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  #21  
Old 12-11-2018, 12:19 PM
n816kc n816kc is offline
 
Join Date: Jan 2005
Location: Winter Haven, FL
Posts: 213
Default the Koch chart

will provide a pretty good estimation of performance adjusted for density altitude once you have test data for 'normal' conditions. I keep a laminated copy in the plane, comes in handy.

https://www.faasafety.gov/files/gsla...%20branded.pdf

Thanks for the fix Carl!
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Last edited by n816kc : 12-12-2018 at 07:22 PM. Reason: link added
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  #22  
Old 12-12-2018, 04:17 AM
Capt Capt is offline
 
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Location: Australia
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Default

I never operate out of marginal strips so the t/off run or distance isn't of concern for my RV8/180/CS but it's always interesting to read others figures they come up with.
As just mentioned, DA makes a big diff as does surface type, tire condition/pressure and obviously prop type and condition, add pilot technique the numbers can be somewhat different. As long as pilots realise that TORA and TODA are different calculations
Im still amazed at some pilots who I run into occasionally think their 180 HP produces just that at all elevations inc FP props, the latter is very worrying!
Stay safe, Vans made us a very capable machine BUT it does have its limitations.
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  #23  
Old 12-12-2018, 01:22 PM
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Eddie P Eddie P is offline
 
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Location: Reno NV
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Quote:
Originally Posted by nigelspeedy View Post
I did an experiment to find out by doing a series of takeoffs using flap 0, 10, 20 & 40 degrees.

....

Cheers
Nige
Thanks Nigel enjoyed reading through your data. It all makes sense and supports relatively established claims. Having some quantitative data on the other hand is the right kind of knowledge. It gives us all a more complete picture, increases accuracy in decision making, improves knowledge.. thank you!
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  #24  
Old 12-14-2018, 06:47 AM
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YellerDaisy YellerDaisy is offline
 
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Hi Nigel,

I realize this is an old thread but am wondering if you might be interested in measuring another set of tests?? Particularly since you appear to have some interesting equipment.

The test: Distance to take-off and clear a 50' obstacle.

The technique: Lift off at minimum flying speed and immediately begin the climb.

I cannot speak to the RV-8 but with an RV-4, this is lifting off at approximately 45 kts IAS, pulling the nose up quite dramatically as the airspeed passes thru 48 kts IAS, and climbing at 50 kts IAS until the obstacle is cleared.

The logic behind this is that at high density altitude, the time required to accelerate to Vx eats up a great deal of distance. Obviously, you have to sneak up on the speeds to ensure safety.

I would be very interested in hearing any suggestions you have for ways for the common man to measure/record this test. The problem we have faced is the 50' altitude. The altimeter and visual from the ground is imprecise, GPS too slow.

Thanks Nigel!
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  #25  
Old 12-14-2018, 11:50 AM
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nigelspeedy nigelspeedy is offline
 
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Location: Tehachapi, CA
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Default More Takeoff testing

Hi JD,
I was thinking that it might make a good subject for a KitPlanes article. The original testing I did was to help me decide on the optimum profiles for my aircraft. But as one poster pointed out I did not make any corrections to a standard weight or altitude so they are not meant to be compared to the Vans numbers. However, knowing the actual test conditions the corrections are not that difficult to make.
In my RV the actual Vx is as close to the stall as I can fly. But this slow just after takeoff is not a great idea, some margin is needed for safety even if this is at the expense of a slightly longer distance. In practice I use 1.2 * Vs as my best angle of climb speed.
If your field is so short that lifting off tail low and being at 1.2 Vs at 50' wont work then I think we need to wait for the STOL RV.
Cheers
Nige
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  #26  
Old 12-15-2018, 06:28 AM
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YellerDaisy YellerDaisy is offline
 
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Quote:
Originally Posted by nigelspeedy View Post
The original testing I did was to help me decide on the optimum profiles for my aircraft.
I completely agree. Knowing exactly how my airplane would perform under a specific set of circumstances was also the reason for my testing.

Quote:
Originally Posted by nigelspeedy View Post
In my RV the actual Vx is as close to the stall as I can fly. But this slow just after takeoff is not a great idea, some margin is needed for safety even if this is at the expense of a slightly longer distance. In practice I use 1.2 * Vs as my best angle of climb speed.
Agreed and that is excellent to hear.
My research leading up to my testing showed that there is a wide range of numbers being used by folks. At this point, I can't recall exactly what I found but it appeared to me that many were using a Vx that was very high - I believe 70 kts IAS was a common. In my airplane, 70 kts is 1.6 Vs. This struck me as odd which caused me to begin testing since waiting for the airplane to accelerate from 1.2 (52 kts) to 70 kts at 10,000' density altitude requires a good bit of time and distance.

Quote:
Originally Posted by nigelspeedy View Post
If your field is so short that lifting off tail low and being at 1.2 Vs at 50' wont work then I think we need to wait for the STOL RV.
That's not the case - it does. The issue was KNOWING how the airplane would perform before attempting to operate off this airstrip. This led to a bunch of testing using a variety of configurations which led to a technique and numbers that are far different than one will typically read about here and elsewhere (in regards to RV's).

I suspect the instrumentation you were using was beyond was is commonly available but have to ask - what were you using? Is anything like that available to the rest of us? Meaning: recording the point at which 50' is reached - more specifically, the distance from the start of the takeoff roll to that point? I'm asking as we found this to be rather difficult and had to enlist the aid of a drone stationed at the 50' point (which took a good bit of trial and error).
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  #27  
Old 12-18-2018, 09:20 PM
jefferts jefferts is offline
 
Join Date: Dec 2012
Location: Boulder, CO
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Default Equipment

I'm pretty sure he said he was using 4 Novatel ProPaks......We use the same unit at work, they're great, but not really hobbyist equipment, a bit user belligerent and they are not cheap...I think(IIRC) ~$5k each.....however, this is a great thread !!!
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  #28  
Old 12-19-2018, 06:27 AM
BillL BillL is offline
 
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Location: Central IL
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Default Please elaborate

Quote:
Originally Posted by YellerDaisy View Post
I would be very interested in hearing any suggestions you have for ways for the common man to measure/record this test. The problem we have faced is the 50' altitude. The altimeter and visual from the ground is imprecise, GPS too slow.
I used the GPS information recorded in the G3X, are you saying it is too slow to be reasonably accurate, at least compared to the next available source/method?

Could you please elaborate on why it is/might be inaccurate.

Thanks.

JD & Kevin, thanks for references on take off & climb (35/50')corrections. I sent a note to Kevin H and he referred me to a 1951 Air Force publication (Herrington) on flight testing. It has some empirical equations with corrections for 1.weight 2.Density altitude 3.Surfaces 4.Wind, and 5.Incline. The general references typical for weight are proportional to ^2 but Herrington said it was ^2.3. The flight manual is available in print for 1953 update or PDF (free) for the original text. Herrington states (paraphrased) the TO phase is so complicated based on first principles and so much variation due to pilot technique and skill, that empirical methods are best used.
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Last edited by BillL : 12-19-2018 at 06:56 AM. Reason: added response to previous post requisition references
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  #29  
Old 12-19-2018, 06:45 AM
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YellerDaisy YellerDaisy is offline
 
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Quote:
Originally Posted by jefferts View Post
I'm pretty sure he said he was using 4 Novatel ProPaks.....
Very interesting - thank you!
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  #30  
Old 12-19-2018, 07:08 AM
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YellerDaisy YellerDaisy is offline
 
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Quote:
Originally Posted by BillL View Post
I used the GPS information recorded in the G3X, are you saying it is too slow to be reasonably accurate, at least compared to the next available source/method?

Could you please elaborate on why it is/might be inaccurate.
Apologies Bill, I was terribly vague. I was attempting to say that attempts to manually note height and position (using instrumentation and outside visual clues) was too slow/inaccurate. All required far too much attention from this pilot when workload is high and things are happening quickly (wheels up to 50' is 3-4 seconds).

I was really looking for pointers on how a 'common guy' can gather this data with reasonable accuracy (and, hopefully, without investing thousands of dollars). You have clued me into using the GPS track/recording functionality. I can see how this would work quite well. While no longer a pressing issue for me, I will still explore this as I'm a bit of a data junkie. Thank you!!
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