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May 27, 2009, 01:38 PM
An itch?. Scratch build.
eflightray's Avatar
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).
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May 27, 2009, 01:48 PM
Registered User
Could it mean putting a T section strip on the rudder, like a Gurney flap each side?
May 27, 2009, 02:56 PM
Light and floaty does it
Work in Progress's Avatar
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.
May 27, 2009, 02:58 PM
Registered User
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.
May 27, 2009, 03:01 PM
Light and floaty does it
Work in Progress's Avatar
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...
May 27, 2009, 03:14 PM
Registered User
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.
Last edited by MCarlton; May 27, 2009 at 03:20 PM.
May 27, 2009, 04:15 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Quote:
Originally Posted by 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, ........
Donít 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
Last edited by JetPlaneFlyer; May 27, 2009 at 04:22 PM.
May 27, 2009, 04:22 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Quote:
Originally Posted by MCarlton
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 .
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
Last edited by JetPlaneFlyer; May 27, 2009 at 04:37 PM.
May 27, 2009, 04:36 PM
Registered User
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?
May 27, 2009, 04:47 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
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
Last edited by JetPlaneFlyer; May 27, 2009 at 04:57 PM.
May 27, 2009, 06:01 PM
Registered User
rofujiyama's Avatar
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.
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
Last edited by rofujiyama; May 27, 2009 at 06:20 PM.
May 27, 2009, 07:11 PM
Registered User
vintage1's Avatar
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.
May 27, 2009, 08:00 PM
Average User
M Ashmore's Avatar
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.
May 27, 2009, 08:28 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by MCarlton
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)

Quote:
Originally Posted by Al Bowers
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...
Quote:
Originally Posted by Don Stackhouse
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
Last edited by nmasters; May 27, 2009 at 08:48 PM.
May 28, 2009, 03:18 AM
Registered User
vintage1's Avatar
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

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.


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