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Old Apr 04, 2002, 04:09 PM
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JetMang's Avatar
San Diego, California, United States
Joined Feb 2002
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Dan,

You are mostly right, but you didn't quote me entirely. I didn't mention anything about parasite drag. Induced drag is the drag due to lift which is porportional to the square of CL with a proportionality factor (drag-due-to-lift factor). So, by increasing the weight, the required lift force increases, CL increases, etc......

The point I was trying to make was that for a model airplane ducted fan jet, there truly is a happy medium between weight and forward penetration of the airplane. It's all about momentum. I'm sure you've noticed that when flying your jets around, you'll get a noticebly faster high speed pass when you bring the airplane around in a shallow desending turn as opposed to just flying around at the same altitude. As the fan unloads and the momentum of the airplane picks up, so does the speed. A very light DF airframe doesn't have the momentum that a slightly heavier airframe does. Bare with me, I'm not saying add weight to the airplane by any stretch, I'm simply making an observation. Also, there certainly is a point of diminishing return where the airplane just becomes TOO heavy which you deffinately don't want.

The interesting thing is, on the heavy lift airplane I designed way back, the more weight that went into the airplane, the faster it flew...
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Old Apr 04, 2002, 04:50 PM
It wasn't me...
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Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally posted by JetMang
Dan,

You are mostly right, but you didn't quote me entirely. I didn't mention anything about parasite drag. Induced drag is the drag due to lift which is porportional to the square of CL with a proportionality factor (drag-due-to-lift factor). So, by increasing the weight, the required lift force increases, CL increases, etc......

The point I was trying to make was that for a model airplane ducted fan jet, there truly is a happy medium between weight and forward penetration of the airplane. It's all about momentum. I'm sure you've noticed that when flying your jets around, you'll get a noticebly faster high speed pass when you bring the airplane around in a shallow desending turn as opposed to just flying around at the same altitude. As the fan unloads and the momentum of the airplane picks up, so does the speed.
I know you didn't mention parasite vs. induced drag. You only said that to go faster you must minimize drag. But as you know there are two components to drag, induced and parasite. And as you know, induced drag has a far greater effect than parasite drag, which is why I brought it up. And the best way to reduce induced drag and fly faster is to reduce weight.

Granted, I can put my jet into a dive and pick up speed, and the heavier it is, the faster it will race to the ground, (which caused more by gravity, than by momentum) but I can't do that the entire flight. At some point I'm going to have to pull back into level flight and at that point, the lighter airplane will fly faster than the heavier one due to decreased induced drag.

Case in point. We had two guys with Kyosho T-33's at our field. One had an 8-cell sub-C battery pack and a 4-cell receiver pack. The other had a 7-cell sub-C pack with a BEC ESC.

The one with the BEC ESC always flew faster and better, no matter what the conditions were.

The advantage that the additional weight gives you in a dive is far outweighed in the other regimes of flight like take-off, climb, manuevering, cruise and max speed, and landings. In all of these cases, a heavier airplane does not have the all-around performance that a lighter one will.

Dan
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Old Apr 05, 2002, 10:14 AM
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JetMang's Avatar
San Diego, California, United States
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Dan,

I feel as though you have taken me out of context a bit. If you want to "dive" your airplane and let it "race to the ground," then feel free, but that's not what I described. I said a SHALLOW DESCENDING TURN, not a dive to the ground...Also, considering that the simplified definition of momentum is VELOCITY*MASS, and considering that weight is MASS*ACCELERATION OF GRAVITY then I would say that the airplane will build momentum in that shallow turn. As you level out of the turn, that momentum will carry you through at a higher rate of speed than an airplane that is a little lighter (up to a point).

Remember I said that THERE IS A POINT OF DOMINISHING RETURN, right? When you are dealing with smaller airplanes like the T-33, the weight penalties tend to be more exaggerated. ie. Adding a couple ounces (or even less) of weight to a smaller airplane will tend to negatively affect the performance much quicker (push you to that point of dominishing return much quicker) then adding a couple of ounces to a bigger airplane (say 1/10 scale). A 1/10 scale airplane has much more margin, and a little more weight will not hurt you, but help you (up to a point as I said earlier) So, considering your example, the RX battery has added too much weight and you have reached that point of dominishing returns to where the airplane then becomes underpowered for the weight. Back in the day, Larry Wolfe of JHH built and flew a "Skyblazers" Sabre that came out very light. Less than 8 lbs I believe. That airplane did not have that great of forward penetration because it was so light. Also, it would float on landings. Don't get me wrong, it flew very well, but it flew light. After that airplanes demise, the next Sabre he built which came out heavier, but had the same setup, flew noticebly better with much better forward penetration. Also, the airplane would settle better on landings. I'M NOT SAYING ADD WEIGHT TO YOUR AIRPLANES...

I hear what you are saying and agree that reducing induced drag helps, but as I said, THERE IS A HAPPY MEDIUM BETWEEN WEIGHT AND FORWARD PENETRATION OF THE AIRPLANE. That has been my experience. All I ask is that you hear what I am saying. Thank you.
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Old Apr 05, 2002, 12:36 PM
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Las Vegas, NV
Joined Dec 1996
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JetMang, I think you are assigning too much importance to the penetration ability of model aircraft. That's a factor I rarely consider when designing a model. If I were designing a PSS (Power Scale Slope) glider that I wanted to get nice big loops out of then I would consider adding weight to it so that in dives I could build up a lot of momentum to carry the model through the loop. In that situation with the wind blowing hard up the side of a hill you have lots of energy you can harness. To maintin the same height you can angle the nose down more and travel faster witout losing height than you can when the wind is blowing half as much. The large energy amount means you can increase your wing loading and use the higher weight to increase your momentum, which gives you larger loops.

Here's a good example, a Zagi 400 on the slopes vs at a level flying field in no wind. I've had several Zagis, with some heavier than others. The lighter ones fly better on level ground in no wind. Loops are bigger and the climb rate is higher. If the wind is blowing at say 15 or 20 mph, I can still fly but it's not as fun because I'm getting blown all over the place. Now if I add weight to the airplane it has more mass and higher wingloading and doesn't get affected by gusts as much. It's still not very fun to fly in wind like that. Now if I were riding in the airplane I'd probably like the heavier one better because I wouldn't be getting tossed around as much, but we're talking model aircraft here. Now when I take the Zagi to the slope, dive it and try to loop it, most of the time it flops out of the top of the loop because as I loop into the wind and pull up, at the top I'm coming toward myself and the wind is behind the model, so airspeed, which is slow anyway at the top of a loop, drops too much and a tip stall out of the loop. This is also caused by low momentum. As I dive into the wind and pull up, my speed relative to the earth is low due to flying into the wind. High airspeed, low groundspeed. My kinetic energy is rather low. I can dive and turn some of my potential energy into kinetic energy and then try to loop, but most of the time it's a floppy loop if I makes it over the top. Now if I go and add 16 ounces of ballast to the airplane, I think it should loop better because when I dive I'll build up more momentum to carry me over the top of the loop. That's good for slope flying but on flat and level ground the Zagi with 1 extra pound of ballast wont fly very well if at all.

For normal flying, lighter is better. Another case is my scratchbuilt T-33. It used the Kyosho wing and tail and rear fuselage, but with a new forward fuselage that was more scale and had area ruled intakes. When I first flew it it was about 8-10 mph faster than the stock T-33s that I was flying formation with. It crashed later on due to BEC failure and after repairs it came out weighing over 4 lbs. The weight increase was noticable in flight, the loops weren't as big as they had been and climb rate decreased. I had the extra speed over stock but with the added weight vertical performance went down, and I like to emphasize vertical maneuvers in my flying style. The plane had more weight, but the loops were smaller, so adding weight did not improve my vertical performance.

I've recently totally rebuilt my Wattage F-22 EDF conversion. I redesigned the ducting so that I could move the battery farther back to correct a major CG problem. I had four ounces of lead in the tail, and now it's gone. The loops are bigger, I no longer struggle over the top, and climb rate has improved. Four ounce on this model was something like 15 percent of the total weight, and it definitely shows.

Weight is the enemy in EDFs.
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Old Apr 05, 2002, 01:36 PM
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Plane Crazy's Avatar
Reno, Nevada
Joined Jan 2001
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I generally stay away from discussions when engineering scholars pull out the formulas. I am more of a seat of the pants flier.

But this is a discussion on Gravity. If the plane is climbing, gravity is slowing it down. If the plane is descending gravity is accelerating the plane.

My comment: "The lighter the plane.... The faster it will go" relates only to the fact that a large EDF plane will ALWAYS weigh more than the same large glo fan plane. If you can remove any additional wieght from the airframe to allow for the added weight of the batterys it will not weigh as much as it could if you did not try and cut some wieght out. You are not really making the plane lighter because you are replacing the wieght you removed with battery wieght. Air penetration is hardly a problem with EDFs because they are heavy anyways.

When you are putting 4 to 6 pounds of batterys alone in an airframe the RTF weight can get pretty high and your wing loading goes off the chart.

I was never trying to cut down the quality of Larrys kits. JHH makes very nice clean designs. The main point is that Electric ducted fans don't require the same structure that glo fans do.

I am going thru and removing weight from the JHH BAE Hawk ARF right now. I have removed 7 ounces of wieght from it so far with out weakening the structure. So this means I can put 7 ounces of batteries back into the airframe. Larry wants to see how I can do converting it to an EDF and I am sending him a report on my conversion when it is completed.

Gordon
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Old Apr 06, 2002, 02:32 PM
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JetMang's Avatar
San Diego, California, United States
Joined Feb 2002
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Gordon,

You are absolutely right. I didn't mean for the discussion to explode like it did. I guess I just took the comment too literally or something. That's one of the things with the internet, you can read something but you might take it differently then was originally intended.

As far as I'm conserned, what we were all saying is true. They are all considerations that one would want to think about when building an airplane and are dependent on what you want to do with the airplane. That's the great thing about aircraft and aircraft design, there are so many different variables to the equation. I think you said it best though, by taking out weight, that's more margin for batteries.

I can't wait to see how that Hawk goes with that wemotec fan. That thing is sweet!
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Old Nov 20, 2006, 03:20 PM
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edf express's Avatar
Birmingham, AL
Joined May 2005
194 Posts
Heavier or Lighter, I want my airplane to fly like I want it. I dont see what the big deal is about the weight, to me its about where that weight is and how it balances the plane. I am definitely more about the balance than anything else...

Many hard lessons learned by overlooking this....
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Old Nov 20, 2006, 07:50 PM
EDF rules... :)
AirX's Avatar
Joined Nov 1999
13,648 Posts
This thread is over 4 years old, there are better and more up to date threads on large fans. Back then the power was limited, now it is practically not a problem getting Gas DF perfromance from an EDF.

Eric B.
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Old Nov 20, 2006, 09:33 PM
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edf express's Avatar
Birmingham, AL
Joined May 2005
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Quote:
Originally Posted by AirX
This thread is over 4 years old, there are better and more up to date threads on large fans. Back then the power was limited, now it is practically not a problem getting Gas DF perfromance from an EDF.

Eric B.

I know, I am making sure I follow the whole process. I dont want to just pick what is new and good, but I need to understand why the choices are what they are and where fellow edf-ers are coming from.
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Old Nov 20, 2006, 09:56 PM
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St. Mary, Maryland, United States
Joined Dec 1996
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Just don't choose a motor from a 4 year old thread
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Old Nov 20, 2006, 10:02 PM
EDF rules... :)
AirX's Avatar
Joined Nov 1999
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Amen...

Eric B.
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