Fully symetrical airfoil selections for a 300MPH 90" Turbine???

[Admiral_Red]

I am assisting a friend in a new venture, the jet will be 105" in length with a 90" wing. All up weight will be 39lbs. Targeted wing area to be about 1400 sq/in. I am looking for airfoil suggestions to put it into the 300mph range that are fully symetrical. Can some of the more aeronautically inclined fellows give me some suggestions on airfoils?

Thanks,
James

[Ollie]

With an average wing chord of over 15 inches, the reynolds number of the wing at 300 MPH will be in the neighborhood of 3.6 million. That means that you can take advantage of NACA 6 series laminar flow airfoils if you are willing to contour the airfoil accurately and make the surface glass smooth and wave free. No rain drops, bug splatters or paint line discontinuities allowed. These airfoils will give you a 50 to 60% reduction in profile drag in their low drag bucket. They will have about a 60 to 80 % reduction in profile drag compared to the same airfoil with standard surface roughness. That means you can go at least 10% faster for the same thrust if the parasitic drag is no more than 50% of the total drag. If the parasitic drag were zero, you could go 22% faster.

Which airfoil to choose depends on whether you plan to use flaps to slow the landing and takeoff speed. It also depends on how thick you need to make the airfoil to contain a spar of adequate height. Something like an NACA66-015 or 66-018 might be appropriate with flaps. Without flaps a mean line cambered airfoil might be necessary rather than a symmetrical section.

A plane of this size could develop something on the order of 2700 pounds of lift force if the airfoil was suddenly forced to its maximum lift coefficient by a vertical gust or an inadvertant elevator input at 300 MPH. That much lift can put serious requirements on the bending strength and stiffness of the wing spar. If the wing is swept back the torsional loads can also be huge and washout will be bent into the wing by the bending loads due to lift even if the moment coefficient of the airfoil is zero. A swept back wing has to be extra stiff in both torsion and bending.

At 300 MPH the kenetic energy is more than an order of magnitude greater than the most powerful rifle bullet. You are contemplating playing with energies that demand a professional level of structural engineering for safety or a deserted desert proving ground and thousands of dollars to cover the cost of a spectacular crash. At 300 MPH even a model this big will be beyond visual control range in less than 5 seconds of straight line flight.

[Admiral_Red]

Ollie thank you for your response. Here are a few more details about the project that might flesh-out some of the lacking details.

All of the template design will done in Compufoil and modeled to the closest of tolerences. The finished product will be a hollow molded structure produced with the utmost of accuracy in molds. The initial wing flown will be a typical foam composite structure to prove viability of the design.

Flaps will be incorporated into the design, and the only thickness concern is at the root, where 3" of clearance is needed to clear the custom Robart's without blisters on the top of the wing. The planform will incorporate a single taper wing with a straight trailing edge. Washout is to limited to 1-1.5 degree to minimize torsional loads.

Construction of the final wing will incorporate a Nomex/kevlar sandwich spar, and as I said, it will be molded in a female mold. The skin will be composed of a mixture of Kevlar, Carbon, Rohacel, and fiberglass.

The aircraft test area has a 7000' runway, nothing to hit for a 5 mile radius, and plenty of monetary backing is at hand.

What is the control surface percentage on the 66 series? I was just looking at them in Compufoil.

James

[Ollie]

Wow, we have come a long way since the ten cent rubber models I built as a kid almost 60 years ago.

You will need at least 0.084 X 0.5 inch crossection, precured carbon spar caps at the center of a 3 inch thick wing. You will also need shear webs that can withstand at least 1350 pounds of shear force at the center of the wing.

The NACA66-021 should do the job for the root section and you could go as thin as the NACA66-012 at the tip without running into a serious tip stall problem with a degree or so of washout if the taper ratio isn't too severe. The washout will add some induced drag at speed. The sweep forward of the 25% chord line will add some additional tip stall margin.

[Admiral_Red]

In reading the tests of the 66 series airfoils, I noticed that the split flaps should be 20% or the airfoil with maximum lift generated at 60 degrees. I have not had the time to finish the documents, but I am assuming that the aileron should be the same percentage, agreed?

The root is a 66-018 that gives me the thickness needed at 23inches in chord, and 7.75" at the tip. Once again, straight traling edge.

James

[Sparky Paul]

Where are you going to fly this thing?
What type of equipment?
How much redundancy?
300 mph.... it will get real small real fast!

[Admiral_Red]

It will be flown on at least double redundancy on all RX/Batts, JR 10x, 42lb turbine, and flown at the club field. As I said, there is 7000 feet of runway. We have the alternate runway of a less than active airport. It is a rural airport and we have communications with all aircraft coming in and out of the field and are traffic control as we are the only ground control at the airport. We also have frequency scanners and will not fly unless it is clear of chatter.

Servo on the list include digitals in each elevator half, rudder, Graupner 1/4 scale on the ailerons, and digitals on the split flaps.

The pilot/builder is a high end turn key builder that has been building jets for people for many years. Many of his current sport jets have seen 300+ verified by radar. This one is big enough for GPS verification due to the vast room in the fuse.

James

[Ollie]

James,

There is a servo torque calculator at:
http://www.csd.net/~cgadd/eflight/calcs_servo.htm
An aileron with a chord 20% of the airfoil chord is a good compromise between aileron effectiveness and servo torque requirement. Assuming an aileron span of 1/4th the wing span, 20% chord and 10 degrees deflection, the servo load is only about 50 ounce-inches at 300 MPH. However, you don't want to try to deploy 60 degree split flaps at such a speed as the torque requirement is over 1600 ounce-inches. You should calculate the red line flap speed for the flap servo you intend to use. Even if you could deploy flaps at 300MPH with a monster servo, the maximum coefficient of lift is more than doubled and the wings would probably clap wing tips if something else didn't fail first.

The straight trailing edge worries me. A straight 26.8% chord line would be a much better choice from a tip stall and structures point of view. BTW, 26.8% is the aerodynamic center of the NACA66-018 airfoil at a reynolds number of three million.

Just a reminder, the aerodynamic forces go up as the square of the airspeed. What works structurally for this size model at 100 MPH may not work at 300MPH where the forces are nine times larger. Structures that we take for granted need to have their strength and stiffness recalculated for the higher speed. Rules of thumb for models just won't do in this case.

[Admiral_Red]

You caught me while I was replying. So in your opinion, the planform should be double taper leaving the 26.8% line straight across. Why does the straight trailing edge worry you?

J

[Ollie]

The sweep of a wing aerodynamically is measured to the line of aerodynamic centers. Any sweep back of the line through the aerodynamic centers contributes to tip stall tendencies and to torsional loading of the wing structure. Slight sweep back can be tollerated. Why put in a feature that is a disadvantage?

[Sparky Paul]

You'd do well to plot some flight paths, to find what bank angles for instance would be required to keep the plane within sight. At that speed it may be in a constant turn around the pilot.
A "pylon turn" at the end of a straight pass will take up a lot of room, and put a very load on the plane.
The fast jets I've seen take up so much sky they can't be fun to fly, while worrying about losing sight of it, either out or up!

[Ollie]

What Paul said!

Without explicitly stating the purpose of the model the design decisions just float around without any useful reference to judge them by.

[heli frapper]

300MPH goes 1 mile in 11.9 seconds

in the vertical dive from the top of loop staring at 1000ft
to the deck will take some three seconds

a typical football pitch is used in 3/4 of a second

one french member of my ex french club with pulse jet exceeds 230mPH especialy in dives vidio game stuff

will try to find out wing foil but suspect it was modified tear drop
similar to 1970s combat wings for controlline

stephen europe

[JimF]

Yikes,

Heli beat me too it, but some rough math says 7000 ft at 300mph takes 15.9 seconds. Assuming I can see the plane 3500 ft away, thats under 8 seconds to airshow center.

Gotta figure its not going to be 300 mph WOT all the time. Maybe cruise at a lazy 200 mph;)

Jim

Having lived just downwind of South Javelina Fart, Tx, I can tell you there is a lot of open space out that way.
:D

[Arbo]

James,

What ever came of this project?

Paul

[Ollie]

I suspect it died from swallowing a large reality pill coated with practicality. If it hasn't happened yet, it will eventually.

[Admiral_Red]

Last time that I talked to my friend about it, he was still working on the stab mold. I'll give him a call and get an update, he's in South Javelina fart, and I am in San Antonio.

James