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James Maher wrote:
I would hope that you both will reconsider your approach to the cooling system. I believe that I understand what you are attempting to do but perhaps you did not consider the aerodynamic ramifications.
Whether you believe in Newton or Bernoulli you must consider that any air exiting through the upper(low pressure area) of a wing will have a spoiler effect on the airflow.
I'm afraid that this will cause a substantial loss of lift on the surface that provides 60% of the lift for the aircraft.
While the low pressure will aid in extracting the warm air from the radiators there is a price to pay for this and it may very well be a high price.
You are adversely affecting the aerodynamics of the aircraft with this approach.
The exit will not be a big hole in the top of the wing. It will be gills that will blow air across the top surface. Larry had good success getting the correct flow after tuft testing. No rooster tails for me, please 8*). There may be a slight spoiler effect at low AOA, but there isn't the need for high lift at low AOA. At high AOA, it will act like the blowers that are used on many STOL designs. Adding some energy to the flow right before it goes turbulent. This will be little different than vortex generators. The effects are further mitigated due to the fact that it is only a portion of the strake, and the outer portion at that. Most of the lift is generated by the canopy/turtleback portion near the center of the fuselage.
At the very least you should consult John Dyke on this.
I don't think that would be useful at all. I could get no response from him at Sun'n'Fun, so I don't expect anything more would come through email.
Besides the diminished lift, it could also cause a change in the Center of Pressure which could adversely affect the weight and balance of the aircraft.
Yes. But not by much. There just isn't enough air that can flow through a 30in^2 hole when compared to the whole strake. Let along the entire fuselage, or even the entire wing. Let's assume that all the lift is generate from 12 lateral inches at the front of the fuselage and 12 lateral inches just after the trailing edge. That is, all the lift comes from a band across the fuselage only 12 inches wide. Taking one side of the fuselage (48" wide), that is 576in^2. So we take away 30in^2 of lift. Just punch a big hole in there. We've only affected 5.3% of the lift area. In reality, the effect will be much less, because there will be back pressure from the heat exchangers and I will use less than 30in^2 of opening, if I can get away with it.
Now, out of that 5%, the possibilities are that extra volume would relieve some of the negative pressure gradient along the top surface aft of station 4, effectively making the tail aerodynamically heavier. Or the velocity of the exiting air could be such that it increases the flow velocity in the boundary layer along the upper surface. In this case, the negative pressure on the upper surface would go more negative, aerodynamically lifting the tail. In reality, both forces will be at work to cancel each other. I expect some small change in the lift characteristics, but no more than can be expected though the normal variations that occur from builder to builder...or even the variation that occurs when the wing skin deflects.
In short, putting the exit gills here is no more dangerous than putting them on the top of the cowl, or removing the hinge post. Any changes will be slight, and playing in the margins. That being said, it is the sort of thing those first few test hops are meant to confirm.
Please reconsider all of the effects of this before making such a dramatic change.
My concern is for your well being only.
I am sending this off list as it is not my intention to embarrass you, only to express my concerns.
I hope you take this as constructive as I do not mean to criticize.
Please. If I wanted someone to pander to me and agree with everything I propose, I would only have these discussions with my wife and children. I have changed direction many times on advice from you and others. I wouldn't keep coming back for more if I didn't enjoy it 8*). If I can't give a decent answer to your well thought out questions, then I should stick to model planes. Begging your pardon, but I'd really like to respond to this in the open. There are people on the Flyrotary list that have forgotten more than I'll every know about this stuff. You've raised valid concerns, and I'd really like to know if I'm doing something stupid.
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,|"|"|, Ernest Christley |
----===<{{(oQo)}}>===---- Dyke Delta Builder |
o| d |o www.ernest.isa-geek.org |
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