Neutral Point shift due to propeller effects

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Matt G.

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Neither Performance, Stability, Dynamics, and Control of Airplanes (Pamadi) nor Airplane Design Part 7: Determination of Stability, Control, and Performance Characteristics (Roskam), nor Perkins and Hage [The three stability and control-related books that I own) seem to have much of any useful information about the effect of thrust on the neutral point. My Stability and Control notes from college have a series of equations for the direct and indirect power effects on longitudinal stability, but unfortunately, the charts used for a couple of the terms are for a very specific WWII-era Hamilton Standard prop only (and came from some NACA report of the early to mid '40s), so they won't do me any good for the design of my scale T-28. How does one determine these things for a GA aircraft?
 

Tom Nalevanko

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I am not sure about the best way to go about estimating the neutral point but I can relate one estimation. When I built my Stallion, Martin Hollmann told me that the NP shifted back around 12 in. under full power. It is a 6 place high wing similar to a Cessna 210 with a 310HP prop on the front.

The plane is in my Avatar picture.
 

Matt G.

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Aft...12"? Wow...unless high wing vs. low wing makes a difference, I find that rather hard to believe, as everything I've seen indicates the Neutral Point shifts forward with the application of power, thus driving the aft CG limit. Otherwise it wouldn't be particularly important, and the aft CG limit would be defined by the stick-free, no-power NP. 12" also seems to be quite a bit for a plane with a 4'ish MAC.
 

Tom Nalevanko

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Oops I meant forward on the neutral point. I was thinking in a reversed manner vis a vis the CG; thus being a problem. Sorry.
Tom
 

orion

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Power definitely has an affect on stability and thus you have the neutral points often calculated with power on and power off (tractor generally being destabilizing). I'm not in my office at this point so I do not have the information in front of me but I do recall that when I wrote the original version of the design/analysis programs we use for this aspect of the development, I had to use four or five different references to in order to derive the complete set of equations and the variables that define them. The chief among these were Perkins and Hage, Hoerner, McCormick, and Torenbeek (you can look within the sticky reference thread in the aero section to get the book titles).

Regarding magnitude, I seriously doubt that the power effect on the Stallion is a 12" shift in the neutral point. Most conventional configurations generally run only in the few percent MAC range and given most static margins, are rarely a significant consideration, unless of course the plane's static margin is narrower than recommended.
 

Matt G.

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Orion-

Thanks for the reply. That's what I was afraid of...it seems that more and more often, there is no one (or two, or three, or four) book(s) on a given subject that covers it completely enough to perform a thorough analysis. I'll have to read through the pertinent sections of Perkins & Hage, and I just found a used copy of McCormick on eBay for a reasonable price, so I'll see how far that gets me. I thought my relatively extensive collection of textbooks from college would be enough to get the job done, but I see that's certainly not true...I've bought 7 more just this year, and I still don't have enough:lick:

This subject is of concern for me at the moment, as it appears, at least this early on in the design process, that getting the longitudinal stability of my scale T-28 to work out is going to be a bit of an issue. I need to refine some of the numbers I've used in my calculations, but so far I'm getting a stick-fixed NP of about 35% MAC. That brings me to another question: What is a reasonable minimum static margin? I have "SM = 0.2 for good stability" written in my stability and control notes from college, so that's one person's judgement/opinion. I can see I would have a hard time keeping the CG near 15% MAC and having this still look like a T-28, as both occupants and any baggage space I may be able to create will be aft of the CG. I think I now understand why there aren't many true-proportioned scale replicas out there...
 

orion

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A static margin of 20% is almost a lawn dart. A stick fixed neutral point of 35% is OK and is about average. However, see if you can tweak things a little - it's not uncommon to see neutral points in the 40% range, and there have been a few that came close to 50%. I'm guessing you haven't as of yet calculated your forward allowable limit, which just might be the 15% mark or thereabouts. If so, that would of course not allow you any CG shifts unless you allowed yourself to have a smaller static margin.

I've seen some planes fly quite well with a margin as small as 5%, although upwards of seven to ten percent is more common. The bottom line of all this is as you say, it is relatively difficult to scale a large, military type point design and convert it to something smaller and with more of a GA mission in mind. Virtually all good ones will be a bit off scale on both, the vertical and horizontal tail areas to make things work just right.
 

Aircar

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How about neutral point for directional stability ? and power effects theron --given that the vertical surface is usually assymetric unlike the horizontal tail and loss of directional control from the yawing effect at high angle of attack on the prop (gyroscopic and unsymmetric ) --as I understand it the first boundary with power effects on high powered WW2 fighters (and T 28 type ) was a roll/yaw departure due to vertical tail stall (roll on their back if the throttle opened too fast at low speed high aplha )

We once had a US Navy T28 come into Williams County airport (Bryan OHIO) to pick up a tiny part for a HP glider kit that one of the personnel was building at Patuxent River naval air station (might have misspelt that ) --they mentioned that they towed gliders with it !!
The radial engine choice for a scaled homebuilt version might not be that large I guess --are you aware of the Australian Rotec radial in this regard ? Maybe you need more power --the Russian radials or something else -- radials are really civilized piston engines and just nice to fly behind ( I used to hitch rides on DC3s and a Beech 18 when at Essendon airport in the 80s --we had a dozen or more DC3s still running freight or tourist flights, two Bristol freighters (Centaurus sleeve valve radials) and a few oddballs like a DC4,Avro Anson and some other radial powered aircraft --all sounded so much softer than anything else )
 

Matt G.

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How about neutral point for directional stability ? and power effects theron --given that the vertical surface is usually assymetric unlike the horizontal tail and loss of directional control from the yawing effect at high angle of attack on the prop (gyroscopic and unsymmetric ) --as I understand it the first boundary with power effects on high powered WW2 fighters (and T 28 type ) was a roll/yaw departure due to vertical tail stall (roll on their back if the throttle opened too fast at low speed high aplha )

We once had a US Navy T28 come into Williams County airport (Bryan OHIO) to pick up a tiny part for a HP glider kit that one of the personnel was building at Patuxent River naval air station (might have misspelt that ) --they mentioned that they towed gliders with it !!
The radial engine choice for a scaled homebuilt version might not be that large I guess --are you aware of the Australian Rotec radial in this regard ? Maybe you need more power --the Russian radials or something else -- radials are really civilized piston engines and just nice to fly behind ( I used to hitch rides on DC3s and a Beech 18 when at Essendon airport in the 80s --we had a dozen or more DC3s still running freight or tourist flights, two Bristol freighters (Centaurus sleeve valve radials) and a few oddballs like a DC4,Avro Anson and some other radial powered aircraft --all sounded so much softer than anything else )
Yes, I'm well aware of the Rotec; in fact, I'm designing this around their R3600. I have a thread for it in the 'Warbird Replica' section. If I can't make it work with the Rotec, then this project will not get built, as I can't afford one of those Russian radials (or their thirst for fuel compared to smaller engines).

I haven't started the lateral/directional stability calculations yet...I'll do that after I get the control stuff added in to my longitudinal stability calculations. Then I'll have to go back through everything and start trying to determine more realistic numbers as the design progresses for certain parameters that are not well defined at this point.
 

Matt G.

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Well, my copy of McCormick showed up, and the section regarding power effects on longitudinal stability didn't really tell me anything I don't already know, but the book seems to still be worthwhile, though- there's a lot of other good information in there about things I haven't even thought about yet.

So for the time being, I'm using the data for that specific Hamilton Standard prop in Perkins and Hage for lack of anything better. I would like to get data that is more pertinent to the aircraft I am designing, but for now I feel I can get away with using these charts because a) the prop used to generate those charts is probably very similar to what a T-28 was originally equipped with, and b) my scale T-28's prop would have thinner blades than a real T-28 prop scaled down to 70%, so in using this data, I am slightly overestimating the propeller effects. Somebody tell me that's a bad idea if it is.

In doing that, I have calculated a NP shift of about 1.8%MAC forward for a windmilling prop, which seems reasonable. My calculated full-power shift is nonsense though...22% AFT. That cannot possibly be right. Either I've got an unrealistic number in my spreadsheet, or there's an error in the calculation, or I need to find an alternate method of calculating this quantity to check it. The equation I'm using is out of my stability and control notes from college, and seems to be my professor's combination of a couple different methods. I can post it if someone wants to see it. There appears to be a massive dependence on the vertical location of the prop with respect to CG, and even varying that number slightly (6" up or down) drastically changes the calculated NP shift. Hopefully someday I'll figure this out.
 

Norman

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Shouldn't the pitch do to the thrust line offset from the center of mass only show up during acceleration? IE if a mass is not accelerating it doesn't produce a force
 

Aircar

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For a tractor design pulling power off and prop airbraking will reduce the dynamic pressure at the tail which should move the neutral point forward --angling the thrust up wards will tend to reduce the amount of nose up trim required which lowers the stick force gradients (less stable)and NP forward --dynamic effects of power on acceleration would be additional to the aero'static' ones as Norman surmises and the swirling flow around the fin in particular seems to be the critical one for these sort of aircraft (the wing in front of the horizontal tail will remove some of the swirl )

BTW why select the T28 to scale down ?
 

Matt G.

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Shouldn't the pitch do to the thrust line offset from the center of mass only show up during acceleration? IE if a mass is not accelerating it doesn't produce a force
No...thrust is an aerodynamic force, not an inertial force. Ignoring the propwash effects Aircar is talking about, if the thrust line is offset vertically from the CG, it will create a pitching moment about the CG that must be balanced out by the horizontal stabilizer, and that moment will vary with the amount of thrust, so when changing the thrust, there will be a pitch change, and the aircraft will need to be re-trimmed after the thrust change is made.

For a tractor design pulling power off and prop airbraking will reduce the dynamic pressure at the tail which should move the neutral point forward --angling the thrust up wards will tend to reduce the amount of nose up trim required which lowers the stick force gradients (less stable)and NP forward --dynamic effects of power on acceleration would be additional to the aero'static' ones as Norman surmises and the swirling flow around the fin in particular seems to be the critical one for these sort of aircraft (the wing in front of the horizontal tail will remove some of the swirl )

BTW why select the T28 to scale down ?
That probably explains why the T-28 has the engine tilted downwards 5 degrees. From what I can see in my 3-views, it also has the hinge line for the elevator rather far aft, which I think would also reduce the stick force. The other thing that's killing me right now is an apparent 15% forward shift in NP when calculating stick-free stability. If I play with the elevator geometry a bit (move the hinge line forward so I get Chalpha and Chdelta coefficients <0 and reduce the elevator chord and span slightly) I get a more desirable 7.5% forward shift.

As far as 'why the T-28', read my initial post in the thread I started for the project in the Warbird Replica section.
 

orion

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Just to give you a double-check value, the low wing twin we're prototyping right now has a 2% NP shift (forward) with power. And yes, the stick free neutral point can shift by quite a bit. For the same twin, the stick free NP shifts from just a hair past 50% (stick fixed) to 43.9%.
 

Matt G.

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Orion-

Thanks! Is that 2% shift on top of the windmilling prop, no power shift? I am tempted to assume a forward shift of 5% or so with power application for now. I have spent most of the day digitizing charts and updating numbers in my spreadsheet. I have obtained a slight improvement of the stick-fixed and stick-free neutral points, and I clearly need to determine the vertical distance from the CG to the thrust line as accurately as possible, as varying that slightly makes the full-power effects go from nonsense to reasonable. In my spreadsheet I am seeing that the direct effects are destabilizing, and the indirect effects are stabilizing for this configuration and my numbers. Does that make any sense at all?


Now the forward CG limit is killing me as well; I'm currently getting 21.4%MAC out of ground effect and 35.1%MAC in ground effect, so I have a non-physical CG range :( The difference between those two seems ok, but the out of ground effect limit seems a bit far forward. I suspect I'm going to have to enlarge the horizontal stabilizer somewhat to get all of this to work.

I'm starting to consider designing a sailplane and building that as a first project, as I can throw away all the power terms in my stability and control spreadsheets, and practice my sheetmetal skills before building the scale T-28.
 

orion

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Some of your numbers don't seem to make sense, especially that forward CG limit. Even with everything considered, most forward CG limits are well forward of your values, often coming in somewhere in the teens percent MAC, in GE. In this case there's really no need to look at free air values since the critical envelope is set by the operations near the surface.
 

Matt G.

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Yeah, the forward CG limit is total nonsense at this point; the Perkins and Hage method gets me 19.7%MAC out of GE, and a ~10% aft shift for in GE; another method from my college notes (which most likely came from Pamadi, but I need to double-check the source) is giving me -13.1%MAC (!) out of GE. Neither one of these makes sense, although the shift for out of GE to in GE seems believable. I'm only looking at 'free air' values because the equations in all of my resources first calculate the forward limit out of GE, and then correct it for in GE.

In an attempt to determine what is going wrong with the full-power NP shift, I plotted CL vs. NP shift at full power, since according to the equation I am using, this is a function of CL, AOA, Thrust coefficient, and horizontal tail CL. I separated the direct effects and indirect effects so I could see what was driving the odd values. It seems that the direct effects are highly dependent on the vertical distance between the thrust line and CG; since I don't have an accurate number for this value, I'm considering it to be zero for now, as it looks pretty close to that by inspection. Consequently, the direct effects result in a relatively constant 4%MAC forward shift throughout the entire CL range (0.25 to 1.8). The indirect effects vary widely from a ~25%MAC forward shift at CL=0.25 to a 35%MAC aft shift (?!) at CL=1.8 and seem to vary directly with the horizontal tail lift coefficient. My tail lift coefficient values do not make sense, either, as at cruise CL of about 0.3, CL_tail = -0.175, but at CLmax (1.8 is my preliminary value at this time) the tail CL=0.56, which is an 'up' load on the tail, which makes no sense. This explains the positive stick force gradients I have calculated, but I'm really confused as to how/why there is an up load on the tail at stall when the aircraft has a positive static margin. I suppose there is probably an error somewhere in my spreadsheet, which is not likely to be fun, easy, or quick to find. :(
 

orion

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Just a quick note: It is entirely feasible to have the free air forward CG limit to be in front of the leading edge, completely off the wing. Most of my designs have that condition but as you said, once you account for everything else (ground effect, prop position/power, flaps, tail size, etc.), that number often shifts aft. Just out of curiosity, what type of flaps are you using, how much span do they cover and what is your tail volume coefficient?
 

Matt G.

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Let's see...currently planning on plain flap, covering about half the span, and the tail volume is 0.56.

I really appreciate all of your help and expertise...the more I learn with this, the more I realize I don't know.
 
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