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RE: Aerodynamic considerations for a speed plane design - 3/31/2008 5:21:01 AM   
combatpigg



Posts: 9783
Joined: 11/22/2003
From: arlington, WA, USA
Status: online 		Yes but in my case I'm working with a .40 sized piston with a half inch stroke changing direction 500 times per second. I'm not sure that the net result [in mph] of side mounting my rear exhaust engine would be significant, even if there was a couple hundred rpm difference.
Lowering drag has to take priority over a fractional HP gain.

< Message edited by combatpigg -- 3/31/2008 5:23:02 AM >

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RE: Aerodynamic considerations for a speed plane design - 3/31/2008 5:28:40 AM   
iron eagel



Posts: 1264
Joined: 7/15/2004
From: Middleboro, MA, USA
Status: online 		A couple hundred rpm is just that, more power=more speed, depends upon pitch also. But beyond that the prop is going to be spinning in a more perfect circle and that would also help some.
Plus you rear exhaust engine can be piped out through the fuselage, no muffler or pipe hanging out in the air stream to create drag.

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The Wrights never crashed, they only had hard landings. I 've had some hard landings myself. AMA EAA AOPA revver #185

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RE: Aerodynamic considerations for a speed plane design - 3/31/2008 5:47:23 AM   
combatpigg



Posts: 9783
Joined: 11/22/2003
From: arlington, WA, USA
Status: online 		I'm a little bit too backed up with projects to draw something up right now. If you know what the Nelson FAI engine looks like [pipe and all], side mounting it will leave you with one of two extreme outcomes.......either something very, very cool.......or something that is extremely dorky and hideous, ugly enough to make an onion cry

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RE: Aerodynamic considerations for a speed plane design - 3/31/2008 7:08:11 AM   
BMatthews



Posts: 8943
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		A BWB would be a good design to start with. But keep in mind that small frontal area is king. That means no thick wings and the blending should take place over a very small distance. Almost a double delta planform. Also if you're smart you'll find a ducted fan engine that has the power and RPM and combines that with a rear exhuast setup so the pipe can be in the wing itself. Some fancy composite center spar design will allow the pipe to pass through and the loads to be slipped around with a carbon fiber hoolahoop around the pipe and connect the spars on either side.

Also there's material out there that supports the idea that a wing to fuselage joint of 90 or more degrees in angle doesn't need a fillet. The Corsair took advantage of that finding and had no fillets at all.


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RE: Aerodynamic considerations for a speed plane design - 4/1/2008 5:20:38 AM   
iron eagel



Posts: 1264
Joined: 7/15/2004
From: Middleboro, MA, USA
Status: online 		Bruce,
A double delta planform sounds like a good idea to me.
Lets see the cheeks blend into chines that go into the main delta is what springs to mind here.
As far as a motor I have a brand new OS 91 VF-DF with a tuned pipe and remote mix valve think that would work?
I want to if possible keep the exhaust internal and just pipe it out so a composite spar was already in the back of my mind as far as the basic structure goes. Where I really am interested in the BWB concept the spars will probably be built into a series of formers that will make up the fuselage and wing root. The internal structure is going to take some time to design but should not be all that bad to build.
I would like to see if I can keep as much of the control setup and linkages, as possible, within the confines of the fuselage. That may take some jumping through hoops but not impossible.
I may start putting together some sketches soon, but I am in the middle of a build at the moment, and have to build a Senorita for my daughter for this spring. That will be pretty stock and a fast build, but I also have my oddball design I would like to at least get the ½ size test plane built for this year as well. So this speed plane may not actually hit the bench before mid summer but I can work on the design here and there.

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The Wrights never crashed, they only had hard landings. I 've had some hard landings myself. AMA EAA AOPA revver #185

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       Post #: 30

RE: Aerodynamic considerations for a speed plane design - 4/1/2008 6:40:10 AM   
combatpigg



Posts: 9783
Joined: 11/22/2003
From: arlington, WA, USA
Status: online 		High Plains is gonna put his slide rule away and go help someone else if you start talking deltas.
For any serious speed project, I've washed my hands of them. For all the effort that Croundy put into that beautiful monster delta of his, he wasn't rewarded with even as much speed as a typical FAI pylon model.

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RE: Aerodynamic considerations for a speed plane design - 4/1/2008 7:18:46 AM   
HighPlains


 

Posts: 2663
Joined: 3/2/2003
From: Over da rainbow, KS, USA
Status: offline 		I put my Pickett slip stick away years ago, for an HP 45. Damn thing cost me as more than my Kraft KP-4 SS radio. That's single stick, not spread spectrum.

Deltas are slow, but their saving grace is that they are easy to land. But if you are using gravity to add power, you lose efficency when you have to pull out.

If you look at the results of the German speed events from a couple of years ago, you will find that the pylon based 6.5cc powered model topped out at 208 mph. The 15cc only did about 226 mph with 2.3 times as much displacement. If the engine power output is somewhat proportional to displacement, then I would have expected at least double the power with the larger engine. That should have been enough to push a plane with simular drag to over 260 mph.

So you can conclude that either the larger engine just is not making the power, or the drag of the airframe was much higher than the racer.

The pylon ship has a very high aspect ratio wing, so it will maintain the dive speed better though the traps.

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RE: Aerodynamic considerations for a speed plane design - 4/2/2008 2:29:41 AM   
iron eagel



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Joined: 7/15/2004
From: Middleboro, MA, USA
Status: online 		I guess the induced drag of a delta is an issue then?

Still there has to be a better way than an over powered glider?

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The Wrights never crashed, they only had hard landings. I 've had some hard landings myself. AMA EAA AOPA revver #185

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       Post #: 33

RE: Aerodynamic considerations for a speed plane design - 4/2/2008 2:42:06 AM   
HighPlains


 

Posts: 2663
Joined: 3/2/2003
From: Over da rainbow, KS, USA
Status: offline 		Not really. At least when a pull-out from a dive is considered, induced drag becomes important. At the slow speeds that RC speed designs fly at, starting with a clean glider and making mods will get you there, while deltas and the like will be left in the dust.

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RE: Aerodynamic considerations for a speed plane design - 4/2/2008 3:00:03 AM   
combatpigg



Posts: 9783
Joined: 11/22/2003
From: arlington, WA, USA
Status: online 		Well, if the timing zone and the "pre-timing-after-the-dive" zone are long enough...it will take a moderately loaded and efficient wing to carry it through with the least amount of drag at the end. Last summer I ran the Nelson .40 on a fairly clean delta and you could really see the speed bleed off quickly...inspite of the plane weighing less than 4 pounds.
The fastest designs in the world didn't just happen, they have been a work in progress that has taken decades to evolve.
To break the mold just a little bit would be to build a side mounted speedster, then do what it takes to blend the engine into the wing.
My guess is that it would take twice the effort and money to build a .90 sized plane, I'll stay with the .40...I know it will fit in my Jeep.

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RE: Aerodynamic considerations for a speed plane design - 4/2/2008 3:56:52 AM   
BMatthews



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Joined: 10/4/2002
From: Burnaby, BC, CANADA
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quote:

ORIGINAL: iron eagel

I guess the induced drag of a delta is an issue then?

Still there has to be a better way than an over powered glider?


In turns yes it is. Pulling G with a delta produces a lot of drag

And have you noticed how all the latest full sized forumula 1 racers are getting away from the stubby Cassutt look and beginning to look a LOT like our model sized over powered hotliners? You can't buck City Hall, the IRS or Physics. A fast glider is going to be the fastest model overall regardless of how you slice and dice it. It may be different if the speed of sound was scalable but for each viscosity of liquid there's an ideal planform for going fast. And for air it ends up being a higher apect ratio wing on a fuselage that's only just barely able to contain the engine and the equipment. So really an overpowered glider is a very good option.

Another option might be a blended body swept flying wing. You get the aspect ratio along with potentially a very clean design. Using symetrical sections means you don't need very much twist for pitch stability at all. A thicker root section that holds the engine and gear and that quickly blends to a swept flying wing with tip fins. The required washout in such a wing combined with the tip fins ensures that the usual vortex formation at the wing tips will be minimized at the same time you're obtaining your pitch stability. And that way you also get your sexy wing fairings.

Sort of like this? Span 54 inches wing area around 500 sq inches speed cowl set up to hopefully hold a honkin' DF engine in the .60 to .90 range or similarly a control line speed engine of the same size. Pipe is buried in the flare of the wing and exits at the trailing edge. Airfoil would be one that promotes laminar flow for a longer portion of the chord to help minimize drag. Normally such airfoils don't work for us but the Reynolds numbers on this one should be good enough to support such a choice. Some number crunching would be needed to find out and some testing before using this. The wing would be made using a double skin with something like a structural foam core to produce upper and lower shells and the molds would be made using CNC machining to ensure total airfoil accuracy to a high degree. The tip fins and curved joiners would be added on after. As shown the wing starts out at 1 inch thick. That's about 8% as it sits now. I'd want to do some testing and figuring to see if there's gains to be had going to more of a 6% section. No point in going any thinner as then flutter may rear it's ugly head.

Whatcha all think?



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Free Flighters go that extra mile........

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       Post #: 36

RE: Aerodynamic considerations for a speed plane design - 4/2/2008 4:01:56 AM   
iron eagel



Posts: 1264
Joined: 7/15/2004
From: Middleboro, MA, USA
Status: online 		The reason for the big engine is that I have it...
Yes the airframe would be larger but you have more room to work with at least inside the fuselage.

It looks as if I have a "lot" to learn as far as model speed airplanes.

So other than deltas what other type of forms have been tried?

Earlier, I was asking about different types of full scale design concepts, and wonder how they might be applied to models.

_____________________________

The Wrights never crashed, they only had hard landings. I 've had some hard landings myself. AMA EAA AOPA revver #185

(in reply to combatpigg)
       Post #: 37

RE: Aerodynamic considerations for a speed plane design - 4/2/2008 4:23:35 AM   
iron eagel



Posts: 1264
Joined: 7/15/2004
From: Middleboro, MA, USA
Status: online
quote:

ORIGINAL: BMatthews


quote:

ORIGINAL: iron eagel

I guess the induced drag of a delta is an issue then?

Still there has to be a better way than an over powered glider?


In turns yes it is. Pulling G with a delta produces a lot of drag

And have you noticed how all the latest full sized forumula 1 racers are getting away from the stubby Cassutt look and beginning to look a LOT like our model sized over powered hotliners? You can't buck City Hall, the IRS or Physics. A fast glider is going to be the fastest model overall regardless of how you slice and dice it. It may be different if the speed of sound was scalable but for each viscosity of liquid there's an ideal planform for going fast. And for air it ends up being a higher apect ratio wing on a fuselage that's only just barely able to contain the engine and the equipment. So really an overpowered glider is a very good option.

Another option might be a blended body swept flying wing. You get the aspect ratio along with potentially a very clean design. Using symetrical sections means you don't need very much twist for pitch stability at all. A thicker root section that holds the engine and gear and that quickly blends to a swept flying wing with tip fins. The required washout in such a wing combined with the tip fins ensures that the usual vortex formation at the wing tips will be minimized at the same time you're obtaining your pitch stability. And that way you also get your sexy wing fairings.

Sort of like this? Span 54 inches wing area around 500 sq inches speed cowl set up to hopefully hold a honkin' DF engine in the .60 to .90 range or similarly a control line speed engine of the same size. Pipe is buried in the flare of the wing and exits at the trailing edge. Airfoil would be one that promotes laminar flow for a longer portion of the chord to help minimize drag. Normally such airfoils don't work for us but the Reynolds numbers on this one should be good enough to support such a choice. Some number crunching would be needed to find out and some testing before using this. The wing would be made using a double skin with something like a structural foam core to produce upper and lower shells and the molds would be made using CNC machining to ensure total airfoil accuracy to a high degree. The tip fins and curved joiners would be added on after. As shown the wing starts out at 1 inch thick. That's about 8% as it sits now. I'd want to do some testing and figuring to see if there's gains to be had going to more of a 6% section. No point in going any thinner as then flutter may rear it's ugly head.

Whatcha all think?



Bruce,
I like it, but I do not know all that much when it comes to models. That looks something like a full size aircraft that is in development now for GA, it is called the Atlantica by Wingco.
Isn't a flying wing is supposed to fairly low drag?

I would like to ask a question, why was something like a canard dismissed so quickly?
In GA they are amongst some of the quickest piston powered airplanes, sometimes called a poor mans jet. One airplane that I would offer as an example is the Velocity quick and long range for 70 gal of fuel.



_____________________________

The Wrights never crashed, they only had hard landings. I 've had some hard landings myself. AMA EAA AOPA revver #185

(in reply to BMatthews)
       Post #: 38

RE: Aerodynamic considerations for a speed plane design - 4/2/2008 4:38:36 AM   
combatpigg



Posts: 9783
Joined: 11/22/2003
From: arlington, WA, USA
Status: online 		That's a great looking drawing, Bruce! I'm clicking and saving that one...uh...just in case your computer crashes.

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Led Zeppelin is NOT "old fogie" music.

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