ckombo

My Feedback: (2)

So, I am new to the speed-plane world, so please bear with me on my limited understanding of the physics behind what makes 1 aircraft faster than another.

Basically, i have a .75 OS 2 stroke that i will be putting in on one of the following planes running a 10x10 prop or an 11x10. The planes are:

1. Sundowner 50
-63" wingspan
-7lbs
-Thin airfoil

2. Phoenix Extra 300 (40-size)
-58" wingspan
-6lbs
-Thicker airfoil

Can you all help me understand which plane would ultimately fly faster? Even though the Extra has a larger airfoil, does the fact that 5" shorter wingspan make up for it in drag coefficient?

Perhaps a Jackal would be a better "speed" plane, but i jsut dont think i can fit the .75AX in it...

Pylonracr

I see a few people have read this and no one has responded, so I will begin with a very basic answer to get people thinking. The basic physics of the situation is this.

Speed = thrust - drag.
The thrust is fairly easy on a basic level, prop efficiency, rpm unload in the air, prop blade airfoil at any given speed, etc. all contribute to thrust.
Drag, on the other hand, gets complex quickly. Static drag is caused by direct wind resistance, ie, Frontal area, airfoil thickness, cylinder head in the airstream, etc.

Induced drag gets involved in a hurry. This is resistance caused by air vortices at the wing tips, drag caused by turbulant air behind wing trailing edges,canopies,any protrusion into the airstream(this drag is where drafts come from in auto racing). In short, anything that can disrupt laminar airflow by causing a boundry layer seperation makes induced drag.

Oversimplified - Thin, skinny airplanes shaped like bullets go fast. Thick airplanes slow down on downlines and usually make pretty good sport fliers.

Hope some of this helps.

Scott Mooney
Scott Mooney Racing Engines

Crazy4Flight

My Feedback: (540)

Bob 268 pounds

Pylonracr

Nice one BOB.
I had to read the header a couple of times before I got it.

airraptor

My Feedback: (66)

bob i was going to do the same thing.


175lbs before a contest.

here are my thoughts which 98% will agree with.

Sundowner you should stay with a 46 size case engine ie: 40-55 AX size engines. the 75 will be to heavy, carry to much fuel and not be any faster than the proper sized engine.

Your 75 would do much better in a GP revolver type plane or world models mustangs.

speedracerntrixie

My Feedback: (29)

I agree here. The OS 75 is pretty much a dog engine. It's best installed in a .60 size Extra, Edge or Yak. A few of the locals have a WM Mustang with a 75 ( We have a race class for this ) and my .45 powered Macchi eats them alive lol.


The Sundowner with an OS 55, Rossi 53 or Jett 56 ( I think it's a 56 ) would give you maybe 110. The easiest way to go fast ( 150ish ) is with a WM Mustang, Kyosho Spitfire or VQ Toni with a YS 115. Mods are required to the wing center, stab and firewall.


Check the vid http://www.youtube.com/watch?v=f12ir...mbedded#at=214

ckombo

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Thanks for the info guys.

Also, i disagree with the comment that the 75AX will make the plane no faster than a 55AX. Iactually already installed it in the EXTRA and i only needed 1 ounce on the tail to balance it. Its really not THAT heavy.

If its all about Thrust - Drag, than i just dont see how the 75AX will be "no faster" than a 55....but what do i know...still learning i guess.

What is it about a Mustang that makes it so fast? Isn't it gennerally a 'heavier' plane than others? Yet i see a lot of racers using the mustang...

Nice video, how do i learn more about those mods?

airraptor

My Feedback: (66)

in your extra yes you may have gained more speed. As other have said you need more thrust than drag. Your extra is a draggy plane and the 55 can only spin max size prop of 12x4-12x5 the 75 can go to a 12x6-12x9 prop.

Now for the Sundowner the 55 AX is 18.5 ounces and the 75 is 26.5 ounces. This is 8 ounces in the nose of that plane. add in the larger engine mount, and more fuel and you are looking closer to 12 ounces more foward of the CG. you will need around 4 ounces in the tail to balance this. You have just added 16 ounces or one pound to this plane with no other mods. yes the plane will be faster in a dive but then may have flutter or other problems. On the straight and level you will have to fly at a higher angle of attack to carry that extra weight and no you are just as fast as the lighter plane with a 55 AX in it. add to this the faster landing speed no you dont have as near a fun plane to fly...........

You can get the sundowners down to 5 lbs easily.

The mustang is set up so we can run inverted engines. this allows us to fully cowl the engine and have a streamlined fuselage. The world models have around 600 sq inches i think so they can carry more weight and not be affected as much.

On that 75 your bought the 95 AX is just a tad lighter and has two more mm in stroke which means alot more power in the same size package.

daven

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to answer your question, which will be faster?

I don't think you would see much difference for either to be honest, both will top out around 100 mph.

speedracerntrixie

My Feedback: (29)

I doubt the Extra would see 100. If done right any of the scale aerobatic airplanes are all drag. This is of course done intentionally so they won't gain alot of speed in a dive. The 75 is well suited in this kind of airplane as it will develope peak power fairly low on the RPM scale. Throw a 14X6 on it and practice some aerobatics. If you are serious about a speed plane you need to get the right combination of airplane, engine and prop.