stek79

Hello guys,
I have a question for the most expert here!

This is something that has been asked on pattern forum some time ago, but no replies were given... where are the experts? [&:]

The question was: there is a tendency to design pattern plane with high thrust line placement (if we see the thing from the side) and with low stab... anyone has ideas about that?

I tried an explaination, which is the following: perhaps the designer wanted to split the later surface of the fuselage in two partitions, such that they are of the same area. Why? If this could be possible, no right thrust (or, at least much less) could be setup, with the advantages that this brings.

Am I on the right track?

I have another question about spiraling slipstream, but first let's see if someone can help with this first question!

Thank you all and MERRY CHRISTMAS!

rgunder

If I am understanding your question correctly, you are asking: a) Why is the thrust line high relative to the vertical location of the CG? and b) Why is the horizontal stab mounted low relative to the vertical location of the CG? Is that right?

Right up front I will admit that I am not familiar with pattern competition rules and to what degree the rules drive the design. That being said I have some general observations. The first and foremost is that the predominance of similarity among designs in the pattern class, or any class for that matter, is driven at least in part by winning designs and the "If it isn't broke, don't fix it" ideology. What I mean by that is that if an airplane is winning in competition, there will be a good number of designs that follow that use similar proportions/layout. This can be seen in most areas of competition.

The second thought that comes to mind is related to the high placement of the thrust line. With the thrust line placed high, reductions in power will result in a nose-up pitching moment. Again, I do not know whether or not that has some benefit in pattern competition.

It is not much, but maybe a good starting off point for discussion of the topic.

stek79

Thank you Richard, and yes, you are understanding correctly my questions.

I too think that the designs are driven by top pilots, but if they do something in their airplane designs perhaps there is a reason...

multiflyer

rgunder explained the effect of high thrust placement correctly. But thrust direction has a great effect too. I don't see a tendency to place the thrust lines up high On pattern planes. They typically try to keep it centered. The low horizontal helps inverted flight trim. If the horizontal is right in line with the wing then it sees the same downwash from the wing during upright or inverted flight. The downwash is always helping the nose up. This decreases the elevator trim difference between right side up and inverted flight slightly.

Multiflyer

stek79

To better explain the "high thrust line" on new design, take a look at last CPLR design:

http://www.braeckman.de/Bilder/F3ABilder/oxalys2.jpg

The thrust is not centered anymore on this new plane, but actually as you say the stab is pretty much on the wing line... ok! I always thought why designers didn't put stab on the same line of the wing, since in an aerobatic plane upright or inverted behaviour should be as close as possible...

Thank you for your imputs, the issue is getting more clear. Two more things I would be glad to know better:

1) My theory about "side area splitting" has some sense?

2) Why, in your opinion, we are going from a mid-wing design to a more low-wing (see oxalys above...)?

Thank you guys again!

multiflyer

That’s a nice looking plane. One thing to keep in mind is that small raising/lowering of wing, thrust line, and such does not have a very large effect at all. Often placement of things depends on other practical considerations. A mid wing might be more trouble to build if the design is using a one piece wing. Plug on wing panels are easy to locate anywhere. But one piece wings can be built lighter weight for the same strength. One piece wings attach to fuselages in the low or shoulder locations. Fuselage is lighter and stronger if the wing saddle is not cutting deeply into the side area. The plane in the link you gave no doubt has a relatively high thrust line because it makes the front styling look good. Inverted engine fully enclosed with good fuselage lines. Lowering the engine would make the cylinder stick out the bottom, or would require turning the engine sideways. Changes due to engine location really become significant for much larger changes like a pylon mounted engine. Basically all pattern or IMAC or 3D designs have the same general positioning.

I am not understanding your question about side area. Basically side area manipulation effects knife edge flight, yaw stability, and roll coupling with yaw. More side area forward makes it easier to maintain level flight in knife edge, but detracts from the others if not distributed well. Here again they have a need to keep the plane looking good too. Human nature is human nature. It's just a fact that planes that look great fly better, even if they might not handle quite as well. As long as a really good looking plane doesn't behave noticeably worse, then it's better. We all know how that goes.

Multiflyer

Rotaryphile

Ideally, the thrust line should be close to the center of drag, so that no downthrust (or upthrust) is needed to prevent pitching up or down when the power is increased or decreased. Side area is very important in overall handling, since it damps yaw. Increasing the vertical tail area will make any airplane handle better when it is close to stall, but side area up front is also important, since the vertical tail needs area up front in order to develop the aerodynamic couple that is needed to resist yaw. If the fuselage has little side area, inertia steps in to supply the force couple, resulting in tail wagging, particularly noticeable if rudder is not handled just right at the exit of a stall turn. I did a lot of experimentation with adding a lot of side area up front, in the form of little vertical airfoils attached to the wings, and covered cabane struts on bipes. I found that it was possible to get virtually hands-off knife edge (no rudder input at all), but overall handling became a bit strange, with the airplane not tracking well in yaw. This is really just the equivalent of a tail-heaviness in pitch, transferred to the yaw axis.

I used to put the horizontal tail in line with the wing, but have run into a problems with two or three airplanes with such layouts. The symptom is an infrequent, uncommanded sudden pitch up or down when flying at a certain lift coefficient. I think that the problem is that the small burble that trails back from the wing may flip from one side to the other of the horizontal tail, resulting in a sudden small change of tail lift. I fixed the problem by increasing wing incidence by maybe half a degree; slightly decreasing the incidence would probably also be a fix. I now put the horizontal tail at least half an inch out of line with the wing, and the pitch problem has never recurred.

stek79

Good infos guys!

Multiflyer, my theory about side area is the following:if we look at the airplane from the side, let's draw a line on the thrust line. There will be some surface above the line (rudder fin tipically), and some surface under the line. If the two amount of area are equal (actually a weighted sum must be performed, in order to consider the distance of every piece of area from the CG) then we need no right thrust, since the spiraling slipstream does not produce the tipical yaw moment that requires right thrust. I'm sure about that last fact, since I've flown a depron biplane with a symmetrical fuselage around thrust line, and it didn't require any right thrust.

That will be a good result, since one of the most difficult trimming to do on a pattern plane is the yaw axis...

rmh

Build a lot of flat foamies with more side area above wing-then some with more area below wing -
Having tried many of these - it is now pretty clear that best setup is with more area above wing -not much- but as opposed to below wing -the overall setup is better .
On flat invereted spins -the stability was always better -so was flat turning
also keep lateral area as far forward as yaw stability permits - the model will do rudder commands with far les input -
theorize all you like - then put it into practice .
The results are what really count
Pattern planes - I have had much experience with - and for all the maneuvers this same setup works very well.
If the thrust line is a bit above wing -no big deal .
The biggest problem on Pattern design is getting CG correct for speed the plane is flown.

multiflyer

stek79,

I think I see what you are getting at but I think you have the wrong sense of the distribution?

Spiraling airflow around the fuselage presents itself to all protruding surfaces making them all see an angel of attack. This in itself is actually good. The protruding surfaces work like stator veins straightening the prop wash. The reaction is a small rolling moment towards the direction of prop rotation. This helps stay the engine torque. Prop rotates right, airframe torques left, stator action twist right.

The imbalanced vertical area in the back is what causes the yaw problems. A slight yaw is produced from the vertical fin because it only sticks out on the top side. A right rotating prop slip stream pushes top mounted fin right causing some left yaw. An equal sized sub fin is what would eliminate this. Sub fins aren't popular because of ground clearance during takeoff and landing. Full size single engine plane designs often mount the vertical fin offset left slightly for this reason. A small amount of right engine thrust can help with this, and works when flying inverted too. Thrust points the other way but vertical fin is inverted and getting pushed the other way too. Note that multiengine planes with wing mounted engines do not yaw in reaction to spiraling prop wash. Area getting torqued on is balanced.

Spiraling slip stream is only one of the propeller effects that right thrust can help. The larger reason for right thrust is P-factor at slow speed. If the prop disk is not exactly perpendicular to the air stream, the blades will see different AOA on opposite sides of the disk. This shows up the most during slow flight. Nose high attitude tilts the prop arc back causing the descending blade (on the right) to take a bigger bite out of the air, and the rising blade (on the left) to take a smaller bite. Basically offsets the center of thrust to the right causing a yaw to the left. Right thrust compensates. As speed builds and nose lowers the effect reduces and the right thrust eventually becomes a nuisance, but aerodynamic stability is much greater now. So, adding more right thrust is a good compromise – but for flying right side up only. During inverted flight the right thrust is now left thrust but the P effect is still the same.

This is why aerobatic planes usually have just a little or no right thrust, while trainer and sport planes usually have noticeably more - they just spend more time right side up and are optimized for that.

A better solution of course is counter rotating props or pure jet power. There is no way to make one prop perfect.

Multiflyer

stek79

Multiflyer,
with all the respect, you are wrong.

It is well known why right thrust is introduced, in this is ONLY for spiraling slipstream and NOT for P-Factor!!! If you are not convinced about that, I can give you a link of a good explaination of a well known aero engineer, which states that very clearly.

Also, it is wrong also the fact that aerobatic planes have usually no right thrust: my pattern plane, which is designed by one of the F3A top pilots of the world, has FOUR degrees of right thrust. Obviously, you could build it without right thrust, and there are well known pilots that do that (namely, Chip Hyde), but they introduce another trick to compensate for spiraling right thrust.

I agree about the contro-rotating props though!

Dick, I appreciate your input, from your experience when such a side area distribution is constructed, it can be said that less right thrust is required?

Again, thank you all guys.

rmh

The thrust offset-my experience-can be reduced if more power is available - all thrust corrections I have tried - are only needed to compensate for change in trim as speed changes.
This may sound odd -but I will restate it
if speed remains constant - trim and forces remain much the same
also the ligher the model wing loading -the less offset is needed
So I elect to use very little offset and simply set rudder as needed -which is very litle
On the downline prop speed changes ( from pulling to braking) you can mix throttle to rudder offest .
The lateral area distribution helps most in any yawing maneuvers - only tiny yaw produces quite a bit of lateral force
It all goes back to basic : least weight with most power --allows constant speed - with least amount of trim and or power change needed to hold this speed in any direction up /down/ 45 etc..
So-- a light , relatively draggy model with lots of thrust is going to "stay the course" best.
Draggy does not mean a big front on the model that is just silly- typically more flying surface area is best way to add parasitic and induced drag..
If wishes were fishes -a little parachute being towed along would be workable ---(drag placed further aft )

stek79

Dick,
I agree with the fact that trims are speed dependand, and this is the key problem IMHO. If it would be possible to reduce right thrust with some almost-symmetrical fuse lateral surface, then no rudder trim and such moments on the yaw axis, which could let the plane fly the same at slow speed - max throttle or max speed - idle throttle... at least this is my understanding, if I'm wrong please tell me!

From what I understand, there is also a tradeoff on the yaw axis, between stability and good KE behaviour: the former requires a more aft surface, the latter a more forward one. Am I understanding you correctly Dick?

stek79

Since we are on the topic, I might add another question.

Often I hear some top pilots speaking about good pattern airplane behaviour in the wind. Another thing to consider when desinging side fuse surface I think... but what is a good windy behaviour? I crosswind, the plane should point its nose into the wind, in order to compensate it? Or should remain neutral?

rmh

careful-the plane can't see any wind.
what YOUinputto hold against drift tho is easily seen -I far prefer almost unstable setup as the model is easier to hold in that slight yaw.
you have to cut n try till it feels right for you but in wind the best fix -fly faster -if possible

multiflyer

Hello all, Hope everyone is having a great Christmas day.

Stek79,

Thanks for the respectful reply, and the challenge to my input. After all, discussion is why we are here. Good stuff. Please post the link to the engineering explanations to which you refer. I am interested. Meanwhile let me clarify my post.

2 points to begin with.

First is the overall design aim. There are 2 different directions involved here - optimizing for right side up flight only, and optimizing behavior while trying to preserve the same good behavior flying upright as well as inverted. Aerobatic designers vacillate between these. One argument is that although inverted maneuvers are performed, the majority of flight (takeoff and landing at the very least) follows the upright sense, so an upright design bias is better. Piloting is made easier for majority of maneuvers at the expense of more corrections required while inverted. If equal emphasis is placed on inverted handling, design elements tend toward only those that help upright and inverted equally. Piloting requires some corrections but piloting technique becomes the same each way. Also the sequences flown in different competition classes influence this. Classes have varying degrees of emphasis on inverted performance. In my above post I tried to differentiate between these design intents.

Secondly I did not say pattern planes use no R thrust. I said they typically use less than trainers or some sport planes. Basically due to the above reasoning. I think your example of 4 degrees does not represent the norm? Note Dick Hanson’s comment from his post above: “I elect to use very little offset and simply set rudder as needed…”

Now regarding P factor. I say right thrust does help to compensate for this. Paraphrasing my above: “for props spinning clockwise, at high AOA thrust is shifted right, yawing nose left. Right thrust line angle pulls nose back right.” Simple as that. Please explain how this is not correct?

Also note that “high AOA” means “towards the sky” whether upright or inverted. High AOA towards the ground, whether upright or inverted, reverses P effect. And (only while flying upright) down thrust helps compensate for (positive) P effect at high (towards the sky) AOA even more by zeroing out blade AOA differences. So this is part of why R and down thrust are common on trainers and such. Trainers are “trainers” because they incorporate design features, like R and down thrust and many others, which make them easy to fly right side up. Consequently trainers don’t fly as well inverted.

Regarding right thrust and spiraling slip stream for aerobatic designs. And here I mean a design that will fly as well inverted as right side up. Specifically from my post above: “A slight yaw is produced from the vertical fin because it only sticks out on the top side. A right rotating prop slip stream pushes top mounted fin right causing some left yaw. A small amount of right engine thrust can help with this, and works when flying inverted too. Thrust points the other way but vertical fin is inverted and getting pushed the other way too.” You stated that “It is well known why right thrust is introduced, in this is ONLY for spiraling slipstream and NOT for P-Factor!!!” I disagree. Explanation by example as follows:

Envision a perfectly symmetrical design. With a right hand rotating propeller set at Zero degrees R and down thrust angle. Thrust line, wing, and horizontal tail, all exactly on centerline. Fuselage and vertical fin mirror images above and below (emphasis on an equal size vertical fin below centerline also). CG at the neutral point.

Begin in level flight at cruise speed. The aircraft is of course pitched up slightly to produce the needed lift. Will the aircraft tend to yaw left slightly? If yes, then due to P effect or spiral slip stream?? Explain how??? (I say yes, and due to P effect as I noted above)

Now introduce some right thrust - for whatever reason. Then roll to inverted level flight at cruise speed. What is that right thrust doing now? This illustrates the problem with right thrust for “aerobatic” design.

Multiflyer

stek79

Multiflyer,
thanks for the wishes, I had a nice Christmas, and you?

Now back to the discussion...

I have to thank you for your last example, I didn't think about that situation: you're right, in this case the spiraling slip stream doesn't produce any yaw, so let's eliminate it from our thinking. I understand your point and I agree with you that in this case R thrust would hurt, and in fact with such a design no right thrust would be needed, otherwise you can get bad yawing while inverted, as you correctly pointed out. Ok. To support this, the symmetrical depron biplane I've flown (I cited it in the previous posts) didn't have R thrust built. Ok, it will experience some yaw due to P-factor (again, I agree with you), but this can't be corrected with right thrust so - no right thrust. To summarize, we should not use right thrust to correct P-factor!


Now, regarding right thrust in patterns and my 4 deg... I don't think I am an exception with respect to the general airplane setup. I see right thrust built into all pattern planes I've seen and heard of. For example, let's look at the Focus II pattern airplane. The great Don Szczur, which has been 2003 FAI USA Champion with that plane, started in the past a thread about its building. From that thread, it is stated that the airplane kit comes with 3 deg of right thrust (and 1 of down):

"The good news is that the angles are all there for you. If you slide the pre-assembled piece into the slots it will give you the correct side and down thrust . Take care to put it in the right way up. The plane is supplied with 3 degrees side and 1 degree down."

Ok. Guess what Don did with respect to right thrust? It added a bit more! If you want more infos, here Don is also suggesting to modify engine mount in order to give more right thrust:

http://www.rcuniverse.com/forum/m_14...Cthrust/tm.htm

That said, I'm still convinced that right thrust should be reduced.. . in fact I started this thread because I was thinking that with fuse mods the slipstream effect could be reduced -> and so right thrust. Nevertheless, it is needed if we want to draw straight uplines. We could certainly reduce it in order to avoid bad yaw in inverted and during corners, but then the uplines... where do they go??? Ok, ideally we could correct them with rudder, but if the champions put right thrust... I think it is better to have hands off uplines and then correct during corners.

Finally, the promised link, a VERY VERY GOOD reading by George Hicks, an aero engineer of Gulf Stream, that also has a forum here on RCU (unfortunately it seems he is not present anymore on the forum [] - too bad!!!!!!!!!!!)

http://www.horizonhobby.com/Explore/...ArticleID=1101

Good reading!!! After that, I have another interestig how-to written by him...

multiflyer

stek79,

I read the article by George Hicks on the Horizon site you posted. Basically he says the points I made just about verbatim. So I am not clear where the disconnect is? I did find 2 statements from George's writing that may be causing confusion??

Your original question was about high thrust line possibly reducing yaw from spiral slip stream. To which I basically answered right idea but wrong distribution. Raising thrust line to get equal side area above and below is not enough. As I wrote above "The imbalanced vertical area in the back is what causes the yaw problems." Equal area above and below is needed at the tail - back where a significant lever arm exists to force a yaw. Area below centerline located forward will not balance the vertical fin above located rearward. So basically I'm saying since fuselage area is near and about the CG it has relatively little moment arm to produce yaw. Vertical fin area located way in the back is at a large arm and side force here will create more significant yaw. I'm saying the vertical fin area is what must be balanced off to eliminate yaw from slipstream spiraling, and raising the thrust line won't accomplish this. Adding a sub fin, at the tail, of equal area, will.

Regarding Right thrust and P effect. I think what threw you off is my statement above: "The larger reason for right thrust is P-factor at slow speed." George’s states "The spiral slipstream is also the ONLY reason we need to put right thrust in our engines." George's statement is a little strong and needs to be taken in context. He goes on to say regarding spiral slipstream "This effect is pretty much the same whether the airplane is flying upright, inverted or on its side. It always tends to make the nose yaw to the left and to compensate for this we offset or angle the engine's centerline." He is speaking here from the perspective of a compensation that works equally for both upright and inverted. When he says it is the ONLY reason Right thrust is NEEDED, he is referring to "needed" for an "aerobatic" type design.

I am saying that “additional” right thrust, beyond the amount to compensate for spiral slipstream, can be added to compensate for P factor, but in the positive direction only. In this regard I am speaking of “additional” benefit for right side up flying only. Regarding P effect George writes: "This uneven disc loading produces a nose-left yawing moment when the disc is at a positive angle of attack and a nose right yawing moment when the disc is at a negative angle of attack (for a clockwise rotating propeller as seen from the cockpit). Since the direction of the yawing moment changes from upright to inverted flight we should not correct for this effect with right thrust." Obviously George is speaking here to the design objective of preserving equal upright and inverted behavior.

My point is that altering the thrust line further, can be used to compensate for P effect also, or any other effect for that matter. But this type of correction is specific to a certain flight condition only. The condition to which I referred is High (positive) AOA. The biggest example that we all must deal with is slow flight. I'm saying adding "additional" right thrust is a good compromise for right side up flying only to compensate for P effect during slow flight, strong pull ups, and steep climbs. Above I wrote "P effect...Basically offsets the center of thrust to the right causing a yaw to the left. Right thrust compensates. As speed builds and nose lowers the (P) effect reduces and the right thrust eventually becomes a nuisance, but aerodynamic (directional) stability is much greater now. So, adding more right thrust is a good compromise” – but for flying right side up only. During inverted flight the (additional) right thrust becomes left thrust but the P effect is still the same.

When I wrote that P effect is the larger reason for right thrust. I might more clearly have phrased that compensating for P effect also requires a larger "application" of right thrust. I mean to say that the majority use of "extra" right thrust, for all types of models, is to make them handle better from slow to cruise speeds, flying right side up only, since this is where the majority of flying happens, by all pilots of all skill levels. An aerobatic design is a specific application and the more appropriate amount of right thrust there is enough to compensate for the spiral slipstream only and no (or little) more.

Bottom line is this. A larger amount of right thrust improves upright handling at the expense of inverted handling. A smaller amount of right thrust benefits upright and inverted handling equally, only for designs that have imbalanced vertical fin area. An equal sized sub vertical fin will eliminate the need for any right thrust compensation of spiral slip stream. Right thrust remains useful to compensate for positive P factor, at the expense of all situations that generate negative P factor. Because our planes don’t have large sub fins at the tail, about 1 or 2 degrees of right thrust is optimum for aerobatic/pattern design in order to preserve upright and inverted handling equality. When around 3 degrees or more is used, the up lines become easier, but a tracking difference between inside and outside loops starts to develop. Also a tracking difference between upright and inverted 45 degree climb lines begins to develop. Airplanes that will fly right side up only benefit from larger amounts of right thrust. With lesser right thrust, planes yaw left during steep climb and straighten out at faster speed. With larger right thrust, planes can be made to fly straight during steep climbs and yaw right at faster speeds. About 4 to 6 degrees of right thrust produces a good compromise.

Last thing to note, since yaw from spiral slip stream increases as speed decreases, it acts the same as positive P effect at slow speeds and during steep climbs. The magnitude of the P effect by itself is made clear to multiengine pilots experiencing the difference in engine out performance between left verses right engine out condition (non counter rotating props).

Does this close the gap?

Multiflyer

stek79

Good points Multiflyer, thanks. I agree with everything, but one thing. We were both correct, it seems!

Or better, I would like to ask you guys this thing, about which I would like to learn more. Multiflyer, in the first part of your post, you say that high thrust line doesn't solve the "spiraling problem" , because of the difference between lever arm in the distribution: I totally agree. I said infact that a weighted sum must be computed (area * lever arm). You said also that a subfin would do the trick, ok...

But, the question that I have in mind is:the spiraling slipstream is centered on thrust line (which I think) or on the stab line?

If it is centered on the thrust line, which seems to me natural, then if you look at Oxalys tail, you'll see that it is almost symmetrical, with a fin and a sub fin (here with sub fin I mean the tail part under the TRHUST LINE)! This is what I thought at the beginning with my question. In other words, ok that we have to look at the tail since it has a greater lever, but with high thrust line the tail can be splitted in two almost equals surfaces! Do you understand what I mean?

Ok... this seems to be reasonable to me, but we have to take into consideration the stab also, since it can disturb the spiraling slipstream and perhaps it can hide the subfin under it (here I mean under the STAB) ? If this happen, then the stab should be placed higher in order to yeald a more symm tail, but that perhaps can hurt other flight capabilities?

Thanks all, good discussion guys.

LouW

Your description of the aerodynamic forces is correct. And your explanation of left turning tendencies is seen in many books, however it simply isn’t true. The problem is that the rotating propeller is a gyroscope rather than a stationary surface. A force applied to the rim of a gyroscope always appears as a reaction displaced ninety degrees to the applied force. Thus the left turning force applied by aerodynamic force on the propeller disc produces a pitching moment at the hub. P-factor is real but gives a pitching moment not a yawing moment on the airplane. The left turning tendency of many airplanes when operating at slow speeds and at a high pitch angle is due to propeller swirl, period. Arranging side area can reduce or eliminate this if desired.

rmh

The "theories" on this subject are endless and mindless- In playing with my small electric models -I can feel the the twist and pull forces from the props -easily - we hold em lightly with two fingers with power set just above hover - it is a very easy and informational tool.

multiflyer

Stek79,

Gap closed on slipstream. Area distribution is the question. Slip stream does not center on stab line. Slipstream “is what it is” by the time it reaches the back end of the plane. I do not think the spiral necessarily centers on the thrust line either. I think the airflow might tend to use the fuselage centerline as a core to revolve around. I have not seen wind tunnel tests or similar on this. The difference points to where to distribute side area and your original question. I suspect that fuselage side area forward works like stator vein to straighten out flow. This is the reason for my disagreement with your suggestion of a weighted sum calculation. Look at it like the fuse modifies the spiral flow, and the vertical tail reacts to the result. Your idea of high thrust line splitting flow about existing vertical fin is actually correct with no fuselage in-between. I have flown many pusher designs with tail mounted on a small dowel located very low, usually out of the prop flow. When the motor is high compared to the fin they do yaw right instead of left. The H-stab straightens flow also, but I think too far aft to matter.

For a tractor prop on a normal fuselage, I think high mounted thrust leaves little fuse area above to straighten prop wash across top of fuselage, resulting in larger cross flow hitting the vertical fin. The implication of this reasoning is a lower mounted thrust line will leave more fuselage area above to straighten flow over fuse top, lessoning the cross impact on the vertical tail. Running an enclosed exhaust above the fuselage within a long cowling would increase upper fuselage area even further. Additionally, the small fuselage area below would leave greater sideways spiral underneath. A counterbalancing sub fin would become more effective and not need to be as large. Interestingly, what I have just described is the milestone pattern design lineage begun by a guy named Joe Bridi with his Dirty Birdy. Compare to his earlier Chaos series. What do you suppose influenced these changes?

It would be good to experiment with shapes in the way dick Hanson mentioned above. Cut out various slab/profile fuselage test shapes, mount an engine in front, and hold them while running to evaluate. Results should be very telling. Essentially this is a wind tunnel test outside of a tunnel.

multiflyer

LouW,

Gyroscopic forces are another thing to contend with. Your description is correct but I think not complete. Regarding P effect you stated "A force applied to the rim of a gyroscope always appears as a reaction displaced ninety degrees to the applied force. Thus the left turning force applied by aerodynamic force on the propeller disc produces a pitching moment at the hub. P-factor is real but gives a pitching moment not a yawing moment on the airplane." I think it is more correct to say P factor gives a yawing moment, and if the aircraft is allowed to give into this yawing moment (actually yaw), then and only then does the gyroscopic forces come into play producing the pitching forces you describe. Gyroscopic stuff only happens when the gyro axis is allowed to react to an applied force. If heading is held straight, there is no gyroscopic reaction.

At high AOA such as in steady state level flight at slow speed or in a steep climb like a 45 degree up line, (+) P factor is happening (unless there is down thrust that zeros out the prop disk at this AOA), therefore some additional R rudder or R thrust is needed to hold heading (in addition to that needed for the spiral slip stream effects of course). It is only when sufficient R correction does not exist that the plane actually begins to yaw, and gyroscopic reactions are generated.

Specifically the difference is this. Under your description of how P effect acts, some additional “elevator” trim is needed to fly straight during P factor generating condition (such as slow level flight and climbing). Following my description above, some additional R rudder (or R thrust) must be held to fly straight during P factor generating conditions. So, respectfully, your statement that my explanation of left turning tendencies as seen in many books “simply isn’t true,” just simply isn’t true, in my humble opinion.


To all,

I am sensing a general discounting of P effect. I caution against this. P effect is very real and significant. For example, imagine a variable pitch propeller, which varies in such a way as to allow the pitch to increase only on one side of the shaft and to decrease only on the other side. Essentially this is mechanically introduced P effect. This is known as cyclic pitch control, and is Igor Sikorsky’s great insight which made the modern helicopter possible.

Multiflyer

stek79

Multiflyier,
I understand your points about fuselage as a core stator... I found a picture of the Dirty Birdy, interesting plane! It can be said that it requires less right thrust? Did you fly one?

mesae

I believe it is very important to consider the position of the thrust line in relation to the "vertical" position of the center of gravity. With a given airplane, moving the CG closer to the thrust line will decrease the amount of pitch response that occurs AS the power setting is changed, before equilibrium is regained.

You have discovered experimentally that side area ahead of the CG is destabilizing in yaw. I flew a home brew canard several times that would tumble if yawed excessively, as in knife-edge flight. So I quit doing that. It had a very large amount of side area ahead of the CG.

You have also discovered experimentally the reason almost all airplanes, even aerobatic airplanes are designed with the horizontal stabilizer in a different horizontal plane from the wing: turbulence trailing the wing can under some conditions cause minor pitch instability.