stability changes under power

[vintage1]

I thought I'd kick this one into life.. really as a result of two personal experiences, and a whole raft of anecdotal information, mainly concerning the conversion of FF models to R/C.

What this is NOT about is power stalling or anything like it.

What it IS about is models that are stable on the glide, but unstable under high power, or indeed vice versa.

Case 1. This is the one example of the latter. My Prototype Miles Magister with far too rearward a CG was barely controllable at take off and landing, and damaged itself: However at higher airspeed it was stable enough to fly reasonably well.

Cases too many to mention, but the Pushy Cat will do. A vintage style model that is impeccable at gliding, under full power climbouts, or just cruising, but will not stay straight at high power level or diving flight.

The simplistic equations that we use to test for pitch and yaw stability, do not take into account airspeed. It simply doesn't factor in the equations..so how come this is happening?

My take on the pushy cat and other vintage models is this:Aerodynamic stability depends on pitch or yaw perturbations resulting in a net change in airflow across the tailplane.

However if this is in a slipstream that is maintained parallel to the models attitude, much of that stability is lost. The Pushy cats fin is almost completely in the slipstream.

The question then arises as to how it is stable in a high power climb. Here I conjecture that the angle of the model is such that the high wing and center of lift is far ahead of the CG: The RC model unlike the FF one has low down pack weight, and a much lower CG..and to reduce power zooming, a more rearwards one as well. this giving some 'pendulum' stability at higher power and climb angles...and indeed the higher angle of incidence at high rates of climb, is likely to 'blow' some of the slipstream clear of the fin base..

I am also wondering if the same effect, but in reverse, might have applied to the Magister..generally a low wing model with most of the drag and wing area below the C of G..so that as the nose comes down, in level flight, the CG moves a little forward of the aerodynamic center..this making it slightly more stable in level flight.

Both models are pending modification and repairs..so any input is welcome.

Also, the problem seems to be fairly prevalent in a number of high wing vintage style planes as well..although here the high dihedral in RET type flying makes it appear to be a propensity for 'Dutch rolling' as the yaw is instantly translated into a rolling moment.

IN every case the models glide fine. And generally climb fine too. Its high power level flight that sets them off..

[olmod]

With self righting aircraft the degree of" Longtitudal Dihedral" is important as is the tailplane area x moment arm to wing area ,many unstable situations can show up as a result of not having enough "LD".
Faster down wind climbs i have always thought that the wind was pushing the top of the tailplane down and increasing the angle of attack of the main wing.

[HerkS]

Vintage -- good exercise - usually more of an issue for a free-flight design.

Propeller slipstream does reduce both longitudinal and directional stability a bit. How much depends on a lot of factors, but even the induced flow from a jet engine exhaust has a similar effect.

Another longitudinal effect is downwash. At slow speed the downwash angle over a conventional tail induces and nose-up moment effect. At high speed that diminishes because the downwash angle is reduced quite a bit.

Most of us fine tune this aspect of a model by doing a "dive test." Set the model up for a nice stable power off glide - then without changing trim, tip it into a dive, allow speed to build up and then bring the stick back to neutral. The model should slowly pull out of the dive. Pull out too quickly - it's nose heavy. Diverge into an ever steepening dive and it's tail heavy. Change CG position an appropriate bit, retrim for a stable glide and do it again until the slow pullout is achieved. There is an element of dynamic stability in this behavior that is not well addressed by static stability analysis.

Downwash is one cause of this behavior but not the only one. As you've noted the location of the wing relative to the vertical CG location is important. The drag of the wing has a critical effect. Many free flight designs - with a large lifting tail and aft CG would enter dive divergence at the first hint of a speed increase, except that the drag of the high mounted with with large dihedral creates an increased nose up moment due the location of its drag.

Once the power-off dive test tuning is accomplished then you can adjust for power effects by a combination of fine tuning the thrust angles. Usually the power-off dive test creates enough of that mythical "longitudinal dihedral" thingy that the plane will still be stable under power and the nose will often then rise naturally as power is increased. Though a very high thrust line can bring in a strong nose down moment. At that point you can usually fine tune it with thrust angle adjustments.

[vintage1]

With self righting aircraft the degree of" Longtitudal Dihedral" is important as is the tailplane area x moment arm to wing area ,many unstable situations can show up as a result of not having enough "LD".
Faster down wind climbs i have always thought that the wind was pushing the top of the tailplane down and increasing the angle of attack of the main wing.

However with RC we actually naturally move the sticks or trims to get that effect..its CG that we cant change on the sticks!

So the important thing is to get the CG right FIRST!

[vintage1]

HerkS that's a good reply as far as it goes. However I am deliberately NOT talking about general stability as both of you have talked about - I am deliberately restricting to to what CHANGES the stability margin under power, not how to achieve a margin in the first place! That is a topic that has been very well covered here many times, and we probably all agree on how it hangs together.


The Pushy Cat was perfectly stable in a power off dive for example. It was either speed, or slipstream, that caused it to show mild yaw instability.

One other possible case popped up in the scale forum. A model that wants to turn, not fly straight, and shows no preference for direction either..

I believe some WWII fighters were like that as well. No such thing as 'trim for cruise' and let it stabilise...

So I am trying to get my head around the subtleties of how much a 'tail in the slipstream' devalues it, and how much the relation of CG to wing position makes for a 'virtual' change in CG as the aircraft departs from level flight..

There is no doubt in my mind that 'pendulum' stability was a feature of many FF pylon power models that allowed them to operate with very rearward CGs and very little longitudinal dihedral at all..such models did not show excessive ballooning under excessive power at all. And not a few featured outboard 'finlets' on the tail as well. I had always thought these to be mere decoration.. :cool:


So certainly the cases of models showing pitch stability improving under power with a CG HIGHER than the wing, are interesting..

Anyway, I did incease teh Pushy fin arae IN teh slipstream, but it made no difference at all.

The subfin is also a bit bigger on the second proto, but that didn't fix it either.

I reckon some depron finlets may be the next step.

[eflightray]

Fins.

I wonder how often models, and probably plenty of full size aircraft, have too smaller fin area. Especially on scale models. Could that be another of the reasons for speed change instability ?

Some full size aircraft gain larger fins, fin strakes etc during developement, even additional fins on some.

Crickey vintage, you admitting to taking a plane out without first double checking the CG is in the right place :eek: :D

Edit - Posted before I saw post #5

[vintage1]

The magister has been jinxed..it got lost in the post for ages, then WIP took ages to paint it, then the weather wasn't right for ages, then when I finally got a round tuit, I had forgotten where the CG was supposed to be, and it 'looked' all right.

It wasn't far off, and calmer weather might have got it down in one piece..and now its sat there all busted till I get a round tuit again..

basically 2oz of lead in the nose, and repairs is all it needs.

[peterangus]

For a model which is well-behaved at low speed, but misbehaves at high speed, structural deflection could be the problem.

A cambered wing, if it is not torsionally stiff, will twist nose-down at speed.

[Istvan Noszvai]

The geometry of control surfaces influences (or destroys) their effectivity at higher speeds. Big rudder and aileron gaps doesn't cause to much turbulence at lower speeds (lower Reynolds Number) and work well, but could be ineffective at higher speed.
There are a lot of control surface geometries to cure the problem, e.g Frise, Junkers, , Fowler, etc. laminated gap-free, elastic rudders, etc.
Covering the gaps could help a lot at model sizes as well!

The other factor is down thrust. If it is to small, and you can't change it, it's possible to mix the throttle to elevator on your Tx to have the "down" elevator trim in cordination with higher rews.

Istvan

[bwalt822]

As aircraft move faster, in almost all cases their center of pressure, neutral point, and aerodynamic center move further forward which will reduce static margin, as you all know the closer the aerodynamic center is to the center of gravity, the less stable the plane is. This is part of what can cause some types of flutter when the center of pressure of a wing tip of control surface moves too far forward.

Low stability at low speeds is probalby due to too small of an stabilizer and fin, when you go faster these things have more effect.

I have never heard of "longitudinal dihedral" do you mean the pheugoid mode of the plane?

[MCarlton]

I'm having a thought V1, which might be completely rubbish, but here goes;

On the pushcat and other vintage conundrums;

If one imagines a spiral shaped airflow coming back from the propellor, it seems to me that in the layout of the pushycat, that "spiral" will have a job to get to the fin. I am really stabbing in the dark here, but could it be possible that at a lower aoa and higher speed(ie level flight and under power) the propwash/airflow over the fin could be "sucked off" as it were, by the low pressure air on the top of the tailplane, thus leaving the fin trying to work in a sort of vacuum?

When power was reduced, the airflow over the fin reverted back to normal, and the yaw oscillation went away?

Stupid? Probably...

[bwalt822]

yeah, i dont think it works quite like that MCarlton. You might be able to have one side of the elevator stalled while the other wasnt but i dont think this is very likely.

For the pushy cat, did the plane always want to pitch up at high speeds when trimmed at low speeds?

[MCarlton]

As I say, it wasn't based on anything other than perhaps a vivid imagination and a large portion of ignorance :)

Cheers

Matt

[bwalt822]

well you are correct that the swirling airflow would create a low pressure over the tail surfaces and that is what causes the moment on the fuselage. These pressure fields would also influence eachother slightly but in reality you would need some pretty rare circumstances to have any effect.

[kcaldwel]

CG's well below the MAC of the wing (large dihedral angles add to this) can show pitch instability with changing Cl, even if they are nominally pitch stable:

http://www.rcgroups.com/forums/showpost.php?p=7764984&postcount=1

According to Perkins and Hage, "Airplane Performance, Stability and Control",

The main effects of high power (propeller aircraft) are to increase the slipstream and down wash across the horizontal stab. Depending on whether the stabilizer is carrying a up or down load, in the flight condition you are looking at, power may be stabilizing or destabilizing Generally, if the stab is carrying an up load, power will be stabilizing. If the tail is carrying a down load, power will be destabilizing. The magnitude of these effects can be quite large, and shift the neutral point "15 to 20 percent".

Aircraft with large horizontal stabilizers will generally have an upload on the stab during slow flight (high Cl), power silll be stabilizing.

These effects I think can explain most of what Vintage has been talking about.

Good book, that Perkins and Hage. There is a lot of more detail there, and calculations, etc.

Kevin

[eflightray]

Just been reading a Q&EFI mag in which Chris Gold reviews his Saunders-Roe SR A-1.

In the preamble he mentions the original full size aircraft aerodynamics, it would directionally 'snake' which became worse with increased speed, also it had a very harsh stall with little warning.

He writes the 'snaking' was possibly due to the very large forward side area. A partial 'cure' was 'T' stripping the rudder trailing edge, (I'm not sure what that is).

The stall was improved by thickening the wings leading edge.

As the Magister doesn't have a particularly large side area up front, what I've written is probably a waste of time, (he realises). :o

[MCarlton]

Could it mean putting a T section strip on the rudder, like a Gurney flap each side?

[Work in Progress]

That's exactly what it means. It only helps if one of the causes of the snaking is the rudder floating around neutral, possibly moving the pilot's feet through the rudder pedals, or possibly just stretching the rudder linkage. The T strip helps detach the airflow cleanly from the control surface.
Pic here.

And the Magister may not have a lot of side area at the front, but it is a fairly deep section slab-sided fuselage, and like most British aeroplanes of the era, is short of fixed fin area.

[bwalt822]

The snake he is referring could be the dutch roll mode of the plane. Usually dutch roll mode will damp out after a few oscillations but it might be possible to have a neutral or very slightly unstable dutch roll mode. Dutch roll is an indication of the margin of directional stability.

[Work in Progress]

Actually I think he means a pure yaw snaking motion, which was very common on early straight-wing jets. It's the reason the Meteor fin kept growing, and the fiddling with the Vampire / Venom fins. Straight wing low dihedral aeroplane rarely have any significant dutch roll problem. You really need sweep and/or lots of dihedral to get into that particular mess...

[MCarlton]

Hence the spitfire had lateral stability issues throughout its history, and, like the meteor, a steadily growing fin/rudder which never really caught up with the increasing torque/prop size.

Funny how a lot of British aircraft of the "era" were underfinned, I suppose the rationale was based on some aerodynamic theory, but perhaps we only look at these things with the benefit of hindsight, we know what we know because of earlier experimentation and error.

Dutch roll is something I've only ever encountered on a model once, which was a sort of bitza swept flying wing (parallel chord) which I made before knowing much, and kept a touch of dihedral on it. Dihedral, Tailless, Swept, No Washout.

It didn't fly well....

What about the "wedge" fin/rudder on the X-15, is this a hypersonic concession? I wonder if the same thing would work (in a less severe form) at model speeds. Perhaps a fin/rudder tapering from 1/16" LE to 1/2" TE.

[JetPlaneFlyer]

Hence the spitfire had lateral stability issues throughout its history, and, like the meteor, a steadily growing fin/rudder which never really caught up with the increasing torque/prop size.

Funny how a lot of British aircraft of the "era" were underfinned, ........

Don’t you mean directional stability?.. if it was a lateral stability issue they would have increased the dihedral or even reduced the fin size.

I'd guess on a high performance fighter aircraft like the Spit the aim would be to have 'just enough' fin and horizontal stab area.. More tail area = more drag= lower top speed, slower climb and less range.
Most pilots in a dog fight to the death would happily accept a little directional vagueness in straight and level cruise flight as an acceptable trade-off for a few more mph and a little better climb performance ;)

Steve

[JetPlaneFlyer]

What about the "wedge" fin/rudder on the X-15, is this a hypersonic concession? I wonder if the same thing would work (in a less severe form) at model speeds. Perhaps a fin/rudder tapering from 1/16" LE to 1/2" TE.

Yes, this was to counter directional stability issues at supersonic and hypersonic speed which first manifested itself on the earlier X series research aircraft.. Supersonic and hypersonic issues are not generally a problem with RC models :D .
All it would do on a model is add drag but despite this I did use the wedge shape airfoil on my little Rapier power freeflight X-15, purely for aesthetics.

Steve

[MCarlton]

Yes sorry I do mean directional stability, getting my brain crossed!

I suppose you're right, its a compromise between stability and agility, and I suppose the Spit was designed initially as an interceptor, and thus long flight times were not considered, so cruising stability wasn't really an issue.

Just thinking about the X-15, and its flight profile, throws up a question about Reynolds number.

In most equations I have seen, a constant value for air in s/m2 is used, around 70,000 according to one reference. How does that vary with altitude and decreasing air density? I would imagine that as one "runs out" of air at very high altitude, something pretty odd must happen aerodynamically.

Instinct (hunch?) tells me that at 350,000ft, there must be precious little air so the X-15 must have been in some sort of limbo between conventional flight and a sort of stabilised ballistic "flight" more akin to a fin stabilised rocket?

If there was enough air density for the X-15 to actually "fly" aerodynamically speaking, then I would assume the stall speed at that altitude would be pretty high, I would guess it would be supersonic?

[JetPlaneFlyer]

You are quiote right... above a certain altitude aerodynamic control surfaces become inefective. The X-15 was equiped with control thrusters which the pilot used in favour of the conventional controls above a certain altitude.

The X-15 at altitude was not flying as such, it simply followed a ballistic parabolic curve through space.

Steve

[rofujiyama]

http://www.rcuniverse.com/forum/m_7608298/tm.htm Hi , here is a link, i noticed while you were first encountering problems with the pushy cat, awhiles , back. It has very similar quirks,as you describe for your plane , with similar physical dimensions , but with tractor layout. Initially ,lot of the acknowledged trimming procedures were implimented , but in post 22-23 , you will see he finaly , found a solution , to the unwanted characteristics. I am not sure if it will allow your plane to retain the glide characteristics, but hopefully , the abnormally large forward fuselage volume, and depth,vertically, will enable it to retain its thermalling characteristics and RET capabilities.
Seeing as it is a fairly common quirk, as you stated, in a few of the older free flight/rc designs, could this mean it is just part of the designing growth development of thermal ships ?
As to the magister, looks like the flair is a little premature , , , as to lack of response to low speed flight , could ,maybe,possibly ,lol , be that the turbulence comming off the two cockpit windows enlarging at low speed and effectively swamping the rudder. But a simpler recourse would be to crank in some upward deflection in the ailerons and see if it delays the stall on landings. i look forward to you dailing in these ships ,as much as possible , because , they are very good looking designs, and have been curious , to see how you progess through the build. :) :cool:
roland

Oh , in regards to a low/no dihedral swept wing, I dont like the stall characteristics at any speed , but with small circular wing tip plates bisecting an imaginary aileron hingline,both fore and aft , and top to bottom of the wing tip, will dramatically stabilize the wing planform,from entering into a spin, and also allow you to recover from one ,too :D

[vintage1]

The Magister is potentially stable: it was too rear a CG and I only mention it because it was less unstable at speed.


The other machines are not aileron ships, so washout wont make a deal of difference.

I will report back when I have time to apply outboard fins, out of the prop wash.

[phat23]

You guys are talking way over my head, but I'm riveted to this discussion because of first-hand experiences at the flying field.

I've been helping a buddy of mine, an oldtimer freeflight guy, convert his models to electric powered rc. The one plane that has given us the most problem is the Spook.

After launch and during powered climbout, it's a real handfull. It wants to rock from side to side and is very touchy to any control input. After it reaches altitude and power is backed way off, it floats around the sky pretty as a picture. Power off glides are great, too.

Seems that no matter what we change in the Spook setup, this characteristic remains constant.

Mike

The Spook pictured is not my buddy's. It's only a reference.

[nmasters]

Funny how a lot of British aircraft of the "era" were underfinned, I suppose the rationale was based on some aerodynamic theory, but perhaps we only look at these things with the benefit of hindsight, we know what we know because of earlier experimentation and error.

I don't feel smart enough today to wright this stuff myself so I'll quote others: (it's not laziness, I really not that smart)

Imagine you have a box. You can't see what's in the box. But you know the box is open on the left side, and open on the right side, because you see air flowing into the left and air coming out the right. So far so good. So you hold a piece of yarn to the left, and you see the air is flowing STRAIGHT into the left side. But when you hold the yard on the right, the air is coming out at a slight downwards angle. And again looking closely at the box, the box is creating a force upwards that you're having to hold down against.

Now you don't know what is in the box, you can't see inside yet (no peeking!). All you know is that the air is turned in the box, and there is a force because the air is being turned.

Does it matter what is turning the air in the box? No, it doesn't matter. The fact remains that the box bends the streamtube of air, and it creates a force. In fact, the device in the box can be an airfoil, right? But it could also be a propellor set at an angle to the airflow!

Now let's go back to looking at our flying wing. Stand over the wing and look down on it. Let's say we have a single engine pusher (PUL-10 or Ho 33, my favorites :-). Now imagine the streamtube of air that comes from the front into the prop. Further, imagine the aircraft is flying at a little bit of sideslip. Where is the streamtube of air being bent? Right at the prop. Let's say the air is coming from the left, and the prop bends the flow straight along the centerline of the aircraft. So the force created by the bend in the streamtube is to the right. But that force is being created at the prop, and the prop is behind the CG. So the nose of the aircraft is being moved to reduce the sideslip.

Pushers have a stabilizing influence.

Okay, lets' now imagine we're looking at a jet. We bury the jet engine in the wing. The inlet is at the leading edge and the exhaust is at the trailing edge (ala the YB-49). Again, imagine you are standing over the aircraft looking down at it. Again, we have the aircraft flying with a little sideslip from the left. Imagine the streamtube of air coming into the jet engine inlet, getting bent, and pushing out the tailpipes. Where did the streamtube bend? Right at the leading edge of the wing. So where is the force being created? At the leading edge of the wing, ahead of the CG. Here's the important part: what is the result of that force? The sideslip is being made WORSE by the streamtube being bent AHEAD of the CG.

Tractors have a destabilizing influence.

All the above is a HUGE simplification of what really happens. But it illustrates the point. And hopefully, you can see the problem now...



Al Bowers gave us an excellent explanation of the stability issues of a tractor vs. pusher installation. However, there are some other issues that need to be considered.

What really matters in the stability issues is how far the prop is longitudunally from the C/G. For example, if you mounted the motor in the nose, with the prop running through a cutout in the wing so that the prop was located approximately at the C/G (such as that little electric B-2 model from Wattage that was popular a couple years ago), the prop would have relatively little effect on stability, despite the fact that technically speaking it was a pusher. In fact, if this "pusher" prop was ahead of the C/G, it would be just as destabilizing as a tractor prop in that location. Likewise, if you mounted the motor on the tail, such as on the "Sea Wind" amphibian, with a tractor prop on the front of it, the prop could add some stabilizing effects in pitch and yaw despite technically being a tractor installation.

Note also that most airplanes will have to fly at a range of power settings, including idle, so it's usually not wise to use the stabilizing effects of a pusher prop to reduce the size of stabilizing flying surfaces. OTOH, at least you don't have to make those surfaces bigger to counteract the destabilizing effects of a tractor prop at high power settings. Of course, if you keep a tractor prop as close as possible to the C/G , the destabilizing effects of the tractor prop tend to be fairly minor anyway. Because of all of this, the net importance of prop influenced stability issues tends to be fairly small for most aircraft, with the possible exception of flying wings.

C/G issues can be a problem. When you mount the motor in the tail, it typically takes a bunch more weight in the nose to balance it. Even if that can be arranged successfully without resorting to lots of nose ballast, you still end up with a plane that has all its mass distributed in the extremities, instead of clustered as closely as possible at the C/G. This tends to degrade the dynamic stability and the control response. There is the alternative of mounting the motor at the C/G and driving the prop through a long driveshaft, but that opens all sorts of cans of worms in terms of torsional vibrations and resonant frequencies, possible whirl-mode instability (that's where that long driveshaft decides to pretend it's a jump rope), and of course all the added weight of the drive shaft and its supporting structure.

In Al's description of the stability issues, he uses the analogy of the "box" that accelerates and deflects air, and the resulting effect on the airframe. This is fine when looking at the effects of the prop on the airframe, but you must be very careful not to let yourself fall into the trap of ignoring the effects of the airframe on the prop. There is a long and sad history of airframe designs that fell short of expectations because of exactly this kind of thinking.

A prop is more than a "box that deflects air". It is a set of rotating wings that fly through a very complex, helical, non-symmetrical flow field. Their efficiency depends a great deal on just how non axisymmetrical that flowfield is. A pusher prop has to fly through all the disturbed flow coming off of the airframe in front of it, and as a result tends to have significantly less efficiency (usually at least a couple percent less, typically quite a bit worse than that, and in some cases more than 15% less efficient) than an equivalent tractor installation. Yes, there can be some gains in airframe efficiency from not immersing as much of the airframe in the higher speed flow behind the prop, but it is usually a far smaller gain than the losses due to the airframe's detrimental effects on the prop. Only a small portion of the total airframe drag tends to be influenced by the flow of the prop, while all of the thrust from the prop can be influenced by the flow coming off the airframe. Attempts to help the airframe by putting the prop in a bad situation almost inevitably end up being "penny wise and pound foolish."

Immersing the prop in that disturbed, non axisymmetric flowfield tends to also generate much higher vibrational stresses in both the prop and the airframe. Although the lifespans of models are not usually long enough for the resulting fatigue stresses to be an issue, it can be a major problem on full scale aircraft. On models it can result in lots of weird little problems with things like screws vibrating loose all the time, wear in control linkages, etc.

Inflicting a lot of disturbed flow on the prop also tends to worsen the noise, both inside and outside the aircraft. I have some tape recordings of the Lear Fan, a turbine powered pusher, taking off. Despite having the engine exhausts about 6 feet ahead of the prop, it sounded just like the RR Merlin in a P-51 Mustang. Pushers with the exhausts closer to the prop tend to sound more like a chain saw.

Prop efficiency is also very dependent on diameter, which in turn tends to be set by ground clearance issues in many cases. The ground clearance at rotation on takeoff on a pusher tends to be the limiting factor in many cases, and the resulting diameter tends to be less than what the same airplane can handle in a tractor prop. This often results in further losses of efficiency.

In the case of a small model that gets hand-launched, the ground clearance problems are less significant. However, you do have to worry about getting your hand sliced and diced at the moment of release.

Pylon-mounted props, such as the Lake Amphibian, tend to have fuselage clearance issues that limit prop diameter, and also can have problems with nose-down trim change from the high thrust line when you add power, which then tends to limit just how much power you can install before the plane insists on nosing over on takeoff unless you open the throttle gradually. These pushers do have the advantage of having the tail immersed in the propwash, so arranging the thrustline to optimize the propwash's interaction with the tail can help to some extent. We did this very successfully on our Roadkill Series Curtiss-Wright Junior, although you still have to be a bit careful with it at the start of takeoff run on a relatively rough surface. One thing that can help in these cases is the use of tailwheel landing gear (such as the Junior's), rigged with a fairly high fuselage angle when in the three-point attitude. This moves the motor aft relative to the C/G when in the three-point attitude, which helps hold the tail down until you get enough airspeed for the elevators to become effective.

In most cases the pusher installation does not provide as much propwash over the elevators as a tractor installation. Even if the elevators are immediately in front of the prop, they still don't get as much local airspeed during takeoff as a tractor would provide. This tends to keep the plane from rotating on takeoff until long after the wing is at flying speed, greatly increasing the required runway length. Once it does rotate, it tends to jump aggressively into the air, making it difficult to get a smooth liftoff. Many pushers, such as the VariEze and the Prescott Pusher suffer from this. The Prescott Pusher (among others) also suffers from the two shortcomings I mentioned above, which resulted in relatively inferior performance in comparison to tractor aircraft with the same payload and power loading.

Although a set of elevators immediately ahead of the prop do not generally get much help from the propwash for rotating the plane for liftoff, they can definitely get blanked by the stagnant flow in front of a windmilling prop on landing. This was one of the quirks of that 4 1/2 ft. Roadkill Series Northrop XB-35 we've been developing. If you try to land with less than about 1/4 throttle, the windmilling props blank the elevators and you have no elevator authority to flare with for touchdown. This tends to be very hard on the landing gear. Diving in at high speed on final approach doesn't seem to have much effect on this; the only reliable fix is to land with the throttle a little open.

On full scale aircraft the FOD ("Foreign Object Damage") issues are much worse for a pusher prop than for a tractor in most cases. Blocks of ice shed from the flying surfaces and the fuselage on aircraft expected to fly in instrument flight conditions tend to be an especially severe problem. So can tire treads shed during takeoff. This is generally less of an issue for models, although stones on the runway can still be a problem.

The lightning strike criteria for pushers (class IV, as opposed to class I for tractors) are also more difficult to deal with (also not generally an issue for models).

On turbine engined aircraft, the heat and corrosive gases in the exhaust can be a serious problem, especially if the prop has to operate in reverse for braking on landing roll or especially for maneuvering on the ground. One of the most serious conditions for typical turbine pushers is when an aircraft uses reverse thrust from the prop to back itself away from the gate at the terminal building.

There are a host of other factors to consider as well, although these are some of the biggies. What matters is that the designer properly considers all of the issues from both the prop's and the airframe's point of view. The net result can go either way, depending on the specifics of the particular aircraft and its mission. I've designed both pushers and tractors, and each was appropriate in its individual case. In general I've found that tractors are a better choice in the majority of cases, although there are specific cases where pushers have an overall advantage. Flying wings, where even small differences in pitch and yaw stability can often be important, are one of the cases that often favor pushers.




I copied all that just to make this one little point: notice that we don't put the intakes of jets clear up on the nose anymore. Suppose there might be a reason for that?

--Norm

[vintage1]

That was well worth reading..thanks.

It adds yet another dimension to the thinking. Yaw wont be an issue with the pushy cat though. The motor is actually within a few mm of the CG.

Roll coupling might well be though. Th effect of a lot of power would be to work with the dihedral, accentuating roll, due to the high engine position.

I do stress this is anything BUT a high power model though. 'High speed' merely means above 20mph, in this case :D

Norman: The Spook issue is a real puzzle. I've seen these fly at Old Warden, with zero problems. On BIG engines too. Not sure how big. but certainly 90 or larger. What size prop are you running and at what RPM? roughly where is the CG?

My fond hope is that this thread runs long enough to provide some kinds of answers.

So far it seems that we have posited two effects:

1/. The slipstream acts to reduce effectiveness of tails in its path. (But of course to increase the sensitivity to control input).

2/. If the prop is ahead off the general aerodynamic centre, it tends to destabilise, and if behind it tends to stabilise, the model. i.e the thrust vector will pull the model off course, or push it on course, respectively.

[nfhill]

Vintage1, I think you have missed a large effect of propeller position. It's not just a thrust line issue.

Consider a normal powered glider with a folding tractor prop. With the prop folded, the model will have a given stability margin and may be adequately stable. With the propeller spinning, the stability is greatly reduced because the propeller acts as a significant surface forward of the aircraft center. Increasing the propeller diameter reduces the stability. It's as if a large surface has been added at the front of the airplane.

I believe this has recently been discussed in the scale forum as an issue when converting an IC powered model to electric if the propeller diameter is significantly increased. With a small propeller the model is more stable than with a large propeller.

[Work in Progress]

Quite so.

[vintage1]

Vintage1, I think you have missed a large effect of propeller position. It's not just a thrust line issue.

Consider a normal powered glider with a folding tractor prop. With the prop folded, the model will have a given stability margin and may be adequately stable. With the propeller spinning, the stability is greatly reduced because the propeller acts as a significant surface forward of the aircraft center. Increasing the propeller diameter reduces the stability. It's as if a large surface has been added at the front of the airplane.

I believe this has recently been discussed in the scale forum as an issue when converting an IC powered model to electric if the propeller diameter is significantly increased. With a small propeller the model is more stable than with a large propeller.

I hadn't exactly missed it..that's why I was asking about the spook prop. It occurred to me that a greater diameter would blank more tailplane..

But I am less sure about 'area up front' being such an issue..after all, its rather sideways on to the planes of interest. And wouldn't change with power. A simple CG move should compensate.

[Don Stackhouse]

A prop sitting at an angle to its inflow generates a moment (such as "P-factor") due to the differences in angle of attack, and especially the differences in airspeed, on the two halves of the prop disk. This results in differences in the lift of each blade and thrust generated on the two halves of the disk, resulting in a moment trying to turn the plane. On a climbout with a right-handed tractor prop, this tries to swing the nose to the left, requiring right thrust and/or right rudder to counterbalance it.

However, those same differences in local airspeed and angle of attack on different halves of the disk also cause differences in the drag of each blade as it goes around the disk. This causes a force (NOT a moment) parallel to the plane of the disk, just like the lift force of a flying surface would generate in that location. On a right-handed tractor prop in a nose-up climbout attitude, this would generate a force in the plane of the prop disk trying to pull the nose UP, and is therefore destabilizing.

If it was a prop located aft of the C/G, it would be trying to pull the tail up (and the nose down), and therefore would be adding static stability.

How significant can this force be? On the V-22 Osprey tiltrotor aircraft, the wing supports the plane in "airplane" mode, and the thrust of the rotors directed upwards supports the plane in helicopter mode. However, halfway through the transition form one mode to the other, when th erotors are tilted around 45 degrees or so, the combined upward component of the rotor thrust, plus the lift generated by the wing, is not enough to support the weight of the plane. In that situation, a large part of the support of the plane comes from this in-plane force from the prop disks.

So, yes, a prop does act like a combined horizontal+vertical flying surface area, over and above thrust-generated effects like P-factor.

[vintage1]

P factor only really works when the prop disc is significantly not orthogonal to the airflow. In normal flight with a normal aircraft, that is not the case. Simply because to have the wing at more than a given angle of incidence, and hence the prop not 'square' to the airflow, is to stall it.

It's really only significant on vectored thrust aircraft, and tail draggers on the ground. Although it can be a factor as is torque roll and sometimes gyroscopic effects, when the model is rapidly pulled into a stalled situation.

Its definitely not the case here either, as the effect is symmetrical - the tendency to veer has no preference for left or right.

But nice to mention it, as no one else has. Its generally responsible for massive swings on takeoff acceleration..needing a lot of rudder to correct until the tail lifts.

[Work in Progress]

V1,

I agree that the willingness of the Pushy Cat to head off in either direction apparently on a whim suggests that P factor is not the driver in this case.

But if you will permit a digression into P factor more generally, actually I believe it is a more pervasive force than you reckon, and can show its influence in many regimes of flight. The low speed tail-down take-off roll is the 'classic' demonstration of its effects, but it can also be felt in high AoA flight at higher speeds, in full flight. Most taildraggers are not quite at the stall with the tailwheel on the ground (otherwise we would not be able to three-point them so easily) and therefore you will have the P factor in play when, for example, entering a loop, or even more noticeably when you are close to the inverted stall just before commencing a roll-out at the top of a half loop.

It is often disregarded that airflow offset in the yaw plane creates a P factor in the pitch plane, which can make aeroplane behave differently in left and right sideslips (usually you will feel the nose trying to drop more in one direction than in the other direction). It is also worth thinking about this effect in crosswind operations in some large-propped, tail-light types.

I agree with Don's general analysis and the conclusion to which it leads, which is that big nose-mounted props directly destabilise the body to which they are attached in pitch and yaw simply by representing a significant forward-acting surface, and tail-mounted props add to pitch and yaw stability for the same reason. Each of those props, naturally, also contributes other forces which have differing effects in various flight regimes.

[vintage1]

Well yes, I buy the P-factor having effect at high angles of attack obviously and of course near teh stall direct torque effects have great inpout too.

I also buy the effects of what is essentially a thrust vector but I hadn't been able to see why that would affect the Pushy cat, BUT there is a possible hint of a mechanism there.

Namely that under yaw conditions, the thrust line is high and offset from the centre of lateral drag, which would tend to roll the model hard into the yaw..dihedral enhancing effectively. Extra yaw/roll coupling under power.

However the Pushy cat does not have excessive dihedral (although the wing is swept), and I wouldn't have thought that this effect would have done more than make it sensitive to yaw.. it should not actually make it (mildly) spirally unstable.

The effect is more like planes with too little dihedral..once on a slight turn, the tendency is to tighten it. But it comes out smartly with opposite rudder. Which poor dihedral doesn't do.

[Work in Progress]

Coming back to the Pushy Cat, I am thinking that only more test flying will answer some of these questions. I am happy to volunteer as pilot or observer if it will help!

Meanwhile, when you say "will not stay straight at high power level or diving flight", just to be sure of what you are reporting, have you tried it in steep power-off dives, i.e. high speed with the motor off, or only power dives?

Also, does it spin enthusiastically, or reluctantly, or not at all, or has that particular can of worms yet to be opened?

[vintage1]

Coming back to the Pushy Cat, I am thinking that only more test flying will answer some of these questions. I am happy to volunteer as pilot or observer if it will help!

Meanwhile, when you say "will not stay straight at high power level or diving flight", just to be sure of what you are reporting, have you tried it in steep power-off dives, i.e. high speed with the motor off, or only power dives?

Also, does it spin enthusiastically, or reluctantly, or not at all, or has that particular can of worms yet to be opened?

Yes, we should probably do that.

Its never been spun for sure.

A gliding dive, from recollection, was OK, as was full powered climbs.

I only noticed it when trying to make progress upwind, with a lot of throttle and a bit of down. Could NOT keep it straight.

Not sure it DOES spin.

I need to make a new UC first..

[Steve Anderson]

As Mark Drela pointed out before, that plane has a huge amount of vertical fuselage area ahead of the CG (destabilizing). So wouldn't the higher power setting you are describing increase the airspeed along the whole length of the fuselage, even ahead of the prop, increasing the effect compared to gliding at the same velocity?

Wing sweep... Seems to me sweep would decrease dynamic stability in the yaw axis because the forward moving wing drag force increase (increased effective span on that side) would tend to excite oscillation in the yaw axis and require a larger verticle to damp.

Even larger vertical tail?

[Steve Anderson]

[url]As to the magister, looks like the flair is a little premature , , , as to lack of response to low speed flight , could ,maybe,possibly ,lol , be that the turbulence comming off the two cockpit windows enlarging at low speed and effectively swamping the rudder. :D

I thought the same thing about the windscreens. I was also wondering if the huge fillets are effecting the airflow along the sides of the fuselage? Too much profile thickness causing separation?

[vintage1]

As Mark Drela pointed out before, that plane has a huge amount of vertical fuselage area ahead of the CG (destabilizing). So wouldn't the higher power setting you are describing increase the airspeed along the whole length of the fuselage, even ahead of the prop, increasing the effect compared to gliding at the same velocity?

Yep. but that should have the same effect under power..as on the glide.unless slipstream effects change it.

Wing sweep... Seems to me sweep would decrease dynamic stability in the yaw axis because the forward moving wing drag force increase (increased effective span on that side) would tend to excite oscillation in the yaw axis and require a larger verticle to damp.
I cant quite see how that works..


Even larger vertical tail?

I tried that..it seemed to work with a Depron piece on, but when I actually put it in balsa, it didn't.

But that is the test to do, yes.

When calm weather arrives.

[Work in Progress]

I thought the same thing about the windscreens. I was also wondering if the huge fillets are effecting the airflow along the sides of the fuselage? Too much profile thickness causing separation?

The mystery with the Magister is not why it was unstable at low speed - the CG was considerably too far back - but why it was better at higher speed. In any case, you can't do a scale Maggie without the fillets, so they will have to stay.

My experience of open cockpits is that they do have an effect. It was noticeable in the Yak-52 that if the rear occupant had the canopy open there was a lot more drag in the round-out and a shorter ground run. The Stampe is well known to be unhappy doing outside, negative G manoeuvres, with an empty front cockpit. It buffets a lot and loses elevator authority. With a pilot in the front seat it is OK, and it is best of all with the front cockpit faired over and the quick-release windscreen detached. The same goes for the relatively few Pitts S2As that have a detachable windscreen / cover arrangement for the front cockpit, with a separate windscreen and sliding canopy for the rear. (Most of them nowadays just have one big side-hinged canopy that covers both cockpits, though)

When we rebuild the Maggie it would not hurt to put another pilot in the front cockpit, or possibly false-floor it, to see if it makes any difference: however, I will be surprised if the correction of the CG does not fix it.

[Ralph Walton]

Its not just a COG thing is it Vintage? From experience I've found some of my models will be perfectly behaved at moderate airspeeds but in dives and when flown faster they became "nervous" and wouldn't stay pointed where you wanted them to go.

I found by accident that when flying faster, a COG of approx 6mm further foreward was required. After that they flew like they were on rails.
I have no experience with free flight , but I understand that many free flighters had very rearward C's of G due to lifting tail theories amongst other reasons

[vintage1]

Its not just a COG thing is it Vintage? From experience I've found some of my models will be perfectly behaved at moderate airspeeds but in dives and when flown faster they became "nervous" and wouldn't stay pointed where you wanted them to go.

I found by accident that when flying faster, a COG of approx 6mm further foreward was required. After that they flew like they were on rails.
I have no experience with free flight , but I understand that many free flighters had very rearward C's of G due to lifting tail theories amongst other reasons

Well probably a forward CG would fix it, but at the expense of needing up trim to fly it, and then power zooming.

It has plenty of pitch stability..
but not a lot of elevator..

[Steve Anderson]

Yep. but that should have the same effect under power..as on the glide.unless slipstream effects change it.
A fuselage is destabilizing, it wants to turn sideways to the airstream. Gliders with small vertical stabs apparently will fly just fine until the speed is increased, then it will Dutch Roll. Maybe the increased slipstream velocity is increasing the yaw force that the fuselage has and is pushing the plane into greater instabilty? IDK.


I cant quite see how that works..
Sweep increases static stability. The forward moving wing increases the effective span on that side creating a force to stop the rotation. Now this is not in any of my aero books, but, isn't the force strongest when the yaw stops and reverses direction?

Isn't this exactly how you excite an oscillation?

Wouldn't that mean a plane with sweep needs a larger vertical tail for proper damping than if it had an unswept wing?

I think the sweep is part of the problem.




I tried that..it seemed to work with a Depron piece on, but when I actually put it in balsa, it didn't.
Depron...square leading and trailing edges? Balsa airfoil?

[Work in Progress]

Depron...square leading and trailing edges? Balsa airfoil?

Ooh. It's a reasonably long shot, but I like your thinking...

[peterangus]

.......... I understand that many free flighters had very rearward C's of G due to lifting tail theories amongst other reasons

It has become customary to state the CG position as a percentage of the wing MAC, but this is not very meaningful.

For essential longitudinal stability the CG must be some small distance forward of the aerodynamic centre of the whole aircraft. The CG position does not relate directly to the wing.

Some free-flighters have an enormous tailplane which pulls the aircraft AC rearwards, and with it, the required CG position.

[fnev]

It has become customary to state the CG position as a percentage of the wing MAC, but this is not very meaningful.

For essential longitudinal stability the CG must be some small distance forward of the aerodynamic centre of the whole aircraft. The CG position does not relate directly to the wing.

Some free-flighters have an enormous tailplane which pulls the aircraft AC rearwards, and with it, the required CG position.


If we could have this framed and hanged somewhere… Very well put.

[BMatthews]

You guys are talking way over my head, but I'm riveted to this discussion because of first-hand experiences at the flying field.

I've been helping a buddy of mine, an oldtimer freeflight guy, convert his models to electric powered rc. The one plane that has given us the most problem is the Spook.

After launch and during powered climbout, it's a real handfull. It wants to rock from side to side and is very touchy to any control input. After it reaches altitude and power is backed way off, it floats around the sky pretty as a picture. Power off glides are great, too.

Seems that no matter what we change in the Spook setup, this characteristic remains constant.

Mike


Mike you probably should have started a new thread but what the hey....

I've seen a buddy's Spooks (yes that's right, he's a Spook NUT and has done about 5 of them in various sizes and both FF and RC) fly and they are what we call spirally on the edge. In the climb it does not take much to generate a dutch rolling situation. They seem to be on the ragged edge of having too small a vertical tail area. In an RC version any attempt to try to dampen this will likely be out of phase with the natural frequency and just make things worse. The best I can suggest is to make the total fin and rudder area about 5 to 8% larger and/or use dual rates for the rudder. Low for the climb portion and normal for the glide in order to desensitize it under power.

[portablevcb]

I tend to agree with this for kind of a different reason. But, I don't have a lot of FF experience to verify it.

Powered FF the plane is traveling near vertical? That means the wing is providing less lift and the motor is providing most of the climb motive power? In this case isn't the plane acting a bit more like a missile, with stability depending more on relation between CP and CG? Would the addition of vertical tail surfaces assist in moving the CP rearward?

If you then try to fly it like a 'normal' airplane the wing is providing much more lift and it's contribution to stability becomes "different"? Along with the change is CG position relative to the MAC of the wing.

Just guessing here, haven't worked out any numbers.

[nmasters]

I realize that the prop is not the trouble for the model being discussed but as a general power vs stability reference this could be useful (http://www.bd5.com/BedeDesign13.jpg) .