Hello XBe02Drvr,
Wouldn't there be a pretty good chance of flipping the aeroplane with engine / propeller torque when going so slow?
- Ivan.
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This is called "crow-hopping" and has been the demise of many a fine airplane. The solution to this dilemma is to put the power to it at the top of the bounce, take it around the pattern, give yourself time to settle down, then try again.
Well, yes, if you're flying a plane that's got that much horsepower and rotating mass, that's a possibility. The vast majority of single engine trigear aircraft don't. In fact the P-39, P-63, T-28, and the Gannett are the only ones I can think of offhand, and the Gannett isn't really likely as, it has counter-rotating prop and short nosegear. So with the Bell babies and the Trojan you have to treat the throttle a bit gingerly when you're low slow and dirty, like you would any other high powered single. If you're a freshly winged Air Corps nugget, you may find this is something the old Vultee Vibrator hasn't prepared you for. With these high powered jobs the trick is to come back on the stick as you're judiciously feeding in the power so when she reaches the apex of her crow-hop and settles back on the runway she'll be nose high enough to touch on the mains first. At this point you're quite far down the runway and are powering up and starting to accelerate, so you better consider yourelf committed to a go-around. Don't forget you're going to need plenty of rudder at these speeds as the power builds. Best keep it on the ground til you've got enough airspeed to handle the P-factor, precession, and torque that will increase as soon as you're out of ground effect. Good luck!Hello XBe02Drvr,
Wouldn't there be a pretty good chance of flipping the aeroplane with engine / propeller torque when going so slow?
- Ivan.
Propellers ahead of the c/g are destabilizing because pitch or yaw produces a side thrust in the direction of the yaw or pitchHello XBe02Drvr,
The Ryan Fireball and Dark Shark would also qualify as fairly high powered though I doubt anyone really needs to be concerned about having to fly one of those. What a strange pair of birds!
On a different note, I was thinking about the discussion about how the Airacobra had a longer nose than typical and why that was bad for stability.
Here is what I am seeing. Please let me know if you believe I have misinterpreted something.
A longer nose with everything else being equal (which it usually is not), just means that the propeller is further ahead of the Center of Gravity and Center of Pressure. Assuming this is the case, here is what I see the effects would be:
1. The propeller's thrust would tend to increase stability because any yaw and pitch deviations would have a longer moment arm to correct.
2. The propeller's spiral slipstream would have less of an effect on a wing and especially a tail at a greater distance.
3. The effect of P-factor would be less because although the asymmetrical thrust is the same, it is at a greater distance from the CoG.
4. Gyroscopic procession is a little harder to figure out but I believe it would be the same as for P-factor. This is the one I am least sure about.
Of course from a practical view, it is rather difficult to have a longer nose without greater surface / keel area ahead of the CoG so maybe that makes up for all the other effects?
- Ivan.
Propellers ahead of the c/g are destabilizing because pitch or yaw produces a side thrust in the direction of the yaw or pitch
I would call that section W1 as being through the fillet, rather than the root, and so would not be surprised at finding a symmetrical section. I suspect that a section a little further out into the main wing would show something more like a NACA 23015.Hello All,
I just came across this drawing of the airfoils used on the Airacobra.
A symmetrical airfoil seems to be an odd thing to use even if the wing tip does have a regular NACA 23000 series airfoil.
Anyone care to comment on the implications of airfoil selection on maximum Coefficient of Lift and maneuverability in general?
- Ivan.
View attachment 493459
Ivan, you're forgetting Archimedes lever.1. The propeller's thrust would tend to increase stability because any yaw and pitch deviations would have a longer moment arm to correct.
2. The propeller's spiral slipstream would have less of an effect on a wing and especially a tail at a greater distance.
3. The effect of P-factor would be less because although the asymmetrical thrust is the same, it is at a greater distance from the CoG.
4. Gyroscopic procession is a little harder to figure out but I believe it would be the same as for P-factor. This is the one I am least sure about.
Ivan, Kiwi's got a point here. For reasons more structural than aerodynamic, you need as much thickness at the wing root as a you can get away with and not incur too much penalty. The deeper you can make your main spar the stronger your wing, and you've got to have someplace to tuck your main wheels.I would call that section W1 as being through the fillet, rather than the root, and so would not be surprised at finding a symmetrical section. I suspect that a section a little further out into the main wing would show something more like a NACA 23015.
I would call that section W1 as being through the fillet, rather than the root, and so would not be surprised at finding a symmetrical section. I suspect that a section a little further out into the main wing would show something more like a NACA 23015.
Ivan, you're forgetting Archimedes lever.
1) If you think of your Airacobra as a balance beam with the CG as its fulcrum, the further out from the fulcrum your propeller applies a force, the more leverage that force has to displace the beam. Conversely, the countering forces of the stabilizers working through a shorter arm will have less leverage to oppose that displacement.
2) You've got your P-factor back to front. The descending blade on the starboard side (seen from the cockpit) has more AOA to the relative wind in a nose high attitude, thus more "lift" and a yawing moment to port.
3) Spiral slipstream won't have that much difference, long nose or short.
4) That big prop spinning out there is a powerful gyroscope, and when you rotate the nose upward, it's going to apply a left yawing moment which your RIGHT foot better be ready to counteract.
This is where that long nosegear comes into play. The positive AOA it causes on the ground means that you can fly off with very little rotation required, thus minimizing those leverage effects of the prop.
Ivan, Kiwi's got a point here. For reasons more structural than aerodynamic, you need as much thickness at the wing root as a you can get away with and not incur too much penalty. The deeper you can make your main spar the stronger your wing, and you've got to have someplace to tuck your main wheels.
Cheers,
Wes
The tip was a NACA 23009 and the root 0015, according to The Incomplete Guide to Airfoil Usage. The 23009 is cambered.The symmetrical airfoil was popular during the WWII era. I believe the P-39 airfoil was symmetrical from root to tip with the tip airfoil being thinner (15% vs 9%). Lift was achieved by the 2% AOA.
Who, me, slydexic? Never!Maybe and Maybe not...... Please watch this video and see if you still come to the same conclusion
Who, me, slydexic? Never!
Don't have a fidget toy handy, but looked at the video. We did those same things in high school physics 54 years ago. I noticed in the video that when he spun the bike wheel clockwise the whole assembly rotatated left, which corroborates my experience in 13,000 hours in the air. Right foot on the rudder to keep it pointed down the runway.
Instructor: "How do you know that all airplanes are radical liberals?"
Student: "Huh?? Never thought of planes as political. How can that be?"
Instructor: "Simple, just give'em power and they swing to the left!"
My girlfriend, an accomplished motorcyclist, equestrian, and mechanic before I taught her to fly, has a similar affliction applying to communication, not function. Didn't stop her from a flying career, and she retired from American as a 737 pilot. When we were training, we didn't use the words "right" and "left", substituting "cap'n side" and "FOside", which fixed the problem, I don't know how. Multi engine training could have gotten real sticky without that fix.
Cheers,
Wes
PS: Despite all your rationalizations, precession to the right flies in the face of experience.
Propellers ahead of the c/g are destabilizing as there's a side force generated by a propeller in yaw (see Ribner, Herbert, "Propellers in Yaw," NACA Report 820, 1945, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091897.pdf) Ribner's rule of thumb is that a propeller has the same effect on stability as a fin with the same projected area as seen from the side; this means that it's actually worse for coaxial counter-rotating propellers. On the other hand, they're stabilizing if behind the c/g, so an aircraft like the B-35 or VariEze has its stability improved by their rear-mounted pusher propellers
Ivan, where did you come up with that conception of the forces at work? It doesn't look like any of the diagrams I've seen of P factor. What shows up in pilot training manuals is a depiction of a left-pointing arrow with its tail feathers at the propeller hub and a value attached multiplied by its distance from the CG to give a moment. This is contrasted with a drawing of the same aircraft with its piston engine replaced by a much lighter turboprop, which requires an extended nose for balance. Now the left arrow is farther from the CG and has a larger moment value, which translates to a heavier right foot to keep the plane out of the weeds alongside the runway. A number of larger single engine planes (Beaver, Otter, Courier, Porter, Air Tractor, etc) have popularized these turboprop conversions, and occasionally startled their pilots with the vehemence of their leftward swing under power.Attached is one drawing I finished up a few minutes ago. It shows the difference between how I see P-Factor affecting an aeroplane with a long nose versus one with a short nose.
Ivan, where did you come up with that conception of the forces at work? It doesn't look like any of the diagrams I've seen of P factor. What shows up in pilot training manuals is a depiction of a left-pointing arrow with its tail feathers at the propeller hub and a value attached multiplied by its distance from the CG to give a moment. This is contrasted with a drawing of the same aircraft with its piston engine replaced by a much lighter turboprop, which requires an extended nose for balance. Now the left arrow is farther from the CG and has a larger moment value, which translates to a heavier right foot to keep the plane out of the weeds alongside the runway. A number of larger single engine planes (Beaver, Otter, Courier, Porter, Air Tractor, etc) have popularized these turboprop conversions, and occasionally startled their pilots with the vehemence of their leftward swing under power.
Cheers,
Wes