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Old Sep 01, 2010, 12:29 PM
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Alamagordo, NM
Joined Aug 2010
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.40 engine at high altitude?

After doing some research I'm interested in the Tower Trainer 40. Money is an issue, but with the payment plan and all around good price this trainer looks good and it was recommended to me at the flying field. If you're not familiar with the kit, it comes with a 4-channel Futaba 4YF, servos and a Super Tiger GS40. I wanted to know if the Tx is any good but my main concern is the power plant. My area is about 4500 feet above sea level. Is this engine enough?
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Old Sep 01, 2010, 12:49 PM
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United States, CA, Oxnard
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you should be ok with that plane and power plant, but make sure u tune the engine, if i'm not mistaken u have to run richer on the fuel, not sure, but u can goggle it.
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Old Sep 01, 2010, 12:58 PM
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Joined Apr 2008
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4500' is not that high. You'll be fine.
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Old Sep 01, 2010, 01:33 PM
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USA, TX, Grapevine
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You will be OK with the engine. the radio system is OK too.
Technically you do lose a little power as the altitude gets higher and higher. But you will have plenty of power for your trainer to fly OK.
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Old Sep 11, 2010, 09:22 AM
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winchester, UK
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depends what you want to do with it....

I have a '40 in my trainer, and wish I had the '46.

Im at pretty close to sea level.

At altitude you will have less oxygen in the air per unit volume (same percentage at around 21%) but less molecules of oxygen due to the lower pressure. This means that you can efficiently burn less fuel and have a lower power output per unit size of engine.

My 40 is fine for basic trainer activities, but once you get past that and want to do more than loops and rolls you may find its not got enough punch to pull the plane about.

I struggle with power to fly inverted, and can only hold it straight and level inverted, inverted large radius turns are a no-no, it just runs out of pull and altitude.

given that I would get a 46 if I had to choose again, and im at sea-level, I would have a careful think. Its easy to throttle back, or even limit 'wide open throttle' but you cant get extra power from a smaller engine that easily.

Incidentally, there is usually little weight difference and size difference between a 40 and a 46, so you dont really lose anything by going up. My preference would be for an irvine engine.
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Old Sep 11, 2010, 10:49 AM
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as i understand it couldnt he just use a higher %nitro to compensate for the lower O2 levels ? i.e. 15% instead of 5% or 20% instead of 10%

or am i mistaken

skylark
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Old Sep 11, 2010, 01:55 PM
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Quote:
Originally Posted by slingsbyskylar View Post
as i understand it couldnt he just use a higher %nitro to compensate for the lower O2 levels ? i.e. 15% instead of 5% or 20% instead of 10%

or am i mistaken

skylark
You are kind of right and wrong IMHO.

The percentage of oxygen available at altitude decreases by about 10% per 1000 metres, so you are going to have roughly 14% less oxygen available to burn your methanol fuel at that altitude than sea level, so very approximately you are going to lose 14% of your total maximum power output.

Nitromethane when burnt in IDEAL conditions (i.e excess oxygen) gives just over twice the energy output as methanol per unit volume. it requires much less oxygen than the methanol to burn so more can be burnt per stroke of the engine ( around 8 times as much) but per unit volume it puts out much less energy (around 4 times less per unit mass or volume). So under these ideal conditions an engine running just on nitro would develop twice the power of one running just on methanol, but need 8 times the fuel input. This if course isnt possible with a conventional glow engine, as the methanol is required to keep the glow ignition working. As a percentage of the fuel, the higher the nitronumber the LOWER the total calorific value of the fuel.

cutting a long story short, and using some very simplified maths:

10% nitro 18% oil 72% methanol = power output of (20/4)+0+72 = 77
20% nitro 18% oil 62% methanol = power output of (40/4)+0+62 = 72
30% nitro 18% oil 52% methanol = power output of (60/4)+0+52 = 67

When the fuels are burnt in excess oxygen.

At sea level the engine are always running rich (excess fuel, too little oxygen)
At higher altitudes this is even more true though you tune the engine to put less fuel through. Increasing the nitromethane content will help as it will burn without oxygen present in the correct conditions, but it will not override the fact that you need to burn 4 times as much of the stuff to generate the same energy as you do from methanol.

The 20% possible increase in nitro content will not overcome the 14% drop in available oxygen concentration.

I await criticism and pulling apart of my maths imminently

James
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Old Oct 10, 2010, 08:15 AM
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United States, WA, Renton
Joined Jan 2009
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You will be fine. I fly at 6200 ft. It will fly it good enough for a trainer, as you get better you will want to add more power to your future planes. Get it and get itying then start watching RCG for a used .45 and put it on when ready.
Where I am at I like to over power it. Most of my .40-.45 size plane have .61 engines on them.
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Old Oct 10, 2010, 11:21 AM
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Keller, TX
Joined Aug 2005
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Quote:
Originally Posted by slingsbyskylar View Post
as i understand it couldnt he just use a higher %nitro to compensate for the lower O2 levels ? i.e. 15% instead of 5% or 20% instead of 10%

or am i mistaken

skylark
Super Tigre engines are designed for little or no nitro. The .40 is a fine engine, not a powerhouse, but certainly a good choice for the referenced application. Using more nitro than the design contemplates makes for more difficulty in tuning and reliability. Reliability is the #1 requirement for this use.

The difference in O2 levels at 4500 altitude is hardly a factor in performance for what we are talking about, here.

Yes, you are, IMO.
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Old Oct 10, 2010, 02:43 PM
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Fla.
Joined Apr 2005
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That combination should be fine for training. As far as the engine goes it should be OK as said above. If you are ordering from Tower, you might ask if you can up grade to an OS 46 for a small fee.
Unless you are racing or doing something high performance you have NO need for more then 10%. Why pay extra to heat up ( wear out ) your engine when you don't NEED it. Chances are you will ( should ) be flying at low RPM most of the time so why waste your money at this time. ENJOY !!! RED
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Old Oct 10, 2010, 03:38 PM
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Cedar City, Ut.
Joined Dec 2009
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The flying field that I use is at 5800 ft. alt. the fliers here use from 5% to 45% (pylon racers) nitro. I personally run 20% (YS four strokes) and run the same in my Super Tigre G-75. So I don't think nitro content is a major issue.

What I do see is proper prop selection as the more important issue.

The RPM range of the Super Tigre GS-40 engine on 5 to 15% nitro (recommended) is 3000 to 17,000 RPM and the best power is at 15,500 RPM then prop it for 15,500 RPM.

Usually at high altitude you will want to go about an inch smaller than the maximum diameter recommended for the engine and go for the pitch that will let you turn the best RPM or about 15,000 on the ground as the RPMs will pick up about 500 in the air.

Also going to a slightly larger engine won't hurt, and if you are flying in a dusty area I would recommend an air filter and a ringed engine as they are more durable than an ABC type engine under these conditions.

Happy flying, Oscar

A man that hates kids and dogs can't be all bad. But then, I've met some dogs that I liked.
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Old Oct 11, 2010, 05:22 PM
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United Kingdom, Oxford
Joined Feb 2003
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I was a t talk by Maynard Hill back in the 90s at the Chesapeake Bay RC club where he said that an ordinary glow engine worked right up to 27,000 feet with no special tuning, QED.

A.
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Old Oct 12, 2010, 10:03 AM
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winchester, UK
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Originally Posted by AndyOne View Post
I was a t talk by Maynard Hill back in the 90s at the Chesapeake Bay RC club where he said that an ordinary glow engine worked right up to 27,000 feet with no special tuning, QED.

A.
Well this guy is talking . At 27,000 ft there is only 1/3 of the oxygen available at sea level.

Not only would you have a considerable drop in output power, but the air/fuel ratio from the very simple carb would be way off.
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Old Oct 13, 2010, 07:53 AM
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He, the man with all the world records, reported that he did nothing special when achieving the altitude record when asked specifically about the lack of oxygen at such a height.

A.
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Old Oct 13, 2010, 11:49 AM
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winchester, UK
Joined Jul 2010
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Well if that was the case, why in 40 years has no-one surpassed his piston engined record for height?...

I could start with the problems with the glow plug, at air pressures of a third that on the ground, there would be issues with heat dissipation, they simply are not designed to work in that environment.

I could follow on with the issue of methanol viscosity at altitude. At 27,000 ft the temperature is somewhere between -30 (being generous) and -50 Celsius. Methanol has double the viscocity at this temperature compared with a gentle 10 degrees C on the ground. As a result the engine will run leaner as less fuel will be pulled passed the needle valve on each suction or vacuum stroke. This is one of the reasons glow engines need subtle adjustments in tuning each week at the field, the fuel viscosity has a big part in this. You only need to read the guys account of his teams transatlantic crossing to see the worry that the temperature change from day to night caused at a very converative 3,000 feet:

" I went to bed at roughly 10 p.m., fearful that the cool of night would increase the viscosity of the fuel, taking the engine from lean to dead."

This nicely follows on to the tolerances on the carb and icing (not the nice kind on cakes). Given the more extreme difference in temperature between the air and the cylinder there will be a greater gradient across the carb, and tolerances will be off. Anyone (including me) with a crappy MDS carb will tell you just how important these tolerances are. If you've ever flown a piston/prop plane at any altitude will be very pleased that your carb de-icer works, if it isnt working you had better hope that the carb melts nicely on your glide back down so that you at least have a chance or restarting her

I suppose it depends by what you mean with the terminology 'worked' in your quote. If you mean barely runs with a poor power output for displacement, no real throttle response and poor reliability then yes I suppose you could be right.

Another option of course is that his record still stands because no-one knows his secret. I certainly wouldn't go around telling everyone and their dog my secrets if I had a world record to my name, or it probably wouldnt stand for very long. Perhaps that is why it has stood for 40 years?
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Last edited by jimmyhorns; Oct 13, 2010 at 12:08 PM. Reason: forgot to include quote
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