbers are roughly double or more what Van's advertises for an RV-8 (depending on HP) ... is that ... normal?

I thought I'd read Van's numbers were usually pretty spot on ...
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