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How does a gas engine act when hot?

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How does a gas engine act when hot?

Old 07-23-2013, 06:14 PM
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vertical grimmace
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Default How does a gas engine act when hot?

I am just wondering what problems arise when a gas engine overheats? I have a tight install, and want to be able to diagnose symptoms if this becomes an issue.
Old 07-23-2013, 07:02 PM
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speedracerntrixie
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Default RE: How does a gas engine act when hot?

The thing about gassers is that they will continue to run when overheating. The biggest tell tale sign would be sagging at high throttle settings. Truth is that you can run at close to 300 degrees and really not know it. You can always read the plug color but usually if you have run it hot long enough to do that it may already have some damage. Having muffler bolts constantly coming loose is a sign of overheating. If your worried about the install make sure you are directing air through the fins. Its always easier to pull air rather then push it so your exit area need to be at least twice the intake area. Don't wait until you have an issue, plan ahead so you don't have an issue. I'm currently rebuilding a 40% Extra that is a good example. notice I have 2 exits, one at the bottom of the cowl and one just behind the canisters.
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Old 07-23-2013, 07:35 PM
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Ernie Misner
 
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Default RE: How does a gas engine act when hot?

For sure if it sags on power on a long up line it is either too lean, getting hot, or both. I think if you do that often enough it might need a new ring and worse case scenerio it will actally seize up tight and wipe out the pison/cylinder in the process.
Old 07-23-2013, 07:43 PM
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Default RE: How does a gas engine act when hot?

Trixie is right spot on with his take on gas 2 strokes overheating. I have been racing, flying and using 2 strokes for many years. I have had few if any heating problems. In my airplanes I make sure I have adequate air intake and lots of exit area. They will tend to loose power when they get hot, and many times quit altogether. Running them rich can tend to mask this however. I think your main problem with your DR1 is going to be adequate exit area for good cooling. The two holes at the top of the cowl should let a good amount of air in.....
Old 07-23-2013, 08:07 PM
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vertical grimmace
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Default RE: How does a gas engine act when hot?

The engine on my DR1 is actually halfway between the firewall, as I had to box it back. I added a ply plate under the inlet hole on one side to keep the air flowing past the cylinder and not dumping out the bottom of the cowl. For exit air, it can go into the fuselage and out the cockpit as the plane is mostly open. I added cooling louvers on top of the fuselage where the head is as well.

This airplane is ready to fly now, and I have had some trouble with the engine. Today, I was going to fly it, but it just stopped running and would not restart. I had fuel and spark. It was running perfectly, but I think it got hot. I was doing a lot of testing on the ground. Range test, making sure needles were set, and taxi tests. Probably ran for 15 min on ground. I could not get it restarted at the field. When I got back to shop, it fired right up.
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Old 07-23-2013, 08:38 PM
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Default RE: How does a gas engine act when hot?

Is the equalization chamber plumbing to keep balanced air pressure to the carb?
Old 07-24-2013, 04:34 AM
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Default RE: How does a gas engine act when hot?


ORIGINAL: Ernie Misner

Is the equalization chamber plumbing to keep balanced air pressure to the carb?
I am not exactly sure which part you are talking about, but the tube you see is a flexible exhaust. It is mounted by elbows to a Bennett muffler with the stingers cuttoff
to lower the profile. This allows the exhaust to exit the bottom of the plane.
Old 07-24-2013, 06:57 AM
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Default RE: How does a gas engine act when hot?

Some of the DR1s had larger holes in the cowl, and, is thier any way you can open up the bottom at the area between the landing gear, to let more of the cooling air out? You might try running the engine without the cowl and see how it does. As an aside, have you got the CG set yet?
Old 07-24-2013, 08:51 AM
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Default RE: How does a gas engine act when hot?

Every gasser I had that got hot was during a maiden flight and then I'd have to tweak the needles to cool it down or install an air baffle. When mine would start to get hot it would start to mis and within one more lap it would die.
Old 07-24-2013, 11:16 AM
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Default RE: How does a gas engine act when hot?


ORIGINAL: speedracerntrixie

The thing about gassers is that they will continue to run when overheating. The biggest tell tale sign would be sagging at high throttle settings. Truth is that you can run at close to 300 degrees and really not know it. You can always read the plug color but usually if you have run it hot long enough to do that it may already have some damage. Having muffler bolts constantly coming loose is a sign of overheating. If your worried about the install make sure you are directing air through the fins. Its always easier to pull air rather then push it so your exit area need to be at least twice the intake area. Don't wait until you have an issue, plan ahead so you don't have an issue. I'm currently rebuilding a 40% Extra that is a good example. notice I have 2 exits, one at the bottom of the cowl and one just behind the canisters.
What I've been doing is adding a hood or dam in front of the exit holes to act as exhaust louvers. Outside Air deflects away from the hole and creates a negative pressure condition which in turn creates the "pull" air flow inside the cowl or fuse Shawn is talking about..

In addition for best results, the inlet side needs to have some sort of ducting (I have used depron, 1/32" balsa sheet, even cardboard) to guide incoming air directly over the fins. I pay attention to avoid large gaps between airframe or cowl and the engine crankcase. These large holes can reduce airflow through the fins because air will flow through this path of less resistance first. I also close off cheek holes unless there is a cylinder inside the cheek.

Good cooling is effected by both proper treatment of the incoming air (ducting) and proper treatment of the exiting air. BTW.....my exiting area is seldom if ever greater than 100% the entry area; the hooding and louvering of the exit holes is that effective
Old 07-24-2013, 02:28 PM
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Default RE: How does a gas engine act when hot?

Pe Rievers also has some good drawings of cowl baffles on his site
Old 07-24-2013, 02:44 PM
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Default RE: How does a gas engine act when hot?

I am going to try it now with the added baffle tomorrow. I think it will be fine, I am just wanting to know what to look out for. I think i did get hot, but I was doing excessive
running on the ground. I have a duct in middle wing to direct the air into the fuselage. THe one problem is with the side mount carb, adding a baffle there is an issue because of the need to access the carb to choke it. So for now I only have a baffle on one side of the engine. I will look and may add another before going out to fly again. I will fabricate a choke here eventually. Really a pain those carbs with no chokes!

Oh, the CG was perfect out of the gate. Most of my weight (rx batt, tank, servos etc) are right on the CG. That heavy old mag engine worked out great for that.
Old 07-24-2013, 03:02 PM
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Default RE: How does a gas engine act when hot?

After looking at your pictures and thinking about it a while I came to the conclusion that there really is no good way to baffle this installation. The two small intakes are just in the wrong place and the whole bottom of the cowl is open. I understand that you mounted the engine upright for scale appearences anf your plan is to install a scale dummy engine. That being said I think I may have a viable solution. IMO if it overheated at low power settings on the ground it will just be worse in flight. That Quadra already has a cooling fan on the front of it. All you need to do is fabricate the shroud. This would be an easy task using blue or pink insulation foam to carve a plug, cover with packing tape and lay up some glass over it. It works for whatever tool the engine was originally intended for, why not here?







Old 07-24-2013, 03:30 PM
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Default RE: How does a gas engine act when hot?

from what i am seeing you will constantly have overheating issues with that install. your best bet is to invert the engine, ya i know it will suck for the scale looks but a dr1 deadstick is NOT FUN!
Old 07-24-2013, 04:10 PM
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Default RE: How does a gas engine act when hot?

Try temporarily removing the bottom vent covers and fly it. They may be taking up hot air exit room.

Also I have owned 3 different brands of gas engines OS,Brison, and DLE. ALL of them warnageist prolong ground running because they over heat.

Old 07-24-2013, 04:20 PM
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Default RE: How does a gas engine act when hot?


ORIGINAL: Lightspeed1551

Ditch the bottom vent covers and fly it. They may be taking up hot air exit room.
If I understand the theory, those vent covers create a low pressure area which provides a rapid removal of air within the cowling and therefore is more effective in removing hot air than just an open area by itself. Is this not so?

Mike
Old 07-24-2013, 07:10 PM
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Default RE: How does a gas engine act when hot?


ORIGINAL: Lightspeed1551

Try temporarily removing the bottom vent covers and fly it. They may be taking up hot air exit room.

Also I have owned 3 different brands of gas engines OS,Brison, and DLE. ALL of them warnageist prolong ground running because they over heat.

The louvers are on the top of the plane. Just behind the engine.
Old 07-25-2013, 05:02 AM
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Default RE: How does a gas engine act when hot?

ORIGINAL: speedracerntrixie

After looking at your pictures and thinking about it a while I came to the conclusion that there really is no good way to baffle this installation. The two small intakes are just in the wrong place and the whole bottom of the cowl is open. I understand that you mounted the engine upright for scale appearences anf your plan is to install a scale dummy engine. That being said I think I may have a viable solution. IMO if it overheated at low power settings on the ground it will just be worse in flight. That Quadra already has a cooling fan on the front of it. All you need to do is fabricate the shroud. This would be an easy task using blue or pink insulation foam to carve a plug, cover with packing tape and lay up some glass over it. It works for whatever tool the engine was originally intended for, why not here?
Yes, vertical grimmace, what happened to your engine are the symptoms of overheating: the rings cannot expand anymore into their gaps and put tremendous force onto the cylinder wall, creating high friction and more heat, reason for which the engine cannot start again for a while.

As the quoted post explains, you need forced ventilation around those fins.

That is a slow airplane and air forced by its speed only goes from areas of high pressure to areas of low pressure.
If you were the air, would you go around those fins while taxing and flying?
If so why and how much?

When you engine is putting out 1 HP, it is generating the heat equivalent to 22 light bulbs of 100 watts each.
Not so much at idle, but that gives you an idea of how much cooling air these engines need.
Old 07-25-2013, 05:57 AM
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Default RE: How does a gas engine act when hot?

Where can one pick up those cooling louvers? Hardware stores don't seem to carry them any more? I know you can buy some nice shiny ones from RC suppliers, but they are quite expensive for 2 cents worth of pot metal.
Old 07-25-2013, 06:27 AM
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Default RE: How does a gas engine act when hot?

Make your own custom fitted louvers.1/64" birch ply works great. You really don't need much area to effectively cool any of the engines we use in our models currently. 2 sets of about 5 square inches each is plenty
Old 07-25-2013, 07:34 AM
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Default RE: How does a gas engine act when hot?

Those exhaust louvres work well when the plane is moving, but if you were stationary or slow taxiiing, there is no pressure reduction at the louvres. That might explain your overheating. You might have to have a secondary exhaust outlet that is just enough for cooling when the engine is idling until you get airborne. JMHO.

Beautiful plane, BTW.
Old 07-25-2013, 07:46 AM
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Default RE: How does a gas engine act when hot?


ORIGINAL: irocbsa

Where can one pick up those cooling louvers? Hardware stores don't seem to carry them any more? I know you can buy some nice shiny ones from RC suppliers, but they are quite expensive for 2 cents worth of pot metal.
http://www.taildraggerrc.com/giant-scale-hardware.html
Scroll down the page they have two sizes available.

Old 07-25-2013, 09:33 AM
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Default RE: How does a gas engine act when hot?

That's not too bad of a price. Thanks!
Old 07-25-2013, 10:44 AM
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Default RE: How does a gas engine act when hot?

sounds like the engin is getting hot enough to cook the gas out of the carb. Make a little scoop on the bottom of the cowl to vent the air up the front of the engin. Also you could make a disc that fits on the front of the cowl with that size hole, make the two holes larger and rotate the disc so the large holes are open when you fly. when you want scale you rotate the disc back to have the holes the correct size, or have the front of the cowl removeable for flight.It is obvious you have skillz and should be able to modify the cowl to help the heat. At flight time the model does not have to be scale especially if it leads to overheating issues. Not that i fly scale!
Old 07-25-2013, 11:27 AM
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MTK
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Default RE: How does a gas engine act when hot?

Louvered or hooded exits work as long as the prop is turning and makes air flow past them. Certainly higher air flow results in higher negative pressure production.

However the lower airflow at idle doesn't mean overheating...don't forget the engine is at or near idle which means less heat generation and lower cooling demand

If some don't understand what I mean by "hooding" you can see a few snaps at my Derivative thread in the Pattern Forum. On my latest piped DLE55 design, inlet area is around 7 squares directed onto the fins while exit area is only8 squares. There is NO issue with cooling no matter how extended the vertical in 90 degree environment; that's how effective this is. Try it, it's straight forward!!

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