Density Altitude

[sierrasplitter]

Summer is coming ! Had my first taste of high density altitude saturday . Flew up to Big Bear CA , L35 field elevation is 6752 . Me and a Buddy on board , 75 lbs of baggage , no back seat , full tank on the left and half tank on the right . I calculated 630 lbs with fuel
73 degrees, making a Density Altitude of 9000 ft
wind was 090@10g15 Runway 8
Used 1850 feet of runway to get off the ground , and had a climb rate of 200-300 fpm
150hp Pacer
I don’t think I would go if it was much warmer out

prop is a stock 74 inch with 62 pitch I think
would like to have a wee bit more climb

how does yours perform at 9K ?

[Gilbert Pierce]

For the reasons you stated above I have always had a 58” pitch. I value climb over cruise speed.

[sierrasplitter]

Thats what Im thinking too. What is a ballpark cost of a prop re-pitch ?

[andya]

I once tried a 62 inch pitch prop on my 150 hp 22-20.
Did get a little better cruise but it came evident that it was going to hurt
take off and climb performance. I flew it out of a grass strip that probably could have used some cutting
but it accelerated much slower than my 60 inch prop I had at the time.
runway elevation wasn't even a high mountains strip, it was about 650 feet farm strip.
I also had a 58 pitch for a while and take off performance and climb was noticeably less than my 60" prop
and cruise was not affected near as much.

[deandayton]

Thats what Im thinking too. What is a ballpark cost of a prop re-pitch ?

I had mine repitched a few years ago. I don't remember exactly what they charged, but it was a few hundred dollars. It's a fascinating process to watch

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[59pacer]

I think that comparing the performance of 2 props can be a bit 'fraught', unless they are both brand new from the factory (and even then ??).
Metal prop blades will bend forward slightly, and flatten the pitch, with use, so the pitch that's stamped on the hub can be 'nominal' at best. Then add the results of dressing out dings etc over a period of time, repitching etc..
My 61" pitch Sensenich went in for overhaul after 2000+ hours, and was scrapped. It looked great, had minor dings only in the 1000 hours I flew it, but was too thin in places, and too narrow in others-- outside overhaul limits. The first 1000 hours were on a Tripacer, the last on a Pacer, so I can guess when the repairs to stone damage occurred.
The new replacement 74DM has 60" pitch. The aircraft climbs and cruises faster than the 61" . The overhauller suggested that I wait and see the performance of the new prop before repitching it. Good advice as it turned out.
Discussing possibly repitching with him, he said all he really needed to know is how many rpm I wanted to increase or decrease, as the number on the hub was not much use.

[sierrasplitter]

Thanks 59Pacer. I just spoke with Sensenich and the Prop was bought in 2014 and is a 74DM6/58 so its a "climb Prop" according to the website . With a 60 listed as their cruise prop for my plane. It has less than 500 hours on it and I have never put a file to it.
So, I need to go back up under the same temp without a PAX and see How She does. I dont know if I should expect much more at 9K but was happy to at least find out its a 58.

[Tailwind_Fan]

I'm flying out of a field that's at 4001 ft with a 4020 ft long runway, on the morning of the 19th. I flew with some friends up to Lake Isabella for lunch. I had Travis (who's both a USN Test pilot and CFI) up front with my and Katerina in the back. Full fuel. Reported DA was about 7k ft... acceleration was slow and finally lifted off about mid-field and the climb was sluggish (2-300 ft). I used the short field technique of setting the brakes then going to full throttle to ensure minimum runway used. The climb was so slow we weren't going to clear the hill to the West, so I navigated around it with a slight turn to the South, following the highway. I managed to make it up to 6500 ft on the way North, and 5500 ft South after lunch but it was a slow climb.... I really wish there was a better way than the Koch chart to calculate takeoff distance and climb performance....

-Alana

[dgapilot]

I'm flying out of a field that's at 4001 ft with a 4020 ft long runway, on the morning of the 19th. I flew with some friends up to Lake Isabella for lunch. I had Travis (who's both a USN Test pilot and CFI) up front with my and Katerina in the back. Full fuel. Reported DA was about 7k ft... acceleration was slow and finally lifted off about mid-field and the climb was sluggish (2-300 ft). I used the short field technique of setting the brakes then going to full throttle to ensure minimum runway used. The climb was so slow we weren't going to clear the hill to the West, so I navigated around it with a slight turn to the South, following the highway. I managed to make it up to 6500 ft on the way North, and 5500 ft South after lunch but it was a slow climb.... I really wish there was a better way than the Koch chart to calculate takeoff distance and climb performance....

-Alana

Not going to be anything better, short of putting a turbocharger in. With an O-320 at a density altitude of 7000 ft, the best you can get is about 97 HP. Best bet is to plan take off at dawn when temperature and density altitude is lowest. Next would be to install a flatter pitch prop, but even that doesn’t help much.

You can try this https://www.sportys.com/takeoff-perf...-computer.html but it is just another version of the Koch chart, just in slide rule format.


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[J Ryd]

Just be diligent about preplanning departure such as as wide circles to gain altitude or direction that enables safe accent. Here in Colorado many of the airstrips are 7500 - 8500". That easily puts DA at 9000 or above. Keep an eye on airspeed and patience. My prop is pitched to 57. Still nothing magical in performance.

Jan