GSJames

Thanks Dave. No, the earlier pictures with the engine upright was just to put something in the drawing that I could use to get the cylinder head clearance set. Since it's a round fuselage/cowl, it didn't matter at that time at what angle I put the engine. From here on out the engine will be rotated to a more realistic position. It will be final positioned later in the process.

Now it's time to add the canopy. I am counting on it to make the 5" depth requirement, BUT I don't want to add a lot of cross sectional area. By keeping the scale outline and slimming it down a bit, I got a cross section through the peak of the canopy that meets the 5" requirement but still has only 12.36 sq in. ( I met the 12.5 sq in requirement at FS 7.0). I'm fairly pleased with the fuselage so far. The wing is mostly finished, just gotta pick an airfoil and then I have to work on the tail surfaces and the wing/body junctions. After I get a 3D model that I like, I'll make a 2 dimensional 3-view drawing for submission to the approval board. Then I wait. No sense in going any farther than that until it is approved.

I'm going to take a little break now and work on my 424 airplanes.

diggs_74

Looks really good Gary.. Maybe I can get this one done a little faster than the AR-6.. It's suppose to be 50 here this weekend.. As long as it's not blowing 30mph it's going to get flown.

GSJames

Don't break it... I want one (eventually) Perhaps I should get good at 424 first however...

GSJames

Real radial engines are HEAVY!!! Because of that, radial engined airplanes tend to have very short nose moments. Model airplanes don't have that problem, a Q-40 of a radial engined airplane which had the same nose and tail moments would be very tail heavy. In a perfect world, I would have access to volumes of weight data and would be able to do a weight breakdown to find where the wing should actually be on the model. Unfortunately, I don't have that, so I have to guess. I measured a couple of model race planes and found that the location of the wing a.c. is approximately 29% of the distance from the base of the spinner to the tip of the tail. So, I moved the wing of this model back 2" to at least get it in the ball park of being able to balance. This reduced tail moment is going to degrade stability in both pitch and yaw. The high aspect ratio will also affect directional stability. I'll have to do some quick calculations to determine just how much I'll need to increase the areas of the horizontal and vertical tails to compensate.

GSJames

In the preliminary design process, engineers use a simple method to estimate the required sizes for the empennage. It is known as the tail volume coefficient. I calculated the volume coefficients for both the horizontal and vertical tails of the model as it exists right now. The horizontal tail volume coefficient came out at 0.73 which is right in the middle of the acceptable range of statistical data taken from other existing real airplanes, so it should be OK at the scale size just like it is. The vertical tail volume coefficient came out at 0.025 which is on the low side of the range from 0.020 - 0.070. It would probably be marginally adequate, but I think that I should increase it a bit because we don't want the airplane "fish tailing" as it flies. I can't increase the moment arm, so I'll have to increase the area of the vertical tail, but I don't want to increase it too much or it will look stupid. Here is a top view with the scale horizontal tail in place and the wing relocated aft 2"

diggs_74

Looks great Gary.. Can't wait to start on it..

GSJames

"Does this vertical tail make my butt look fat...???"

So, I resized the vertical tail. The black outline is the original, the red is scaled up 7.5% and the blue is scaled up about 10%. The black gives a tail volume coefficient of about 0.025 which is too low in my opinion, the red about 0.03 and the blue about 0.037. There are real airplanes with tail volume coefficients of about 0.03 so I suspect that this would be adequate, but not extremely stable in yaw. The blue would be very adequate, but I think it "looks" funny. I am tending to go with the red and suggest that the molds of the vertical tail be a separate part in case I've screwed up.

Does anyone have data on the area of the vertical tail of other successful Q-40's that they would be willing to share?

Whaddya think?

Scorpion Racing

I would be tempted to test it with the smaller one, and add on only if it oscillated in flight or did something weird. We are going much faster (scale-wise) and the air molecules don't get any smaller even though our models are!

Just my thoughts,...

diggs_74

Just looking at some of the other Q40's there are, in many cases the V-stab is quite small.. Look at the Vendetta for instance, the V-stab area is quite low compared to the rest of the plane.. Of coarse there are some that will say that's where it's first turn (not yet up to speed) can be a little exciting..

daven

My Feedback: (1)

I'm one of those Lee

diggs_74

I know.. However, even with that toad Jett on my Vendetta and I never had an issue. It wasn't up to speed any of the laps

GSJames

OK, so what's the area of the Vendetta vertical tail and what's the moment arm? I can use that for comparison.

diggs_74

I don't know, I sold mine.. I have a dago here that's got to be pretty close.. The dago is about 6" (bottom of fuse to top of stab) high with a 3" chord at the top and 5" chord at the fuse junction. Now, Dago's are notoriously bad on the first lap.. The first time I ever met Randy Bridge he told me to hold full right rudder all the way to #1 from the push.. It took me about 3 heats to get my thumbs to do it but that's what it took to get a good first lap.. Whether or not that was a problem with the vertical area or something else I'm not sure.

GSJames

Just a thought, would it be legal to put a yaw gyro in a Q-40? I don't see anything in the rules that would prohibit it.

diggs_74

The only issue would be the weight and where do you put it.. I know the new ones are pretty small but...

GSJames

OK, I corresponded with Lee, he chose the small one and will make the others out of balsa if needed. SO, here are some views of the almost complete model. I now have to add the fillets, landing gear, etc and then produce the 3-view. I might need to add some dihedral.

GSJames

On the other hand, I want you to see the view that all of the competition will be looking at...

diggs_74

Obviously you're assuming someone other than me will be flying it

fizzwater2

My Feedback: (61)

with me, that would be their view as they go by me.. again.

diggs_74

I here you there.. I don't get lapped quite as much as I used to but it still happens on occasion..

GSJames

Airfoil selection time. Pick a section, any section.

Scorpion Racing

Gary, I would divide 87.5 by the chord 3" from center on the main wing. That will give you the % thickness minimum you need to meet the rule (7/8" @ 3"). That will rule out a lot of airfoils right there. You can then figure out the Rn (1.2M?) using the average top speeds in Q40 using your chord data. Since your wing isn't tapered that much, I would use the average. With mine, I had to consider root & tip, and how the stall progresses along the platform. I added 1.5 degrees dihedral to mine as well. With your low wing design, make sure the fillets clean the top of the wing to fuselage joint good, and end by the 3" mark. Otherwise, your minimum thickness is measured beyond the wing fillets.

Looking good!

GSJames

Many people believe that THINNER airfoils are "better" than THICKER airfoils. I am not one of those. For example, in full-size Formula One, people for YEARS were always using the thinnest airfoil that they could get away with, some as thin as 6%. Then comes along Jon Sharp with the original Nemesis Formula One. It has a 14% thick airfoil and he proceeds to blow everyone away. In pylon (both model and full size) we spend a LOT of our time in the turn. A section which has less drag at the lift coefficient being generated in the turn will come out of the turn faster. F=ma It's a trade study of course, and to do it right would take a simulation program that accurately modeled the airplane around the course with different sections on the same planform. I'm way too lazy to attempt that, so I'll just look for one that has low drag in the turns without giving up too much in the straights. The Cdo of the wing alone is a fairly small part of the total drag of the airplane. The Cdi is a much bigger part, but still not as big as all the junk that we have to have hanging out in the wind. Rn=1.2 x10^6 is approximately a 8.25" average chord at 190 mph so I figure that's reasonably within the ballpark. I think I'll just get out my dart board...

diggs_74

You lost me after "thinner"

GSJames

Yeah, but how about "dart board"???