BigBadJon

im heading from california to colorado for the summer. my question is going from 1400 ft. to 7600 ft.. do i need to increase pitch on my props? or is it just a matter of retuning the engine? and is there anyone that flies in the conjes county. so i dont have to fly by myself lol.

Trollmaster

Speaking from 5000 ft, your going to have a significant overall power loss in rpms and performance. Add in the DA and half a dozen other things and the engine performance will drop. If you feel your plane is running like crap , it is but at no fault of the engine. You'll have to retune and maybe go down in prop size, not up.

If the engine is losing power, I don't think going up in prop size will help.

You probably should just wait until you get their and try it out first. As long as your aware of the potential problem and "feel it out." Dont try any stupid stunts until your sure the plane will follow though on it all.

I've seen guys come from Florida with Turbines and try flying here and tell us to shove it. So we just sit back in our chairs while they're out in the fields picking up the pieces from over shooting the runway for the 10th time.

Crazy4Flight

My Feedback: (540)

Altitude Density..... thinner air means less oxygen, lean your mixture a bit and still looose power. Also you will have less lift, more airspeed need to takeoff.

Trollmaster

Just curious, what planes/engines ya bringing

KC36330

good thing I'm not from Florida to me the difference was having much less glide and coming up short not overshooting, that was fixed with shorter down winds or use of power on final.

kc

Mike Connor

My Feedback: (5)

The indicated air speed of an aircraft is the same for take off and landings regardless of the elevation. This would result in a higher ground speed and longer take off and landing rolls at high elevations.

Trollmaster

Although I'm "use" to it, this is one reason I am a big fan of Kavan Wheels. You can bring that thing in, all set up for a perfect "overshoot", hit the asphalt and crank that baby sideways and slide like your on ice with no wheel hopping on any size or any type plane.

MJD

My Feedback: (1)

Yep - the big problem is adjusting your visual perception to account for the change, esp. when you are really familiar with the aircraft. I flew a fairly highly loaded little sport .15 canard around Colorado Springs at 6100 feet, and while it flew fine, hand launches were notably more of a challenge (and they were a pain at low elevation) and landings were pretty hot compared to my normal 700 feet ASL.

What you have at 7600 feet is an aircraft with 11.36/14.00* = 18.9% less horsepower that at 1400 feet, that needs to fly at a ground speed of 18.9% more to behave the same way.

MJD


* according to a US Standard atmospheric table

Trollmaster

Thats about right, I recall a few years back calculating it all up to an average of a 3% loss for every 1000ft. I compared numbers and such with those with near exact setups and it was real close. This is why some guys need to be careful when asking for numbers as to where the replies are coming from.

combatpigg

My Feedback: (3)

Does going up in nitro content help compensate for the thinner air without the normal shortcomings, like blown plugs, etc.?

BigBadJon

well my list started out with 2 prop jets, a 30% pitts bipe. and a mach racer delta and an excelleron 50.
there is a runway near Alamosa/ monte vista im told. if not. there are plenty of wide open areas i suppose. lol

oldvet70

My Feedback: (10)

I used to live in Monte Vista and my wife went to school in Alamosa. There was an RC field just west of Alamosa along HWY 160. I have not been there in a couple years but it's been there a long time. There is a club in Monte Vista called the SLV MODEL AIR-O-NUTICS so you might want to talk to them. I'm sure they can help you out.

Mike Connor

My Feedback: (5)

I found this article about nitro content and high elevation at http://www.flyrc.com/articles/tune_your_engine_2.shtml .
Tuning For High Altitude Engine Operation
...Run more nitro. Experienced tuners have learned that some of the oxygen can be replaced by increasing the nitromethane content of the fuel; the nitromethane molecule contains oxygen that is released during the combustion process. If you use 10% nitro fuel at home, bring along some 20-30%....

MJD

My Feedback: (1)

Interesting - after looking at that article briefly, where it said check rpm, I was about to say why on earth would rpm be reduced, because lift and drag are both proportional to air density as well. That may be true, but then I realized that a contrubting factor that does NOT change with elevation is mechanical friction in the engine. Therefore, so it seems to me, the prop load at a given rpm would be reduced by the same factor as the horsepower, but the friction at a given rpm is the same, therefore the rpm would indeed drop due to the reduced horsepower.

As to the effect on the engine, that is a complex question. Ummm..

- returning rpm to the same level as at lower elevation is still a lower horsepower situation.. due to reduced prop load but the original engine friction level
- as you go above that point in rpm, of course prop load increases as - I believe - rpm squared, but internal friction also increases. So to spin the prop faster on the ground to the point where it is absorbing the same amount of horsepower as before, which I think would be the right way to return original flight performance, the total horsepower must be higher to account for the increased enginge friction. So then it stands to reason that the engine would have a higher thermal load to handle, and internal temp could be higher due to original cooling rate but increased horsepower output.

So my first guess is that reproducing the original horsepower might be a hair harder on plugs than it was at the original altitude. Whether or not the difference is enough to care about is beyond me.

Okay, now my head is spinning. I'm not trying to be a smarty-pants here, just thinking through the problem out loud, and I could be all wet on the whole bag.

MJD

Mike Connor

My Feedback: (5)

That post got my head spinning too . My first thought was like yours. The reduced engine power should turn the same prop at the same rpm because of the reduced prop load in the thin air. My second thought is that I do not want to think about it any more because it makes my head hurt. I would like to find an article that gave exact #'s from the same engine at different elevations using different nitro % and props. Combatpigg, see what you started![>:]

Trollmaster

Here's an aspirin for ya. Both my Jett 50 and 90 turn about 1500-2000rpms less with the same prop as Dub recommends down their. Spend some time in Alaska. The performance charts will generally show the aircraft gross weight can be substantially higher than the grows weight of the aircraft.

Mike Connor

My Feedback: (5)

What % nitro do you run and have your tried higher to gain rpm?

Trollmaster

Oh I could go higher if i changed the props around. Just stating what Dub has on his with a certain prop. The Jett 90L with a 12/6 says 14,300. I'm at around 12,600 with the same prop.

Mike Connor

My Feedback: (5)

Actually, the question was about the % nitro you use and have you ever tried a higher % of nitro for better performance.

Trollmaster

Oh, 15% Omega for most of them. Some guys use the 30% on the jett 50's, Makes a big difference too.

Mike Connor

My Feedback: (5)

It sounds like nitro may solve part of the problem of high altitude flying and is easier then trying to turbocharge a RC engine. It also sounds like it takes a Jett at 5000' to get the performance of an OS or TT at sea level.