Which Hacker should be used in "E" Turbax - Page 2 - RC Groups
 Apr 04, 2002, 04:09 PM The RC Geek 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...
Apr 04, 2002, 04:50 PM
It wasn't me...
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
 Apr 05, 2002, 01:36 PM Flying Welder Pilot 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 Last edited by Plane Crazy; Apr 05, 2002 at 06:18 PM.
 Apr 06, 2002, 02:32 PM The RC Geek 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!
 Nov 20, 2006, 03:20 PM Registered User 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....
 Nov 20, 2006, 07:50 PM EDF rules... :) 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.
Nov 20, 2006, 09:33 PM
Registered User
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.
 Nov 20, 2006, 09:56 PM Registered User Just don't choose a motor from a 4 year old thread
 Nov 20, 2006, 10:02 PM EDF rules... :) Amen... Eric B.