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06-08-2011, 05:18 PM
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Join Date: Sep 2006
Location: Douglas Flat, CA
Posts: 589
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Quote:
Originally Posted by ronschreck
A tricycle gear F-1? That's just too ugly to contemplate!
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I can't help but agree, but there are a couple of compelling advantages to retract tricycle gear.
For one, it lets you put the entire main gear aft of the station of maximum depth on the wing. And that gives you a good chance of maintaining laminar flow on that part of the wing back to where the gear doors are.
On a retract conventional-gear airplane with the gear tucked into the ahead of the spar, there is pretty much no chance you can maintain laminar flow behind where the gear doors or gear openings are. There just isn't any way to make the gear door closures tight and smooth enough so they don't trip the flow.
With the gear legs back behind the spar, you can realistically maintain an extra dozen square feet of laminar flow on an airplane the size of the RV-8, and that is nothing to sneeze at. It's worth at least a couple of knots.
Of course, you might lose a bunch of that due to the nose gear weight and installation drag, so it might just be a wash from a speed perspective.
Personally, I think that retractable gear is out of place on a recreational airplane, even a very fast recreational airplane like an RV. It adds a level of operational seriousness that I think is out of place in the RV, and doesn't increase performance or utility commensurately with its added expense, complexity, and risk.
Feel free to point up the apparent hypocrisy in my designing and developing a recreation sailplane with retractable gear. I defend it thus: In a sailplane, a gear-up landing is usually an embarrassment but not a serious problem; depending on the landing surface you might even do so little damage that you can fly again that day. In an airplane, it more often than not constitutes a total loss for the insurance company.
Thanks, Bob K.
__________________
Bob Kuykendall
HP-24 kit sailplane
EAA Technical Counselor
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06-08-2011, 05:34 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Quote:
Originally Posted by Bob Kuykendall
IPersonally, I think that retractable gear is out of place on a recreational airplane, even a very fast recreational airplane like an RV. It adds a level of operational seriousness that I think is out of place in the RV, and doesn't increase performance or utility commensurately with its added expense, complexity, and risk.
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I don't know about that. Last weekend, I was standing by my RV. I think it's pretty good looking. Then a friend with his F1, three blade prop & pointed chrome spinner shows up. His makes my RV look slow! Then a Lancair Legacy (trigear) retract,...... that just set a coast to coast speed record, taxi's up. Now the F1 looks slow..!
That Lancair sure was good looking, as well as fast looking!
L.Adamson --- RV6A
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06-08-2011, 05:41 PM
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Join Date: Jul 2009
Location: Ankeny, IA
Posts: 210
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Laminar flow top vs bottom
My glider experience tells me that the critical region for laminar flow is over the top of the airfoil. It's particularly critical not to trip the flow over the forward portion of the top surface, but the lower surface is much less critical.
I agree that gaining performance with retracts on an RV is a tough proposition, but if they offered the Lancair Legacy in a retractable tailwheel version, I doubt I'd have ever bought a rivet gun.
M
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06-09-2011, 10:01 AM
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Join Date: Sep 2006
Location: Douglas Flat, CA
Posts: 589
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Quote:
Originally Posted by foka4
My glider experience tells me that the critical region for laminar flow is over the top of the airfoil. It's particularly critical not to trip the flow over the forward portion of the top surface, but the lower surface is much less critical...
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The laminar flow on the lower surface is normally thought to be less critical since it usually operates in a positive pressure gradient, and is therefore more robust and less prone to tripping over into turbulent. As I understand it, it also tends to trip straight to turbulent flow, without forming a draggy separation bubble. However, something as big as a gear door seam will trip it.
The surface condition on the wing upper surface is more critical, since it has a negative pressure gradient, and the laminar flow there is touchier and more prone to tripping and separating. But it's really only critical back to about the station of maximum depth; the flow usually goes turbulent around there regardless of surface condition.
Just because the laminar flow on the lower surface is more robust doesn't mean that it is any less important. Every square foot of laminar flow is a small but measurable reduction in drag over the same area of turbulent flow, regardless of whether it is on the top or the bottom of the wing. And as you're aware, on a well-designed and well-built sailplane wing, you can often hold laminar flow on the bottom wing surface all the way back to the control surface hinge line, and with good hinge line sealing you might hold laminar flow onto the control surface.
The counterintuitive thing is that loss of laminar flow on the lower surface can cause more loss of performance than it does on the upper surface. If you trip the flow on the upper surface at the 15% chord station, you lose laminar flow between there and about the 40% station, where it would have tripped anyway. So you'd lose about 25% of the chord's laminar flow. But if you trip the laminar flow on the lower surface at the 15% chord station, you lose laminar flow between there and about the 80% chord station where the control surface hinge is. And that's the loss of about 65% of the chord's laminar flow.
That counterintuitive bit is why it is a bit odd that some glider owners moan about the spar mirroring through on the upper surface, but don't seem to care much that it shows through on the lower surface. In fact, it matters relatively little on the top, since the spar is about where the flow would trip anyways, and it matters a bunch on the bottom, since that threatens the laminar flow between about the 40% chord line and the 80% chord line.
Thanks, Bob K.
__________________
Bob Kuykendall
HP-24 kit sailplane
EAA Technical Counselor
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06-09-2011, 10:23 AM
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Join Date: Jan 2005
Location: Cartersville, Georgia KVPC
Posts: 945
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Bob,
Thanks for the very interesting explanation. Having built a few R/C gliders, I've often wondered why the guys with the super slick composite wings tended to get better overall performance than the guys with the film covered balsa wings did, even though the composite was heavier. This helps explain it.
Quote:
Originally Posted by Bob Kuykendall
The laminar flow on the lower surface is normally thought to be less critical since it usually operates in a positive pressure gradient, and is therefore more robust and less prone to tripping over into turbulent. As I understand it, it also tends to trip straight to turbulent flow, without forming a draggy separation bubble. However, something as big as a gear door seam will trip it.
The surface condition on the wing upper surface is more critical, since it has a negative pressure gradient, and the laminar flow there is touchier and more prone to tripping and separating. But it's really only critical back to about the station of maximum depth; the flow usually goes turbulent around there regardless of surface condition.
Just because the laminar flow on the lower surface is more robust doesn't mean that it is any less important. Every square foot of laminar flow is a small but measurable reduction in drag over the same area of turbulent flow, regardless of whether it is on the top or the bottom of the wing. And as you're aware, on a well-designed and well-built sailplane wing, you can often hold laminar flow on the bottom wing surface all the way back to the control surface hinge line, and with good hinge line sealing you might hold laminar flow onto the control surface.
The counterintuitive thing is that loss of laminar flow on the lower surface can cause more loss of performance than it does on the upper surface. If you trip the flow on the upper surface at the 15% chord station, you lose laminar flow between there and about the 40% station, where it would have tripped anyway. So you'd lose about 25% of the chord's laminar flow. But if you trip the laminar flow on the lower surface at the 15% chord station, you lose laminar flow between there and about the 80% chord station where the control surface hinge is. And that's the loss of about 65% of the chord's laminar flow.
That counterintuitive bit is why it is a bit odd that some glider owners moan about the spar mirroring through on the upper surface, but don't seem to care much that it shows through on the lower surface. In fact, it matters relatively little on the top, since the spar is about where the flow would trip anyways, and it matters a bunch on the bottom, since that threatens the laminar flow between about the 40% chord line and the 80% chord line.
Thanks, Bob K.
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__________________
Moose
VAF #136
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06-09-2011, 10:28 AM
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Join Date: Sep 2006
Location: Douglas Flat, CA
Posts: 589
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Quote:
Originally Posted by L.Adamson
...His makes my RV look slow! Then a Lancair Legacy (trigear) retract,...... that just set a coast to coast speed record, taxi's up. Now the F1 looks slow..!
That Lancair sure was good looking, as well as fast looking!...
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Oh, I completely agree that retractable gear looks totally cool! And I completely agree that at those higher speeds the retracts make for a huge performance improvement.
It's just that for the typical RV, and even most atypical ones, I think that the expense, complexity, and risk of retracts is just not worth the improvement in performance and aesthetics. My belief is that the ongoing issues with A-model noseovers is nothing compared to the carnage we'd see if all RVs had retracts.
That said, I sure would like to see an RV or a Rocket do something good in Sport or Super Sport pylon racing. And that is not happening without pick-em-up-the-feet. But that is a different kind of flying altogether.
Thanks, Bob K.
__________________
Bob Kuykendall
HP-24 kit sailplane
EAA Technical Counselor
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06-09-2011, 09:51 PM
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Join Date: Sep 2009
Location: EP, TX
Posts: 27
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Quote:
Originally Posted by Bob Kuykendall
Oh, I completely agree that retractable gear looks totally cool! And I completely agree that at those higher speeds the retracts make for a huge performance improvement.
It's just that for the typical RV, and even most atypical ones, I think that the expense, complexity, and risk of retracts is just not worth the improvement in performance and aesthetics. My belief is that the ongoing issues with A-model noseovers is nothing compared to the carnage we'd see if all RVs had retracts.
That said, I sure would like to see an RV or a Rocket do something good in Sport or Super Sport pylon racing. And that is not happening without pick-em-up-the-feet. But that is a different kind of flying altogether.
Thanks, Bob K.
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I'm just curious; what do you mean when you say "is nothing compared to the carnage we'd see if all RV's had retracts"?
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06-09-2011, 10:52 PM
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Join Date: May 2011
Location: Madison, WI
Posts: 5
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Quote:
Originally Posted by L.Adamson
I don't know about that. Last weekend, I was standing by my RV. I think it's pretty good looking. Then a friend with his F1, three blade prop & pointed chrome spinner shows up. His makes my RV look slow! Then a Lancair Legacy (trigear) retract,...... that just set a coast to coast speed record, taxi's up. Now the F1 looks slow..!
That Lancair sure was good looking, as well as fast looking!
L.Adamson --- RV6A
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I'll be honest. I am torn between an F-1 and a Legacy. I want the F-1 for fun. But the wife would like to sit next to me. Decisions decisions... 
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06-09-2011, 11:46 PM
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Join Date: Sep 2006
Location: Manstad, Norway
Posts: 866
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When....
.... I was flying in the airforce, we had a saying:
There's only two types of pilots flying RG-planes: those who have landed gear up, and those who will.... 
__________________
Regards Alf Olav Frog / Norway
First RV-7 completed, (bought partly finished from a US-builder) 305 hrs per July 2014, SOLD
Second -7 had first flight Feb 25th 2014. 220 hrs pr July 2019. Life is good!
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06-09-2011, 11:54 PM
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Join Date: May 2011
Location: Madison, WI
Posts: 5
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Quote:
Originally Posted by ao.frog
.... I was flying in the airforce, we had a saying:
There's only two types of pilots flying RG-planes: those who have landed gear up, and those who will.... 
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Guess for 16,000 hours I've been lucky. 
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