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Warbird Downthrust

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Old 02-26-2013, 01:36 PM
  #1
Me410
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Default Warbird Downthrust

Working on my TF GS Corsair on and off for the last three years and I am almost done. After all painting and panel lines, I saw that my cowl mountingwas not perfect, the cowl was alittle pointed upward. I changed the cowl mounts and now the cowl is perfect.
Stupid me, the engine mount holes have long been done and aligned to the old cowl position. Now my engine does not line up through the center of the dummy radial in the cowl. The engine hub sits alittle high.

A little bit of downthrust will center the hub.

The Hellcat is a great flying model and seems to have a fair bit of downthrust.
Would it be a big mistake to put downthrust in a Corsair?

I know downthrust is usually for high wing planes where the drag from the high wing will want to pitch the plane up if there is no downthrust.

Yet the low wing Hellcat has downthrust.

Any help on this guys?
thanks
Tom
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Old 02-26-2013, 02:27 PM
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da Rock
 
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Default RE: Warbird Downthrust

The TF 60 ARF doesn't appear to have any. That's not positive proof since we'd have to compare the wings AOIs too.

Why not just move the motor mount. Last time I did it, it took about a half hour.
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Old 02-26-2013, 03:02 PM
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Me410
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Default RE: Warbird Downthrust

I can't change anything on the actual motor firewall and mount holes. At least not without a ton of work. I made the firewall very heavy duty using a one inch thick piece of hardwood plus extra plywood, big blind nuts all epoxy in on the back side of the thick firewall. All super heavy i know, but I knew I would need extra nose wieght, and turns out I still need to add about a LB of lead. The heavy firewall will also help to absorb some engine vibration from the G62.

Alittle downthrust could be accomplished with two washers.

I suspect the incedence of the wing has alot to do with the need for downthrust on a warbird?
More pos incedence, more downthrust ????

Don't know
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Old 02-26-2013, 03:17 PM
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Default RE: Warbird Downthrust


Quote:
ORIGINAL: Me410

I can't change anything on the actual motor firewall and mount holes. At least not without a ton of work. I made the firewall very heavy duty using a one inch thick piece of hardwood plus extra plywood, big blind nuts all epoxy in on the back side of the thick firewall. All super heavy i know, but I knew I would need extra nose wieght, and turns out I still need to add about a LB of lead. The heavy firewall will also help to absorb some engine vibration from the G62.

Alittle downthrust could be accomplished with two washers.

I suspect the incedence of the wing has alot to do with the need for downthrust on a warbird?
More pos incedence, more downthrust ????

Don't know

Sounds like a strong firewall. lol

Are you really going to use just washers? If you are worried about vibration, and it sounds like it's a sensible worry, it might be a good idea to accomplish it with something substantial.

As for the AOI being a reason for downthrust, the usual culprit is pitch moments. Anything above or below the wing generates the moments and both the Corsair and Hellcat are truly low wing planes. The Corsair's wing makes it definitely a low winger. As for more positive incidence being a problem, in flight the wing assumes whatever AOA it needs. Everything else follows. Since your model is going to be significantly heavy, the wing will need more AOA at all speeds. That's going to point the nose up more than if the plane was design weight. So yeah, it really looks like downthrust is on the menu.
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Old 02-26-2013, 03:29 PM
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Default RE: Warbird Downthrust

Look at your plans....   design downthrust should be shown if there is any in the design.   Prop Certerline to fuse centerline.
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Old 02-26-2013, 04:32 PM
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Default RE: Warbird Downthrust

Here is a quote from a full-scale Corsair article I just read on the internet.....and if it's on the internet it must be true!

"Also, the engine thrust has been increased by +2 degrees (nose down) for better flight characteristics."
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Old 02-27-2013, 09:16 AM
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Default RE: Warbird Downthrust

Hi da Rock: "Are you really going to use just washers" No, I just said that to show how downthrust is a easy fix to my problem. If I do put in downthrust I will make a tapered shim so all of the metal mount flange of the motor sits flush to the firewall.

" Since your model is going to be significantly heavy " Actually , I thought I did very good to get this kit at 27 pounds all up flying wieght ( without fuel ). This plane has full detail, fiberglass, paint, panel lines, full cockpit, full pilot, etc. So for a TopFlite kit I think the wieght is ok. Sure, an ARF would be much lighter and a TF kit that was just Monocoat covered would be lighter, but I am used to the higher wing loading.

So it sounds like you think downthrust would be ok. Thanks for your reply
I will make the shim removable so I could try the plane both ways.

Tevans55 ( EMS 72" Spitfire buddy )
Hi, thanks for the note on the 2+ degrees down thrust.

ThunderBoat42: The plans show no downthrust.


I may just experiment, although I would like to hear more about the subject.

Tom
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Old 02-27-2013, 10:29 AM
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Default RE: Warbird Downthrust

In my experience a couple of degrees of downthrust is always a good idea.
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Old 02-27-2013, 12:04 PM
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Default RE: Warbird Downthrust

I would add the downthrust and test fly it before I went out of my way to make other, more difficult changes. My Hellcat has 3 degrees of down thrust which had me worried initially but it flies great and I've not noticed any negative effects at all. My .02 cents.
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Old 03-02-2013, 09:54 AM
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Default RE: Warbird Downthrust

I have made a shim and I will try the downthrust on the maiden flight, that may be this month if weather amd field conditions are good.

I need to try to measure how many degrees this shim wil give me, two degrees is what I hope to get.

I will post the results.

Glad to hear your Hellcat flies great Chad.

Tom
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Old 03-02-2013, 10:24 AM
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Chad Veich
 
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Default RE: Warbird Downthrust


Quote:
ORIGINAL: Me410

I have made a shim and I will try the downthrust on the maiden flight, that may be this month if weather amd field conditions are good.

I need to try to measure how many degrees this shim wil give me, two degrees is what I hope to get.

I will post the results.

Glad to hear your Hellcat flies great Chad.

Tom
Good luck with it Tom. Unless you end up with rediculous amounts of downthrust I don't think you will have any problems. Certainly not any that will jeopardize the airplane during test flights. I had a Sea Fury which had excessive downthrust and the only time it was a problem was in a go-around situation as it would pull the nose down when full power was applied. Had to be quick on the up elevator if you were very close to the ground to prevent it from skipping the main gear. However, it was probably the best point rolling warbird I've ever flown and I've often wondered if the excessive down thrust wasn't one of the reasons for that!
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Old 03-04-2013, 11:23 PM
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Default RE: Warbird Downthrust

I would say add the down thrust. The TF GS Corsair I bought used last year had it about 3 degrees built into the engine mount and it flew great. When I mounted the engine in my TF 60 size Corsair I added a couple degrees there and same thing. Maybe I'm just used to having it from the planes I learned on. I taught myself to fly with the Parkzone electric warbirds and they all have down/right thrust. Maybe I have just got used to it being there, but now when I mount the engine or motor I always make sure it has some of both and things usually work out pretty well.
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Old 03-05-2013, 06:10 AM
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Default RE: Warbird Downthrust

Here is a real low wing T 28 trainer and check out the amount of downthrust it has. Paul
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