Is there a "rule of thumb" for derating gasoline engine power or thrust for a field elevation of 6,000'

Thanks

Murphey...as in Murphey's Law

I would say that "the rule of thumb" forr 6,000" MSL is "ask Dick Hanson". He is at some altitude as well, and always has creations that seem overpowered, if that existed, but at his altitude it's just right.
DKjens

There are some calculators out there; most say 3%-4% power loss per 1000' gain in altitude.

Power -or thrust --
a bit tricky
The engine has less air to breath --but the prop can churn the air more easily--
So- same prop at any given rpm will produce less thrust than at sea level
How much?
Again, tricky
The only definitive test is a spring scale
turn a given prop at a given rpm at sea level- record - then turn same prop at same rpm at altitude -
compare
Let's say that the test shows 12% difference - ( this depends on corrected altitude density)
Now lets add power till we get same thrust .
How do we do this?
Remember - we are reading prop results -not engine setups
OK let's say-we can simply get more power out of the engine-until the thrust is equal.
Now we have the power (thrust the same)
Will the plane fly the same ?
No.
It will fly faster with same thrust because air has less resistance.
The slow speed performance will still be inferior because it takes more speed to develop same lift.
Actually, being lighter -will make the most difference in how the plane performs.
So --we add pipes to engines and carve off all the weight possible from our airframes.
Some highly touted combos -at sea level -are simply duds at our altitude.
So what is the "rule of thumb"?
I don't have one - just add all the power you can without adding weight-and if possible loose all the weight possible
So far I not seen one too light with too much power.

I am a member of a club in Arvada. Elevation at the field is 5973' above sea level.

Here's what I know from personal experience:

It takes a .60 minumum to hover a 5 pound airplane--and thats gonna be full throttle hovering--not 1/2 throttle

It takes a 1.80 to hover 12 pounds at darn near full throttle. That was a ST3000 on a Hanger 9 25% Cap 232 that weighed 11.75 pounds. Hovering was 3/4 throttle--with not much left for emergency situations.

An OS 1.20 Surpass (non-pumped) will ALMOST hover 10 pounds at full throttle with a 15-6 prop.

A Thunder Tiger .91 will pull a Kadet Senior around plenty good with a 14-6 prop--provided it doesn't weigh much over 8 pounds. I had mine rigged for long distance flight. 3000mah battery and 54oz of fuel. Plane weighed 15-16 pounds loaded for a long distance flight. It took every last bit of a 450' runway to get it airborne with flaperons down at 25 degrees. Not much you can do with a Kadet Senior that weighs 15 pounds--and has a .91 4-stroke--but fly it straight and level for about 2.5 hours. I never tried to land it fully loaded. Always flew it out of fuel and tried to land with only 4--8oz left in the tanks.

My World Models 80" 300S Patty weighs 15.5 pounds with a G-62 on C&H ignition. I can hover at 3/4 throttle with a Menz Ultra 22-10 at about 7200RPM. But, I'm too chicken to do it very close to the ground.[:-]

I put a TH .75 on a 5 pound airplane--Chief 40 size 3D Giles 202. It hovered at 1/2 throttle easily. Propeller was a Pro Zinger 13-5. Full throttle from a hover was like shooting the plane from a cannon. Straight up--in a hurry--for as far as you could see it. Quite a lot of right rudder applied to actually achieve a straight up climb at full throttle--lots of torque on that little airframe. I should have pinned and glassed the firewall--because I rippped it out after about 100 flights.

A Moki 2.10 will yank 13 pounds around--no problem. It will hover about 14.5 pounds--at full throttle--if you prop it right. 20-6 or a 20-8 prop

A Moki 1.80 will hover about 12.5 pounds at full throttle--but, thats with a cline and the tank on the CG

A BME 50cc engine turns a 20-10 pretty good--slap a 22-10 on it--and it's a turd.

A DA 50 will turn a 22-10 around 6500--6800 depending on the day. And the prop. Nobody running a 24-8 on a DA 50 at 6000' elevation. They just won't turn it.

Hope this helps.

Your results are smack on the money with what we see here .
So -- My EFYAK-runs a ZDZ50 with AMT pipe and turns a 22x8 Mejzlic at an honest 7750
also a 23x8 Mejzlic wood clone (Skorepa) at over 7200.
This gives serious instant recovery
My modified 80 single on tuned pipe runs -- a real 7000+ on MenzS 26x10-
again serious power
If I stick a small muffler on either one - power become just adequate.
A24x8 will load the ZDZ down into low 6000 and there goes the max torque -same as happens with the DA50.
these smaller props have to spin to work .
When we did an article on a Funtana 90 -we used a Roto25 and a 17x6 prop -this worked but an identical model with a OS 1.2 -seemed underpowered -so we did static thrust tests - the OS on a 15x6-produced 10 lbs thrust - The Roto was under 13 lbs
You simply can't get enough power and keep the weight down -at these altitudes -to cause a "overpowered"problem.

Dick Hanson:

Have you ever run a G-62 on a pipe at your altitude? Any recomendations or particular combos that work well?

I'd like to try for a bit more power outta mine--but not if it's gonna cost me 2 pounds and only get me 500RPM.

I'm at 15.5 pounds with my Patty right now and I really like the way it flies--but I'd like to see my RPM closer to 8000 with a 22-10.

I'm running a 40:1 mix right now--and it's NOT synthetic. I'd like to keep my fuel mix where it's at. I'm just under 4 gallons on the engine right now. I don't want to switch to 100:1 for a few hundred RPM. My engine likes oil--and it stays plenty cool and lubed at 40:1. Plug looks good.

Thanks

never tried that setup -looks like weight of the model is your primary consideration tho .
So If your ears can stand it -run the most open exhaust possible

Well, I just switched from a B&B muffler with 2 exhaust tubes too a Slimeline Pitts smoke muffler tonight.

I'll tach it in a few days and post a few numbers from 10,408' elevation.

I don't dare fly the plane at this altitude. Too scared. No good runways either. There is a straight road about 3/4 mile long near the golf course, but powerlines right next too the road, so kinda dicey on the take-offs and landings.

Trainer and other planes with light wing loading are okay--but still too chicken to try it with the Extra.

The LT-40 with a TH .75 is plenty good power, but landings with 20 degrees flaps are purdy fast.[X(]

There's a field at about 8000' down in Silverthorne. I might try the Extra down there later in the summer, after I get used to the field and runway. I'm new too that club. Only flown there a couple times. I want to get used too it, before I fly the Extra there.

The runway is a bit short for my taste with the Extra. I think it's only about 400-450' of Kentucky Blue grass. Nice and manicured--thick grass, but I'm used to landing this bird on a 750' concrete runway at 6000' and using ALL of it.

It's gonna take alot of confidence and some knee pads to get the Extra off that grass runway and back down again.[:-]

grass at 10,000ft?
man -you are getting close to the timberline --

No grass around here--at least not enough for a runway.

The Kentucky Blue grass runway is down in Silverthorne at 8000' elevation.

Mostly just rocks and sagebrush 'round here.

Leadville is an old mining town. Gold was discovered here in 1867 and the population went from just a few miners--too 50,000 people in a matter of only several months. The state of Colorado had considered making Leadville the state Capital--as it was 3 times the size of Denver at the time.

But, I think they realized that mining is boom and bust and that the town would eventually run out of gold--and it did. The current population is somewhere in the neighborhood of 2850 full time residents--and around 1000 "seasonal" workers who come every year to work the ski resorts--Vail, Beaver Creek, Copper, Breckinridge, Keystone--for the winter. The "seasonal" workers show up around October and leave in May.

Treeline is about 11,500' +/- around here. My lawn here at the house--10,408'--will just barely grow. Gotta water it a lot and feed it a lot.

Ya gotta figure, that if the trees won't grow--not much else can survive either.[&:]

I'm charging batteries, and I gotta make a tiny repair to the right wingtip. Scuffed the covering last time out.

Then we're gonna fire this beast up and see what she tachs at this altitude.

If possible -- better go for a good electric setup -at least you can get full motor thrust by just selecting props
We decided this year that anything under a 40 ZDZ--becomes an electric
Our smaller electrics -Hackers 20/20 outrunners will fly a 16 oz model -400 squares like crazy -on 1350 -2100 ma pack.
Just a thought -----
We ran a ski lodge one winter -at Brighton (Utah ) and my poor TR3B was a gutless wonder at that elevation.
I never went back-----

I'm too much of a gearhead too ever switch to electrics. I love internal combustion engines--especially 2-strokes. Gassers are even better. I've given up on glow over 1.00 size. Just too much mess and the fuel costs eat my wallet up too bad.

I'm looking at one of those little electric foamies--$150 investment. Something to fly around in the backyard, and practise my 3D.

I was tempted to convert the OMP Boxer Bipe too electric--but it was gonna cost me $500 for the power setup---lost interest in that real quick.

I could buy more than a couple REALLY NICE sport glow engines for that price. Or I could convert 2 more of my chainsaws to Synchrospark ignition and get wrap around mufflers for them--the works.

Electric still seams a bit expensive for the larger setups--and I like my noise makers too much to give them up.

Cool! We went from thrust to timberline to electrics!