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  #11  
Old 02-01-2016, 07:19 PM
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nigelspeedy nigelspeedy is offline
 
Join Date: Dec 2008
Location: Tehachapi, CA
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Default not quite vans numbers?

Kamikaze,
Variations will come down to several factors
The first being how you choose to define 'takeoff distance'. Do you just mean the ground roll or do you mean the total distance to 50'. Often in the kit planes industry you just see the ground roll quoted, or the distance not precisely defined.
The second reason for the difference is going to come down different test conditions (weight altitude, temperature, runway slope, engine & prop).
The third reason is going to be pilot technique. Rotation speed, pitch rate and speed at 50' are all going to make a very big change to the distances measured.
I doubt runway length is ever going to be a limiting factor on how I operate mine. Perhaps if you are planning to operate of short or sloped grass strips at altitude it could be more of an issue.
I think the takeoff performance is great regardless of how my measurements compare to the Vans published numbers.
Cheers
Nige
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Last edited by nigelspeedy : 02-02-2016 at 12:48 AM.
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  #12  
Old 02-02-2016, 08:54 AM
David-aviator David-aviator is offline
 
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Location: Chesterfield, Missouri
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Quote:
Originally Posted by kamikaze View Post
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 ...
Quibbling over 300' vrs 600', it's a matter of pilot technique. Compare that to the Piper you are flying.

Go for any RV and you won't be disappointed.

I've flown a lot out of a 2200' strip and will testify the 8 with FP Catto will get off in less than 1000' hands down. CS is better but FP is not shabby.
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  #13  
Old 02-02-2016, 09:05 AM
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airguy airguy is offline
 
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I fly regularly off a private strip that is 2600' with powerlines at each threshold, so realistically 2300' usable each direction, and in a loaded Cessna 172 it's manageable on all but a 100F afternoon at max gross. I don't expect to have any trouble at all with my 9A on that runway.

My quick 30-second takeaway from seeing the data above was that all the variations managed to produce a takeoff ground run of 550' plus or minus 50'. I was surprised there was that little variation in the ground run with the flaps and attitude variables. It all comes down F=MA, acceleration on the ground run.
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  #14  
Old 02-02-2016, 09:17 AM
Toga 1 Toga 1 is offline
 
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Location: LaGrange, GA
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Default Performance

Awesome Article. Your 0/10* flap, tail up/down performance numbers seem to be spot on with my very "unscientific" experiences. Thank you for sharing.

Tim

Toga1, "Phantoms Phorever", RV-8, "Betty Jane"
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  #15  
Old 02-02-2016, 09:33 AM
kamikaze kamikaze is offline
 
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Location: Ottawa, ON, Canada
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Point taken, thank you! Notably if your measures are not roll, but to 50' ... that makes a big difference.

And yes, I am comparing to my Piper, which is why I want an RV :P
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  #16  
Old 02-02-2016, 09:48 AM
Chkaharyer99 Chkaharyer99 is offline
 
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Location: Pilot Hill, CA
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Nigel,

Thanks again for sharing the results of this test. I will respect your disclaimer and do some tests of my own to see what kind of performance my plane is capable of producing under my control.

In examining the graphs, it appears to me in most cases you were airborne at around 300 ground roll and achieved 50 feet agl in around 500 feet. That's very impressive.

Thanks,
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  #17  
Old 02-02-2016, 01:53 PM
Bill Dicus Bill Dicus is offline
 
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Location: Shorewood, WI (Milwaukee area)
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Default Nigel TO throttle

Thanks Nigel. Our experience is similar, I think. After experimenting with various configurations I also use zero flaps for all normal takeoffs on hard surfaces. 10-20 degrees flap for soft, grass etc. Like you, I don't think the takeoff distance will be significant as a limiting factor anywhere I'm likely to fly.
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  #18  
Old 02-02-2016, 05:23 PM
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BCP Boys BCP Boys is online now
 
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Nigel,
Nice work here. Just a general observation and flying my -7 just over 200 hours. I've never actually documented and graphed my testing like you have done but I have come to realize that my -7 likes to jump off the ground at it's "normal tail dragger angle of attack". I was taught that when the elevator becomes "alive" (when you have stick control), you should pretty much hold that attitude and the plane will leave the ground nicely. Now there is obviously some exceptions to that rule. Long, wet grass for example...where the tail wheel is digging in to the ground and you need to get it off the ground as soon as possible. But runway conditions aside, I wanted to let you know that my experience has been pretty much exactly as your charts state. When I hold a tail low attitude I get off the ground noticeably faster. With that said, weight plays a pretty big part on how well it gets off the ground obviously. When I have full fuel and a passenger, my -7 wants to come off the ground at it's normal attitude but then start to immediately sink causing it to bounce and starts porpoising . I have to hold it on the ground (tail up) until the speed is appropriate for take off. I wanted to bring this up to you to see if you planned on doing any testing with a passenger.

Also, same disclaimer from me.... I'm just sharing what happens with my particular plane. O-360 - Wood prop - VSO 51 Kts
I'm sure it may be way different with a CS prop.

Just some food for thought.
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  #19  
Old 12-11-2018, 09:16 AM
BillL BillL is offline
 
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Location: Central IL
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Default Dust off this excellent and useful thread

I have to belatedly say thanks to Nigel for this data. I am finally getting some more accurate data for my RV7 POH and want to see what was available.

I have 180 hp with the Hartzell composite - the wide blade one. I tried the lift the tail on a cool day , 34F, and got about 450 ft for lift off. The G3X data was used to calculate the position and speeds. VS defined the lift off point.

450' seems high compared to Vans (275) and my estimated weight was 1480 lbs, so it was not heavy. I could probably make a little more RPM on brakes and certainly use the tail low lift off technique for slightly reduced roll. Maybe higher tire pressure will help too. Cold tires rolling drag coefficient can not be good.

I really appreciate the detail Nigel has provided, it will shorten the test work.

One snag is the accuracy of the WAAS gps data, but way better than the next best and it is really easy.

One last thing, if anyone has governing equations to make a definitive correction of TO roll and climb to different temp, altitude conditions, please post or send an email.
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  #20  
Old 12-11-2018, 10:17 AM
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YellerDaisy YellerDaisy is offline
 
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Hi Bill,

Certainly NOT definitive but I have found these rules of thumb to be pretty accurate.
https://www.mountainflying.com/Pages..._of_thumb.html

The item that is missing from this thread, IMO, is Density Altitude (DA). Without definition, I am of the opinion that numbers are useless. Much like saying that my airplane will do 200. 200 what?? Knots, MPH, true, indicated, GPS??
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Last edited by YellerDaisy : 12-11-2018 at 10:33 AM.
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