YA P-38 Build
06-20-2014, 08:26 AM
#51
On my TF Corsair with a DLE 55RA I've run it with the dummy engine and w/o, with the stock cowl and with my cowl flap mod. In every condition the engine ran fine, didn't over heat. I had to make very small H needle adjustments between winter and summer and thats it. Not that its the same thing but its a stark contrast in cowl volume and how the engine responds to it.
On the P-38 cowls Im going cut them between the exhaust ports to make a slot and increase flow, for starters. Then the props as I said before and then we'll see whats going on. If need be Ill try the breather tube after that.
To give you a better idea of how sensitive the engines were acting yesterday just by turning the plane 90deg into the wind to start them we had better results in cooling alone.
On the P-38 cowls Im going cut them between the exhaust ports to make a slot and increase flow, for starters. Then the props as I said before and then we'll see whats going on. If need be Ill try the breather tube after that.
To give you a better idea of how sensitive the engines were acting yesterday just by turning the plane 90deg into the wind to start them we had better results in cooling alone.
Last edited by Chris Nicastro; 06-20-2014 at 08:36 AM.
06-20-2014, 09:26 AM
#52
My Feedback: (24)
Wow, that's pretty sensitive.
Another thought concerning baffles, if you make baffles, so air only passes over the engine fins, and maybe a little around the case. You are effectively reducing the the amount of air passing thru the cowl, or to look at it another way, reducing the volume of air that needs to exit. All the air that flows thru is right were it is needed for cooling, and may help you get closer to your 3 to 1 exhaust to inlet area. You want air to be sucked out while in flight. Since carbs are rear mount, and would be on the cooling air exit side, might help there too
With no baffles, air just packs into the cowl undirected, inefficient, and looking for a way out. Been in aviation a long time, directing and controlling cooling air is a big deal. We usually get lucky on our models, but when things get tight/hot, always good to see how's it done full scale
https://www.google.com/search?q=engi...pme9IitUwSM%3A
Another thought concerning baffles, if you make baffles, so air only passes over the engine fins, and maybe a little around the case. You are effectively reducing the the amount of air passing thru the cowl, or to look at it another way, reducing the volume of air that needs to exit. All the air that flows thru is right were it is needed for cooling, and may help you get closer to your 3 to 1 exhaust to inlet area. You want air to be sucked out while in flight. Since carbs are rear mount, and would be on the cooling air exit side, might help there too
With no baffles, air just packs into the cowl undirected, inefficient, and looking for a way out. Been in aviation a long time, directing and controlling cooling air is a big deal. We usually get lucky on our models, but when things get tight/hot, always good to see how's it done full scale
https://www.google.com/search?q=engi...pme9IitUwSM%3A
06-21-2014, 03:39 AM
#56
There are no additional holes in the cowls to help with cooling.
The rivets where done with a soldering iron and various sizes of brass tubing sharpened and the burnt in to the paint. then the whole plane was weathered with different colors of artist chalk to help highlight them and also the add some weathering to the plane. This was about a 2month process.
Last edited by Greg Wright; 06-21-2014 at 03:41 AM.
06-21-2014, 06:58 PM
#57
My Feedback: (24)
Hey Chris
As a sanity check, ran some quick numbers on CG, just to make sense of the different CG locations. Both the 3/4" fwd of the front spar with gear down, and/or right at the front spar with gear up make sense to me. MAC is located slightly outboard of where wings plug in.
25% MAC would be 3.42 aft of LE, 33% would be 4.52" aft of LE. The middle of the aluminum spar (on my plane) is roughly 4.32, or 31.5% MAC
I would imagine if you balanced it at fwd spars as Greg and Travis said with gear up, it would be close to the 25% mark with gear down.
Would think 3/4" fwd of spar with gear up would make it pretty nose heavy. Maybe that's what they want so you don't slow it down too much and stall it??
As a sanity check, ran some quick numbers on CG, just to make sense of the different CG locations. Both the 3/4" fwd of the front spar with gear down, and/or right at the front spar with gear up make sense to me. MAC is located slightly outboard of where wings plug in.
25% MAC would be 3.42 aft of LE, 33% would be 4.52" aft of LE. The middle of the aluminum spar (on my plane) is roughly 4.32, or 31.5% MAC
I would imagine if you balanced it at fwd spars as Greg and Travis said with gear up, it would be close to the 25% mark with gear down.
Would think 3/4" fwd of spar with gear up would make it pretty nose heavy. Maybe that's what they want so you don't slow it down too much and stall it??
Last edited by 70 ragtop; 06-21-2014 at 07:33 PM.
06-21-2014, 10:03 PM
#58
Great thanks for the numbers. I talked it over with the owner today and Ill be stripping the lead out and doing the CG over at the front spar.
Pretty sure YA's setting is a CYA CG because the plane would sure be nose heavy. I was thinking it would run out of elevator range and would necessitate a fast landing. They even state to take off at high speed.
So today we figured out the engines. The 19X8 props and retune did the trick. I took apart the carbs and decked the intake manifold along with deburring the reed block. What we discovered was that the carbs can easily vapor lock as soon as the temps get up in the 180F+ range. Not sure if all that was absolutely necessary but one reed block had a very small leak.
I ran the engines w/o cowls and set the needles. Strangely the L needle had to be 2 turns out while the stock setting is 1.1. The H needle stayed around 1.5 - 1.75 turns out. I got the engines in sync and the temps were very close.
Put the cowls back on and ran the engines again with some minor needle tweaks.
Taxi test with the brakes was completed. Pretty cool and fun to get used to. These engines are very strong so I had to see how much power I could run up with the brakes on. Not much above idle and you could see the plane straining against the brakes and just before dragging the plane I released them. They work great and I was able to use them a lot before bleeding out most of the air.
I didnt have to cut the cowl so Im pleased about that. Im going add baffeling though to direct the air to the cylinder better. Id like to run some form of intake ducting for the carbs so they get cold air. That will increase reliability so the carb body doesnt heatsink so much. Id even like to see a finned carb body.
This plane sounds like a top fueler as it taxis, lol, love it, its pretty cool.
Pretty sure YA's setting is a CYA CG because the plane would sure be nose heavy. I was thinking it would run out of elevator range and would necessitate a fast landing. They even state to take off at high speed.
So today we figured out the engines. The 19X8 props and retune did the trick. I took apart the carbs and decked the intake manifold along with deburring the reed block. What we discovered was that the carbs can easily vapor lock as soon as the temps get up in the 180F+ range. Not sure if all that was absolutely necessary but one reed block had a very small leak.
I ran the engines w/o cowls and set the needles. Strangely the L needle had to be 2 turns out while the stock setting is 1.1. The H needle stayed around 1.5 - 1.75 turns out. I got the engines in sync and the temps were very close.
Put the cowls back on and ran the engines again with some minor needle tweaks.
Taxi test with the brakes was completed. Pretty cool and fun to get used to. These engines are very strong so I had to see how much power I could run up with the brakes on. Not much above idle and you could see the plane straining against the brakes and just before dragging the plane I released them. They work great and I was able to use them a lot before bleeding out most of the air.
I didnt have to cut the cowl so Im pleased about that. Im going add baffeling though to direct the air to the cylinder better. Id like to run some form of intake ducting for the carbs so they get cold air. That will increase reliability so the carb body doesnt heatsink so much. Id even like to see a finned carb body.
This plane sounds like a top fueler as it taxis, lol, love it, its pretty cool.
06-22-2014, 08:26 PM
#60
My Feedback: (24)
Chris
I will snap a couple pictures, but really nothing to see
I little info on cowling baffling and airflow...ran across this on one of the full size forums
"The bigger hassle with trying to use the ram air is the pulsating flow off the propeller. It creates pulsating pressures in the carb, which causes a mess in terms of air/fuel mixture homogenization and distribution. I have run up some airplanes that act up and run roughly with the cowling off and the airbox wide-open to the prop blast; as soon as the cowl is put back on and the engine is permitted to draw whatever air it needs from the flow running mostly past the filter, it smooths out and runs fine. The older Cessna 182s were famous for such stuff. Old guys have known about this for years. It's why engineers mostly angle the air filter so that it's more or less parallel to the cowling surface, recessed a bit so it doesn't generate a lower static pressure, and why many aircraft use a NACA duct on the side of the cowl. It doesn't generate any significant ram pressure and keeps the airflow smooth.
Look at pictures of fast production airplanes and see how they arrange the engine air intake. See how few have anything sticking out into the slipstream to create ram effect. Those engineers know that the drag created by any such affair would cost far more than any ram recovery would be worth.
The other thing most important about intake openings in the lower cowl: You DON'T want extra air coming around that filter into the lower cowling "to help cool the engine." The whole cowling and baffling setup is designed to create a positive pressure above the engine and a negative pressure below it, to cause air to flow in a controlled manner past the cooling fins. Allowing air into the lower cowl around anything mounted in it or extending through it will reduce the pressure differential and create overheating problems. The Supercub is a classic example of a poorly designed cooling system, with many of them having a huge hole in front of an oil cooler right below the propeller, and another big leaky scoop affair for the induction air. Piper added extra air outlets and deflectors to the air exits to try to improve it, but the real solution is to design it properly in the first place.
Look at this picture and figure out what happens if we let extra air get in under the engine, or if those baffles start leaking."
I will snap a couple pictures, but really nothing to see
I little info on cowling baffling and airflow...ran across this on one of the full size forums
"The bigger hassle with trying to use the ram air is the pulsating flow off the propeller. It creates pulsating pressures in the carb, which causes a mess in terms of air/fuel mixture homogenization and distribution. I have run up some airplanes that act up and run roughly with the cowling off and the airbox wide-open to the prop blast; as soon as the cowl is put back on and the engine is permitted to draw whatever air it needs from the flow running mostly past the filter, it smooths out and runs fine. The older Cessna 182s were famous for such stuff. Old guys have known about this for years. It's why engineers mostly angle the air filter so that it's more or less parallel to the cowling surface, recessed a bit so it doesn't generate a lower static pressure, and why many aircraft use a NACA duct on the side of the cowl. It doesn't generate any significant ram pressure and keeps the airflow smooth.
Look at pictures of fast production airplanes and see how they arrange the engine air intake. See how few have anything sticking out into the slipstream to create ram effect. Those engineers know that the drag created by any such affair would cost far more than any ram recovery would be worth.
The other thing most important about intake openings in the lower cowl: You DON'T want extra air coming around that filter into the lower cowling "to help cool the engine." The whole cowling and baffling setup is designed to create a positive pressure above the engine and a negative pressure below it, to cause air to flow in a controlled manner past the cooling fins. Allowing air into the lower cowl around anything mounted in it or extending through it will reduce the pressure differential and create overheating problems. The Supercub is a classic example of a poorly designed cooling system, with many of them having a huge hole in front of an oil cooler right below the propeller, and another big leaky scoop affair for the induction air. Piper added extra air outlets and deflectors to the air exits to try to improve it, but the real solution is to design it properly in the first place.
Look at this picture and figure out what happens if we let extra air get in under the engine, or if those baffles start leaking."
06-30-2014, 02:45 PM
#62
Nice details, whats the story with your plane, was it damaged?
Were on vacation until the 8th so the week of Im going to be making some updates. Im making a baffle for the cowls and then I want to make intakes that pass thru the fire wall. Lastly Im making vent lines for the carbs diaphram also passing thru the fire wall. This way the carb always has clean cooler air and the diaphram as well. I'll also add a screen in front of the intakes for debris.
We got the engines to work but Im adding insurance since were still not totally sure yet.
Were on vacation until the 8th so the week of Im going to be making some updates. Im making a baffle for the cowls and then I want to make intakes that pass thru the fire wall. Lastly Im making vent lines for the carbs diaphram also passing thru the fire wall. This way the carb always has clean cooler air and the diaphram as well. I'll also add a screen in front of the intakes for debris.
We got the engines to work but Im adding insurance since were still not totally sure yet.
Last edited by Chris Nicastro; 06-30-2014 at 02:53 PM.
06-30-2014, 03:08 PM
#63
My Feedback: (24)
Yes, mine was pretty much destroyed on its maiden by previous owner....flat spin all the way to the ground. DA-50s, having trouble with one overheating, and it quit on the first circuit, rolled over and that was it. There's a video of the crash, and a bunch of pix of the rebuild on the P-38 brotherhood thread. I bought the retracts, and the previous owner threw in the plane...cut the booms in half, and shipped it by bus. Its a little bit of a project
Sounds like you have a good plan. Looking forward to seeing more progress pictures, and some flight videos!
Sounds like you have a good plan. Looking forward to seeing more progress pictures, and some flight videos!
Last edited by 70 ragtop; 06-30-2014 at 09:47 PM.
08-06-2014, 10:45 PM
#64
Some updates on the build.
Removed the engines and installed nice large velocity stacks which should help as heatsinks and to draw fresh air from the next compartment.
Next I added sheet metal heat shields to block radiating heat from the exhaust on to the carb.
The baffle templates have been made and tomorrow Im cutting two sets and beginning to epoxy them to the cowls. Im going to use a very thin contest ply.
Removed the engines and installed nice large velocity stacks which should help as heatsinks and to draw fresh air from the next compartment.
Next I added sheet metal heat shields to block radiating heat from the exhaust on to the carb.
The baffle templates have been made and tomorrow Im cutting two sets and beginning to epoxy them to the cowls. Im going to use a very thin contest ply.
08-20-2014, 08:23 PM
#75
Tackled the machine gun ports. Used carbon tube and installed them pretty straight line of sight.
The hard part was sizing or scaling the gun barrels. I decided on a proportion 1.5X the caliber to get close to the outside dia of the barrel cooling tube. Compared to photos of the full scale it seems to be about right. The ports vary by plane model too so as of now Ive seen 3 different ways they made these ports in the nose.
The gun barrels are going to be made from brass tube and the cool part will be the 20mm cannon. The tube has an 8/32 bolt soldered to it and its going to retain the nose cone. To remove the nose you just loosen the cannon.