Hello XBe02Drvr,
Wouldn't there be a pretty good chance of flipping the aeroplane with engine / propeller torque when going so slow?
- Ivan.
Why a Rear Engine For the P-39?
- Thread starter MIflyer
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Hello XBe02Drvr,
Wouldn't there be a pretty good chance of flipping the aeroplane with engine / propeller torque when going so slow?
- Ivan.
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!
Cheers,
Wes
Hello 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.
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.
swampyankee
Chief Master Sergeant
- 4,031
- Jun 25, 2013
Propellers ahead of the c/g are destabilizing because pitch or yaw produces a side thrust in the direction of the yaw or pitch
Hello Swampyankee,
Just to confirm we are describing the same thing:
If we have a tractor propeller rotating clockwise as seen from the CoG and the nose of the aircraft pitches up, we get a yaw to starboard because of gyroscopic procession. Is this what you are describing?
If not, then what is the cause of the force you are describing?
- Ivan.
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.
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.
KiwiBiggles
Senior Airman
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.
Cheers,
Wes
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
Hello KiwiBiggles,
While I agree with you about the location of section W1, for it to be a representation of the fairing at that point would be very atypical.
In fact most Wing Root sections in drawings I have seen tend to be at the aircraft centerline at which point there clearly is no real airfoil shape or the fuselage itself can be considered the "Fairing".
My BELIEF is that someone had it in their mind that a symmetrical airfoil section was better for high speed purposes but used a cambered wing tip section instead of washout to address tip stall.
Hello XBe02Drvr,
I don't believe I am actually forgetting "Archimedes' Lever" at all. Here is why:
1. I believe you would be correct if the forces in discussion were purely lateral, but they are not.
What you really have is a torsion at the center of the propeller hub. From this torsion, only part acts tangentially to the radius (length of nose) from the propeller to the CoG of the aeroplane.
2. I don't believe I actually specified any direction at all in my post..... Or did my dyslexia do its part yet again?
3. Agreed.
4. Maybe and Maybe not...... Please watch this video and see if you still come to the same conclusion:
As a quicker alternative, one can just pick up that recent fad toy, the Fidget Spinner and do a test.
I just did this with a heavy metal Fidget Spinner and believe the yaw would be to Starboard and not to Port.
View: https://www.youtube.com/watch?v=n5bKzBZ7XuM
As a further explanation of my reasoning for #1, I need to make up some drawings to illustrate.
Your argument is about Wing Thickness. I believe we are all in agreement that the thickness is 15%, so from a structural and strength standpoint, there is really no argument. The Airacobra had its share of problems but I do not believe lack of structural strength was one of them unless you are discussing the demise of Jack Woolams in Cobra I.
The discussion is really about whether the Airacobra had a symmetrical airfoil at the root. The source of the drawings I posted was a set of drawings by Paul Matt for Historical Aviation Album in 1965. IF this drawing is correct, then it might explain some of the maneuverability issues.
- Ivan.
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P-39 Expert
Non-Expert
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.
swampyankee
Chief Master Sergeant
- 4,031
- Jun 25, 2013
The tip was a NACA 23009 and the root 0015, according to The Incomplete Guide to Airfoil Usage. The 23009 is cambered.
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!"
Cheers,
Wes
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Actually my form of "Dyslexia" if you can call it that is mostly a verbal (and to some extent a written) thing. I will sometimes point with my left hand or visualize something to my actual left but describe it as my right side. I don't seem to ever do the reverse. Peculiar.
I think you should watch the video / videos again. For the case of a bicycle wheel suspended on one side of the axle, the suspension point can be considered the nose with an upward pitch moment. In the case of the bearded fellow spinning the bicycle wheel, he was spinning it COUNTER clockwise as seen from the free end of the axle where the cockpit would be. I didn't like the video that was actually done by the bearded fellow because he describes torque as vector along the axis of rotation as DEFINED by the "Right Hand Rule". I remember this business from college physics and even more recently from helping my Daughter with her physics class. It is a good mathematical description but hardly intuitive. Here is the other video which shows much more clearly the actual direction of rotation:
View: https://www.youtube.com/watch?v=ty9QSiVC2g0
Now keep in mind that I am not stating that the aeroplane should pull to the right because there are a lot of other forces acting on it. I am just stating that gyroscopic precession should cause a starboard yaw with a clockwise rotating propeller as seen from the cockpit.
Thanks!
I needed that.
- Ivan.
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.
Cheers,
Wes
PS: Despite all your rationalizations, precession to the right flies in the face of experience.
Hello XBe02Drvr,
The Left / Right thing gets silly at times but not deadly. The absolute worst experience was giving a blind man directions to an empty seat when we were in a subway car. I corrected myself when I saw his reaction to my directions but in less safe circumstances it could have gotten very bad. I also had a friend at one work site that was blind (landmine in Vietnam) and life got interesting at times especially when a taxi dropped him off about a block away from where it should have. He got out of the taxi right in front of me as I was walking back to the office from lunch so he was lucky. I generally don't make mistakes if I am very careful as I was in that case.
One of the amusing things I found out by observation was that he could not actually tell the difference between Diet Pepsi and Diet Coke. He depended on the position of the button on the soda machine and didn't notice when they moved things around.
Regarding Gyroscopic Precession, it is just how physics works. I didn't invent it. If you play with a Fidget Spinner for a few minutes, you will see.
Gyroscopic Precession is probably a very minor force in relation to the forces that cause a yaw to Port.
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. Force from P-Factor can be broken up into a Radial Component in relation ot the aircraft CoG and a Tangential Component. Note that only the Tangential Force is really relevant in causing the aircraft to yaw.
- Ivan.
swampyankee
Chief Master Sergeant
- 4,031
- Jun 25, 2013
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
Thabks Swampyankee,
There had to be more to the story considering the differences between experience and simple calculations of the factors already being discussed.
Time to do some more reading.
At this point, there does not seem to be any sense to illustrating differences in Gyroscopic Precession due to nose le nose length; The differences would have been fairly small.
- Ivan.
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
Wes,
I have not followed this thread in depth so have a question or two based on your last reply.
The turboprop conversions all usually make way more power, which regardless of nose length would cause a greater swing at power up correct? I would think if power remained the same the longer nose would be more stable not less. Conversely a shorter nose would cause a bigger swing if power remained the same, correct?
Cheers,
Biff
