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  #1  
Old 04-26-2021, 02:46 PM
Dune Dune is offline
 
Join Date: Sep 2013
Location: Montgomery, TX
Posts: 15
Default High DA ops.

I would like to learn about your experiences in high altitude / high DA airstrips. There is a specific airstrip I’m thinking of but I’m more than reluctant as there may not be a comfortable margin for error:

Elev: 7300 asl
Runway Length: 2500 ft one end unobstructed, tall trees on the other, asphalt.
Morning temps: 18-20 degC usually calm in the morning

RV-7 180 Lycoming M1B/ Hartzell blended airfoil cs prop.

I’ve flown out of high mountain airports like Mammoth or Big Lake. Much longer runways and different airplanes so I have no frame of reference.

Thanks.
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  #2  
Old 04-26-2021, 04:23 PM
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Ironflight Ironflight is offline
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Whiel we generally don’t worry about DA most places we fly out west with the RV’s, 2500’ is pretty short - if it is truly unobstructed, then maybe....but you might think of trying a restricted throttle take-off on a long runway where you live (compute an equivalent amount of power to what you’d get at the altitude/temperature you’re interested in, and use that much), then measure the take-off distance. That’ll give you some idea of the skill level involved and the margins you might (or might not) have....

Paul
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RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
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  #3  
Old 04-26-2021, 05:05 PM
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RV8Squaz RV8Squaz is offline
 
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Not a bad idea as Paul says above. But for a little more accuracy on your trials, realize that at 7300’ I wouldn’t expect any more than about 22-23” manifold pressure. So when you do your part throttle experiments don’t set any higher than that. Also at 7300’, standard temp is about 0 deg C. That puts you 18-20 deg above standard. A fair trial would have you doing the experiment at sea level, with 22” at 35 deg C. Finally engine power is only one part of the equation, propeller thrust and lift are other aspects which would be difficult to duplicate at sea level.

My general experience has been that the ground run at 7300’ is about triple the ground run at sea level. 2500’ is pretty darn short. It is likely doable with zero obstructions. A solid headwind would help.

Why not take some baby steps and experiment along the way? On my way out to Leadville once, I stopped at 3500’, 5000’, and 6500’ elevation airports to get a feel for it. Still I managed to rotate a little too soon at LXV. The airplane wasn’t quite ready to fly. Not a problem on a 6400’ runway.
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  #4  
Old 04-26-2021, 05:31 PM
blaplante blaplante is offline
 
Join Date: Jan 2010
Location: Southern California
Posts: 290
Default Beware of the slope

Don't know what runway you are looking at, but be very careful of any runway slope. I don't know the exact rules of thumb but any uphill will extend that roll. Also beware the weight. No good being perfectly happy solo and no baggage and then trying the same thing with a passenger.

Only other comment - in my fixed pitch 160 HP - I'd give it a pass.
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  #5  
Old 04-26-2021, 06:05 PM
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jcarne jcarne is offline
 
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Location: Worland, Wyoming
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Quote:
Originally Posted by Dune View Post
I would like to learn about your experiences in high altitude / high DA airstrips. There is a specific airstrip I’m thinking of but I’m more than reluctant as there may not be a comfortable margin for error:

Elev: 7300 asl
Runway Length: 2500 ft one end unobstructed, tall trees on the other, asphalt.
Morning temps: 18-20 degC usually calm in the morning

RV-7 180 Lycoming M1B/ Hartzell blended airfoil cs prop.

I’ve flown out of high mountain airports like Mammoth or Big Lake. Much longer runways and different airplanes so I have no frame of reference.

Thanks.
Quote:
Originally Posted by RV8Squaz View Post
Not a bad idea as Paul says above. But for a little more accuracy on your trials, realize that at 7300’ I wouldn’t expect any more than about 22-23” manifold pressure. So when you do your part throttle experiments don’t set any higher than that. Also at 7300’, standard temp is about 0 deg C. That puts you 18-20 deg above standard. A fair trial would have you doing the experiment at sea level, with 22” at 35 deg C. Finally engine power is only one part of the equation, propeller thrust and lift are other aspects which would be difficult to duplicate at sea level.

My general experience has been that the ground run at 7300’ is about triple the ground run at sea level. 2500’ is pretty darn short. It is likely doable with zero obstructions. A solid headwind would help.

Why not take some baby steps and experiment along the way? On my way out to Leadville once, I stopped at 3500’, 5000’, and 6500’ elevation airports to get a feel for it. Still I managed to rotate a little too soon at LXV. The airplane wasn’t quite ready to fly. Not a problem on a 6400’ runway.
Good points.

Just as a data point I just took off yesterday around this DA. I get around 24.3" on the MAP due to some ram action. (update for clarification: shortly after liftoff at 100KIAS, it would be less on the roll)

I also have the IO-360-M1B in my -7A.

You would quite likely be able to do the takeoff but I certainly wouldn't be comfortable with the margins... Especially at anything other than solo.
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Last edited by jcarne : 04-27-2021 at 08:13 AM.
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  #6  
Old 04-26-2021, 07:31 PM
Dune Dune is offline
 
Join Date: Sep 2013
Location: Montgomery, TX
Posts: 15
Default Thanks

Thank you for all your replies. I have done reduced power exercises on my previous airplane before operating at significant elevations. I wasn’t to happy with the reliability of my conclusions. For one, though I can replicate the power setting that I get at 7000ft and do a SL takeoff with that, there is also the factor of decreased propeller and wing efficiency. So the second whammy is unaccounted for.

In my visualization this was a solo operation with 1/3 fuel, 20 lbs baggage. I weigh in at 170. But note I say my visualization WAS. The replies, which I appreciate greatly, agree with my gut that it is not going to be an enjoyable experience. I’m not looking for adrenaline anymore at this age ��.

I think I’ll just go to TVL instead. You can takeoff two times in that runway length and at gross weight!

Cheers fellow aviators!

Last edited by Dune : 04-26-2021 at 07:39 PM.
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  #7  
Old 04-26-2021, 09:01 PM
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Ironflight Ironflight is offline
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OK, so now that you mention TVL (right in our neighborhood), I want to know which strip you were thinking about!
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Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
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  #8  
Old 04-26-2021, 11:00 PM
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koupster koupster is online now
 
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While you can simulate the reduced power by reducing the MP, that doesn't account for the increased ground speed needed to generate the IAS you need for takeoff. Given 7300' and 20 C (no wind), you'll need 69 knots to generate 60 KIAS for takeoff. So, you need to accelerate to a speed that's 15% higher than at sea level. Density altitude in this example is 9300'.

This Koch Chart can be handy: https://www.takeofflanding.com/
It shows that 2500' in these conditions is equivalent to a 850' long runway at sea level and standard temperature.
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  #9  
Old 04-26-2021, 11:05 PM
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bjdecker bjdecker is offline
 
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NV72 perhaps?
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  #10  
Old 04-27-2021, 06:29 AM
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RV8Squaz RV8Squaz is offline
 
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Location: Senoia, Georgia
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Quote:
Originally Posted by jcarne View Post
Good points.

Just as a data point I just took off yesterday around this DA. I get around 24.3" on the MAP due to some ram action.

I also have the IO-360-M1B in my -7A.

You would quite likely be able to do the takeoff but I certainly wouldn't be comfortable with the margins... Especially at anything other than solo.
Jeremy,

I have an IO-360A1A and constant speed prop on my RV-8. I too get about an inch of ram rise at cruise speed.. There would be zero ram rise at the initiation of the takeoff run and very very little at typical lift off speeds. I wouldn’t count on that one inch on a short, high elevation airport.
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