retnavycdr

My Feedback: (190)

I hope this is the correct forum to ask this question. Has anyone ever seen any formula/algorithm that plots relative elevation above sea level against engine displacement? Our club here in Colorado is at about 5,000 or so feet of altitude, and all of the members use much larger engines than the ones recommended. This is not, in my observations, so they can just fly faster. The relative humidity is about 10-15% and the air is not as dense as at sea level. What I am looking for is some formula that states the increase in displacement/power needed for every 1,000 feet, if anythings like that exists. I was considering going with two OS Surpass 70-II four-stroke engines in the World Models P-82 Twin Mustang, which comes with a recommendation for two 40 two cycles. I want to be able to take off and get around smartly, am not too concerned with raw speed. I know what the left vertical stick is for, and that it does not have to go to the "firewall" very much, if at all.
Thanks for any input on this.

DarZeelon

Roger,


Please see [link=http://futurecam.com/densityAltitude.html]this web page[/link].

The power an engine makes very closely follows the air density.
If the density is half, at 18-20,000 ft., the power the engine makes will also be half as much.

A given prop will be spun at about the same RPM, since prop drag is also reduced by the same rate.
Thrust will be halved, however, since lift from the prop blades is a product of the speed squared.

You will need speed greater by the square root of 2 (1.41 times greater) to take-off.
This, since lift is a product of the speed squared.

I guess you would need roughly twice the power, for similar performance, at half the altitude density.

pe reivers

That is a nice site to compare power with. These also are the data I [link=http://mvvs.nl/prop-power-calculator.xls]incorporated in my thrust calculator[/link].
It boils down to this:
If the power to weight ratio of your plane stays the same, performance will be the same. At higher altitudes, you need the same power, so in fact a larger engine to produce that power.
In my spread sheet you can adapt the altitude, and then play with prop rpm to find the same amount of power as the plane originally asked for. Then you find the engine that turns those prop rpm at sea level.
you will probably find, that for every 1000 feet altitude, your engine displacement should increase by 2%, because this is the change in air pressure (read: available oxygen).

Cyclic Hardover

My Feedback: (3)

I've been flying here in Albuquerque for 10 years now and we over power most everything. There is about a 3% loss in power,speed,rpms numbers for every 1000ft elevation. I have tested this against friends from sea level and it's very close. One thing to consider is go with the higher of the recommended engines. Lets say you bought a 60-90 size plane. Install a 90. I actually install .60s in a 40 size if I can fit it in.

A very close friend has that P-82 with a pair of 46fx's and it passed up a 40 size wm P51 with a 61fx. If it were me even though Ihave seen it first hand, I would put a pair of 61fx's in it, not that it really needs it but what the heck.This guy also has that KPM P38 with a set of 91fx's. Has not flown yet but will want to see that go up.

You guys are a tad more humid than we are. Here on any given summer day, the "DA" can be as high as 8000ft at noon time.
All my planes are overpowered. Some respectively and some not like my WM .40 size P-51 with a Saito 100.

At a guess, a pair of those fourstroke 70's will do okay on the plane

Sport_Pilot

Since the wings develop less lift I think the engine displacement would have to increase more than that. But I havn't looked at your spread sheet. Performance will never be the same. If you size the engine to take off in the same distance, it will have to make the model go faster in that distance to offset some of the lift that has decreased. And of course the engine can do nothing about the increased stall speed and increased approach speed on landing.

And actually prop speed does decrease at altitude, just not as much as the power is reduced.

Sport_Pilot

That seems right if the engine power is decreased 2%. The engine power would decrease to the cube (its displacement) but the prop and wing lift would decrase to the square (its surface area).

Cyclic Hardover

My Feedback: (3)

This is why you need to be careful when you ask somebody here what rpms you should be getting and somebody from Florida tells you what he has. He's not wrong but won't be close to what you get. My general comparison is my Jett90 . Dub Jett post around 14,600 roms with the same prop that I use and I am around 12,000 ish. It's a healthy hit but still a performer.

DarZeelon

Mike,


Give Dub Jett a call - 713-680-8113.

At your altitude, you should be getting about 95% of the sea-level HP from your engine.

Your numbers suggest a 45% deficit at same altitude numbers.
I am quite sure something is not right.

TLH101

My Feedback: (90)

Exactly, less air density creates less load on a given prop. I used to live in Alb, and on a couple of visits, I have brought some of my planes to fly. I get the same RPM there, as here, at sea level, with the same prop.

retnavycdr

My Feedback: (190)

Dar Zeelon, et.al. : Thanks everyone for the great answers. thanks. I will try the P-82 Mustang Twin with the OS .70 II SUrpass engines, mainly because I had two of them, and they are 4-strokes, and I would rather have a little too much power than a little less than is needed. Dar- I had a chance to stay in Haifa for about a week in 1984, while working in the Middle East. Beautiful place. I am glad my passport got "special" treatment so I could return to Jidda, Riyadh, and the other places I was working.
I found all the spreadsheets very helpful.

DarZeelon

I am always glad to help, Roger.

I grew up in Haifa, until I was 10, agree it is a nice city and understand that 'special treatment'...

TimC

Dar, is it also true that the plane will fly at the same airspeed, even with the reduced hp at higher altitudes? It would seem that if the prop will turn at the same speed with reduced power, the plane will fly at the same top speed.

Cyclic Hardover

My Feedback: (3)

Keeping this on laymans terms, I am not going to match wits with the "DAR" since I do respect his knowledge which at a good guess exceeds mine but I will say this and that is Einstein proved that time travel was possible in theory but does mean it can or will happen in reality. I have been at this 10 years and the fact that I am here at this location and many others aren't.

Plus being an ex aviator does not make me brighter than others, just that I "get it." Charts, graphs ,formulas are nice and can get you close but are not always accurate. I have several guys around the states that have identical setups as some of my planes which confirms some of my thoughts on all this.

Turbines are also finicky and react differently depending on the location. The type of air we have here in Albuquerque is totally different than that in Denver and were 400 miles apart and the same altitude. Turbines have problems here and run perfectly un in the Denver area and or California.

Engines are less efficient the higher you go, nothing new about this. One thing that helps is more powerful glow engines in the same size casings as the smaller ones. When I flew in the military, I never had more engine problems anywhere I'd been except right here in New Mexico.

Like I said, you take a summer day here and add some humidity to it and before you know it, you have 7500-8000 ft, your engine is starving for air. Sante Fe is 40 miles away and 7500ft. They can be pushing 8000plus on the DA.

pilot727

I hear you, us guys in utah have to deal with alt. as well and it does make a huge difference, when i first started flying several years ago i was always buying an engine that just did not have the power for the alt. here, so i just went out and bought a ys-160 now alt. is no more problem to me, haha but what a difference to fly some where other then here.

TimC

I've flown at low altitude and it's quite a leap in performance. I'm always amazed at how quick the plane is airborne and how quick it will turn. The same landing speed at sea level will buy you a stall up here. The engine transitions better to my ear and I've always felt turned more rpm. I'm willing to admit the rpm might be the same. I'll make a same day test soon and report back.

blw

My Feedback: (3)

I have to agree with Cyclic Hardover as I am also the same type of ex aviator. We did use the charts for turbines every time we flew, as he mentioned. I don't think we ever computed the effects of temp and DA on airspeed. It was all power based. Unless you take in account the lore that more power was needed in autorotation during hot days, but that was an informal rule of thumb that may or may not have been true. I didn't teach that myself. My first training helicopter was a reciprocal engine and I'm almost 100% sure that we didn't have performance charts. Hardover may remember if he flew those too.

I spoke to a friend of mine who lives in Albuquerque, New Mexico and they could not fly OH-58A helicopters after lunch time due to temps and DA. All training was done in the mornings. Sounded pretty rough on performance. Do you know Mark Friskel? If so, tell him I said 'hi'. We went to flight school together.

Cyclic Hardover

My Feedback: (3)

No, have not run in to him. Its, funny that depending on where you live, you can become anexpert in areas that others will never experience. Here , I had more compressor stalls than you can imagine along with as many near overtorques during the summer months. Nothing like the sound of a overtorque when the transmission sounds like grinding and twisting. Sounds and feels like whenyou grind your teeth. But then when I was in Alaska for 3 years-nothing, but in the winter the power charts came out to allowed the gross weight to be darn near 11,000lbs!!!! 1500 higher than max. No performance issues their.

DarZeelon

Tim,


This is basically so.

However, Mike and Barry will concur that although true air-speed (TAS) will be the same, the indicated air-speed (IAS) is reduced considerably.

Imagine that; you're traveling at 200 mph, but your air-speed indicator says you're going 120...

BTW, the stalling speed (and VNE and VMO...) is in IAS terms... As you wrote, the stalling speed (in TAS terms) becomes visibly higher at lower density altitudes. If your model had an air-speed indicator, it would still show the S/L stalling speed...

NM2K

My Feedback: (14)

--------------


Is the OH-58A a twin rotor, no tail rotor heli? If so, we had one attached to our fire department at Luke AFB. It used to carry a fireman and a fire suppression kit slung underneath for off base crash/rescue.

DarZeelon

The OH-58A has the oldest model, with a two-blade tail rotor, a four-blade main rotor and a V tail.

The NOTAR is a much later development of the same basic design.

In between, the Hughes 500M Defender, with a five-blade main rotor and a T tail served in the IAF as a tank buster.


...Are we not off-subject???

w8ye

My Feedback: (16)

From my experience with real fixed wing airplanes, at 6,000 to 7,000 ft density altitude you are down to the realm of 75% maximum power available from the unsupercharged engine.

But at this density altitude, you can achieve the maximum cruise true airspeed available with just 75% power.

Most light planes with a normally aspirated engine have their maximum cruise speed listed at about this elevation.

DarZeelon

Jim,


I can add to this that most supercharged piston engines have 'sea-level supercharging'.
I.e. the supercharger does not give the engine any noticeable power boost at sea-level.
But it creates the same sea-level intake pressure as altitude is gained, so the engine continues to make its rated output to higher altitudes.

This allows supercharged planes to cruise at higher TAS at higher altitudes... Too bad most of those piston engine aircraft do not have a pressurized passenger compartment...

In turbines, such as turboprops, a similar control is 'flat-rating' the engine.
This means the maximum flat-rated HP can be produces from sea-level to ~24,000 ft.

This again gives a clear speed advantage at high altitudes.

Cambo

My Feedback: (2)

I learned to fly at high altitude. Just remember that a planes wingloading doesn't change. The plane will still fly much faster than normal in the air. A larger engine will help in some cases. The best thing is to get a plane with a low wingloading and large wing for high altitude flying.

blw

My Feedback: (3)

The OH-58A is a modified military version of the Bell Jet Ranger. The rotor system is larger for out of ground effect hovering. Likewise, the tail boom was extended for the larger rotor diameter. The rotor system is 2 bladed with teetering blades.

The original use of "NOTAR" was for the Hughes 500 airframe.

Things get measured in different ways. For example, some turbines are limited to EGT temps (exhaust gas temp), and some to TOT (turbine output temp). Likewise, N1 speeds are measured differently among manufacturers.

The limiting factor for the turbines that I flew was always max torque. Temp and PA would determine max torque for the transmissions for that period of flight. The UH-1H had a max transmission torque limit of 50 psi. I don't remember a 50 psi day myself. Most were around 49 or 48.5 from what I recall. The OH-58A was so underpowered that you would reach 927C, the TOT max, long before you reached torque limits. The OH-58C was a different story and we got a lot of fast spikes in torque that were overtorques.

Hardover, I only had one compressor stall. My first batch of students with night vision goggles, so I was really new myself. Got into the stalls and had to land. I guess I should be dead because as I was at about 50' and hunting for a place to plant it when I saw one big transmission line pole. Those big cross country towers. I pulled an armpit full of power since I didn't see the other poles and I wanted out of there. That engine held power without a hiccup until I found another place. I felt really dumb when I realized what I was doing.

I could be wrong, but TAS and IAS was always close. We used 90 knots as a base speed for flight and IAS was close enough to TAS that we rarely bothered to differentiate, even on cross country flights.

DarZeelon

Yes, Barry.


I made a 'booboo' and was referring to the OH-6 (see photo), a predecessor of the NOTAR.
Since NOTAR was mentioned, I automatically assumed the OH-58A was a previous Hughes 500 version...


About IAS and TAS, at low altitudes and reasonable temperatures, they are very similar.
When very cold and low conditions, IAS is higher.

But as altitude and heat increase, IAS is lower than TAS; even considerably so...


For example, the Airbus 320 has a VMO of 350 KIAS (Knots - Indicated Air-Speed), yet actually cruises at altitude at .77 Mach, which are about 440 KTAS.

At altitude most small choppers fly, the difference is very small, however.