Take off distance in j4xx

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peterd
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Take off distance in j4xx

Unread post by peterd » Fri Sep 04, 2009 11:52 am

There is always quite alot of speculation as to how far do you roll before the wheels leave the ground. I was wondering that if jab owners give exact data as to how far they run in different conditions we could build up a heuristic database that could actually provide guidelines. Factors affecting have to be acurate if posted. Maybe the unofficial jab chairman Sias can collate the data. There are enough flying in different conditions that would be very helpful to me as a pilot. The poh is official but I think we can learn alot more from each other's varying conditions. My focus would be the j430 and j400.
To start with is it a good idea ?
If so which parameters are included and which conditions need to be excluded.
eg. all up weight,oat,qnh,rh,wind hdg/kts, rwhdg etc.prop(STD or sensinich),rpm,flap pos,rotate ias.

What do you guys think.
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Re: Take off distance in j4xx

Unread post by Hennie van Zyl » Fri Sep 04, 2009 12:05 pm

Jab J430
Empty weght = 398kg
Sensenich GA prop
5200ft alt
10 deg C
No wind
full fuel (135l)
One up. 80kg
15 deg flaps
QNH= 1028
2760 RPM static
250 met run
Last edited by Hennie van Zyl on Sat Sep 05, 2009 4:35 pm, edited 1 time in total.
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Re: Take off distance in j4xx

Unread post by BenSmit » Fri Sep 04, 2009 3:04 pm

Hennie
How about doing that same takeoff with more weight on board to compare?

Was that on a grass strip?

I will try to do some on the weekend as report back
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Re: Take off distance in j4xx

Unread post by Hennie van Zyl » Fri Sep 04, 2009 3:22 pm

BenSmit wrote:Hennie
How about doing that same takeoff with more weight on board to compare?

Was that on a grass strip?

I will try to do some on the weekend as report back
Smooth dirt strip. With 75kg pax = 300 met
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Re: Take off distance in j4xx

Unread post by henk-indabush » Fri Sep 04, 2009 3:39 pm

Nice idea, peterd. Now if you were to post the spreadsheet we can all have a place to complete the data and compare.
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Re: Take off distance in j4xx

Unread post by BenSmit » Fri Sep 04, 2009 11:57 pm

henk-indabush wrote:Nice idea, peterd. Now if you were to post the spreadsheet we can all have a place to complete the data and compare.
Something like this?
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Re: Take off distance in j4xx

Unread post by henk-indabush » Sat Sep 05, 2009 6:43 am

Thanks Ben, =D> =D> =D> :idea: it certainly have more value to me this way! Just reading a lot of figures confuses what little grey matter I have left!

One suggestion - What about a columm for density altitude?

And we are agreed that this is for measured T/O distances only, and not for estimated?

So how does one measure it? Have someone on the ground marking the lift-off spot and measure it afterwards with a tape-measure/Google earth? Mark the T/O spot yourself (while taking off) with an imaginary marker and go measure it afterwards? Have someone on a quad or a bicycle or donkey (yes, we breed them fast in the Free State!) follow you next to the runway and mark the lift-off spot?
I do not know which method will be the preferred one, but from a safety point of view, this is a thread with possible hidden dangers. Just a few words of caution:

1. The way of measuring the actual distance, should always be done in a SAFE manner. Do not do something stupid in order to obtain it.
2.These figures are NOT essential to have - there are published figures in the POH which should be used, in a safe manner.
3. Do not follow these figures when you take off. Just because Wynand could take off on 200 M on a given day, does not mean that any pilot can do it given the same set of circumstances.
4. Pushing it. True to pilots ego's, this may lead to everyone trying to outperform (my aerie can take off in shorter distance than yours) each other and may lead to dangerous take-off manouvers. Do not do something stupid.
5. If you really want to take-off in short distances, go for training in short field techniques. There are specific techniques.
6. The figures are for ground roll only, not to clear a 50 ft obstacle. So it's NOT figures to ever be used for calculating runway lenght needed!

I suggest we use the info on the spreadsheet wisely. I do think it's a good idea to have accurate numbers, for interest sake. But we are not trying to rewrite the POH, and the POH figures should always be the guideline, not some spreadsheet on avcom.

I certainly would like to see these figures, for interest sake.
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Re: Take off distance in j4xx

Unread post by peterd » Sat Sep 05, 2009 11:40 am

That is a good point Henk. POH is the rule. As a pilot I would want to know what the jxx does in different conditions. It should never be trying to out do each other on how short you can take off . In the end I would like us to have reliable info on density altitudes from sea level to 8000 ft with weights ranging practically from 500kg to 700kg. (I take it pilots weight 80kg plus low feul 50kg and aircraft 350kg is about the minimum for a flight in any given direction). After few distances at each flight level a pattern will hopefully evolve.
I also think the take off distance must be rounded up to the next 100m. Ie 370m =400m or 540=600m that way error on the side of caution.

Maybe Ben can put a siclaimer on the top of the xls sheet and a density air calculator!
Thanks for all the possitive comments
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Re: Take off distance in j4xx

Unread post by henk-indabush » Sun Sep 06, 2009 7:50 am

I took the liberty of modifying your spreadsheet, as well as adding a (very!) basic calculator for density altitude. Please look at it and advise (or just make them!) any changes. One needs to add the data on the calculator sheet, calculate pressure and density altitudes, and then transfer these values back onto the data sheet. Should be simple enough.
Any improvement on this sheet (especially the calculator) will be welcomed!

A disclaimer is not worth the screen it's written on. I rather put a big warning that it is not an accurate tool. The jabbie users can use it at our own peril.

And yes, rounding up to the next 100 m is a safe idea, but is it accurate? I thought you wanted accurate numbers? Devil's advocate: If my T/O ground roll is 215 m, and I take it as 300 m, are you going to see what you want to see on your statistical database? And why would you want to round it off to the nearest 100 m "for safety's sake" if you are not going to use it as a guideline for future take-offs? In my opinion, this should be done as accurate as possible or not at all, to have any real value.
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Re: Take off distance in j4xx

Unread post by BenSmit » Sun Sep 06, 2009 8:56 am

Hi Guys

Agreed this is just for info sake. The more info the better I always think. The POH is rather vague and I would rather have more info than less.

Measuring distance, I would say someone on the ground marks the spot and measure it on google earth is the safest. (20m either way wont be an issue because if you are measuring your required TO distance in those units, then you are already in trouble)
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Re: Take off distance in j4xx

Unread post by peterd » Sun Sep 06, 2009 9:27 am

Youre right about the TO distance. I agree Henk= write down what you measure. DA calculator is great. The other variable that affects TO dist is the gradient of the runway. With google earth we can tag the elevation of both thresholds if there is a gradient present. Taking off downhill with a tailwind vs headwind uphill stat would be interesting. Ie two fields added . rw heading and height at threshold (RW36/650 and RW36/550). By looking at the runway elevation we can see which threshold was used ? if the gradient is going to complicate it lets leave it out.
My friend and I tested the j430 vs j400 with the exact load to check the difference in TO dist and there is a big difference so we need to know the aircraft type.
Ie: j400 or j450 ,j430.


Once we have all the fields figured out we can start populating the database.
Your comments ?
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Re: Take off distance in j4xx

Unread post by BenSmit » Sun Sep 06, 2009 5:44 pm

There are various factors that influence this data, propeller seems seems to be one of them as the standard does not seem to give the same performance as the GA Sensenich

Honesty, by compiling this we are doing better in terms of knowledge.

Lets face it the POH says VERY little and has HUGE disclaimers in it. "Jabiru aircraft accepts no responsibility for the guidance data and informatiuon provided in this manual" Can only be Australian.

The POH talks only of pressure alt and not DA and says you have to increase by 115 metres for every 1000' of PA. Now this I am not sure I understand, if the TO roll at sea level is 400m, at a PA of 5000' you have to add 115m X 5? or does it mean you start counting at 0 ie 115 X 5 = 575?

Hennie, I dont know how 250m take off roll is achieved, must be the prop? I did standard short field take offs but did not pull her off and could not do better than 380 -390m with a DA of 6200 and AUW of 585kg
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Re: Take off distance in j4xx

Unread post by henk-indabush » Sun Sep 06, 2009 8:21 pm

=D> Now we are heading in the right direction in obtaning valuable info! Thanks guys, for your input into peter's idea.
Agree, the POH, is as I read it, to add the 115 for each 1000 ft = giving you 975 in Ben's example. Which, from my experience, is more than ample and I have never got close to using that much runway at 7000 density altitude (NOT at MAUW) Perhaps that's a good question to ask "George"? Surely Len will be just too happy to help us to interpret the POH correctly.

Indeed runway slope is important, if the gradient is a factor and can be measured, I suggest we do put it in. Otherwise a "slight uphill" or "slight downhill" should do. The comments columm (last one) is valuable to add any of these data and other comments.

I have found that on my 450 the Sensenich made a substantial difference n T/O performance, compared to the standard prop. Have not measured distances accurately previously, but I will start now and post it on the sheet. So have no accurate data for standard prop.

Next question: How to uplaod the latest database? Perhaps Sias is a good call, to keep the latest one, otherwise I suggest we keep it here on this forum, with a post-fix to indicate the latest version. I have posted Ben's last one with the runway slope added as version 1.1, so the next guy who adds info can post it as TO distances 1.2 etc.??
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Re: Take off distance in j4xx

Unread post by BenSmit » Sun Sep 06, 2009 9:32 pm

Well, you can see from my TO data that there seems to be a huge difference in performance. 25% in fact from Hennie's data. However if I look at what the POH says and factoring in that i was not at MAUW and was using a flat, Tar surface with a slight headwind, it is almost correct.

Have a look at this chart, it is my understanding of the basic premise of the POH data, am I correct or missing the point?


I just dragged out my SAAF POH for the P166s and the data there is huge, maybe we should try to develop something similar. It factors in Alt, Temp, weight, wind, runway slope.
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Re: Take off distance in j4xx

Unread post by BenSmit » Sun Sep 06, 2009 9:59 pm

Guys

Why don't we get together at Parys, Kittyhawk or Rhino one Saturday or Sunday and do some quality measurements, we can then do it controlled and safely and get data we can use safely. Maybe use Tar and Grass for the excersize?

Henk? and others?
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