alasdair

This was mentioned in another thread, and rather than hijack that thread I've started another.

I have heard it said that a propeller on the front of an aircraft will tend to destabilise it, but a pusher prop at the back increases stability.

It was just a vague statement, and I don't know where I read it. But in another thread it was said that, in regard to yaw stability,a prop had an effect similar to a little fin. It would destabilise at the front of an aeroplane but help stability at the rear. Neither myself or BMathews have found theory or calculations to back this up, but he had some examples from full size aircraft that seem to indicate that it's true.

Since propellers have rotational symmetry I reckon that whatever theory applies to the yaw stability effect of props should also apply to pitch stability. Does a propeller have an equivalent H-stab area as well????

I have noticed that, when using my usual CG calculation, it works great for trainers and general low powered models, but for an Edge or Extra or F3A aerobatic model the stability seems reduced. It is adequate, but only just. Does a big powerful prop have the effect of moving the Neutral Point Forward??

If so WHY?

I also noticed that the usual CG calculators give slightly too far forward a CG for gliders (not that I've flown one for many years). Do glider pilots just like less stability?? Or is it lack-of-prop effect?

Surely someone from the wide and knowledgeable readership or RCU will know something definitive?

rmh

Bored?

Towing an object is more stabilizing then pushing it
Ever try to push a length of chain?
As for CG calculators
The lighter the wing loading - the more tolerant the CG position-
Stability is a moving target -
stable in what attitude?
a highly aerobatic plane will be more stable in high ALPHA flight -IF the CG is extremely aft
yet - the same craft will be twitchy if flown at a very low AOA
Don't believe it ?
try it .

UStik

Nothing to contribute, but I think [link=http://www.dtic.mil/dtic/tr/fulltext/u2/a801088.pdf]this one[/link] was quoted in another thread here a long time ago.

otrcman

Yes, it is true that the propellor on a tractor type plane has a destabilizing effect in both pitch and yaw. As a matter of fact, this phenomenon is noticeable on jet airplanes with nose intakes as well. I'm away from home and pecking on an iPad right now, but after I get home I'll post a sketch to explain how the phenomenon works.

Dick

BFoote

Stability is a function of dynamics. IE change in momentum. Momentum is mass dot velocity. To change velocity = change in acceleration. Two functions of stability. Natural and dynamic stability. This postulated question has solutions depending on flight orientation is BOTH a function of NATURAL AND DYNAMIC stability.

Depends in which direction you are accelerating in this case for a propeller and likewise which spin direction. For instance slowing down a rear prop is stabilizing whereas a "normal" is destabilizing. The ol' chute in back compared to balancing a broom handle on your finger tip. You can have both scenario's but depends on which flight aspect you are in. Slowing down: Speeding up. In One flight aspect will be stabilizing and in the other destabilizing.

The yaw, pitch aspect has nothing to do with where the propeller is. Has everything to do with dynamics and which direction the propeller is rotating along with the mass/diameter of said propeller. Said propeller, non constant speed props that is(caveat from lift torque of additional power/pitch control added of course), when throttled create an increased torque due to change in momentum. Especially true on landing.

So, the axiom that a propeller has a stabilizing effect on vertical stabilizer area for a front or rear propeller is ENTIRELY dependent on which direction one is accelerating. IF you are slowing down, then a rear propeller will have a stabilizing effect and a front will have a destabilizing effect on yaw stability. Conversely on accelerating a rear propeller orientation will have a destabilizing effect and a front will have a stabilizing effect. Likewise the spin orientation of the propeller will determine which direction additional pitching moment will be introduced and this will likewise clearly be dependent if rear or forward propeller orientation. In other words a clockwise propeller induced pitching moment caused by "reving" a fixed pitch propeller will be the OPPOSITE for a pusher propeller orientation. This is in additional to the induced roll from reving the a fixed pitch propeller due to change in momentum. Right hand rule of dynamics. Chapter 3 of any dynamics book probably.

Nothing is static. Cannot solve the problem through a static drawing.

UStik

I'd trust Dick.

But by now, could you explain the essay linked in post #3?

And how about [link=http://www.aircraftspruce.com/catalog/pdf/13-09032.pdf]this one[/link] (Figure 2-32)? One could easily replace the pitch/side-view situation by a yaw/top-view one.

BFoote

What they say is: Summation of momentum which translates into summation of forces which equates to "effective" side area when a plane is yawed or yawing. Call it conservation of energy if you wish. Conservation of momentum, yadda yadda.

Effectively the first .pdf says: RULE OF THUMB: Effective side area of a yawed propeller = additional fin area. Effectively extra "rudder" with a crude scale factor for the number of propeller blades. That area creating "lift" * moment arm to CG = countering yaw moment. Think of it this way. It is far easier to turn a propeller plane with low revolution speed/power setting than high due to this effect. A fighter needs to be able to climb/turn at the same time = need for increased fin area to overcome this steadying effect under power. All of which is utterly dependent on angle of attack of propeller, velocity, propeller slip factor, yaw angle, drag polar cofficients of the propeller, distance of propeller disk to structures, yadda, infinite yadda, factors. But, this is a rule of thumb and other major moments of inertia and momentum must likewise be added to the pot before stirring for a definitive answer.

All early NACA papers are a great read on practical testing and application. Can't find such papers anymore. Have to pay for them even though the government pays for their development. Those early papers are all gems. Have them all downloaded on CD. At the time, they were state of the art. Today they are hopelessly crude, but end result are that the early papers draw far better finite conclusions or at least rough rules of thumb one can live by to build/design/safely fly an airplane. In the end, unless you are trying to draw the last 5% out of a design, those early NACA papers are perfect for what one needs. They are still today perfectly good for civilian aviation. Well some of them before the 30's are pretty spotty and full of hogwash that we know today is not true, but otherwise, they are great.

In short, it is frankly impossible to describe this unless one has a basic understanding of dynamics as there are a lot of variables involved and multiple different vectors of momentum involved as well. Several of which are dependent on the time domain and are therefore impossible to draw on a static piece of paper.

Hircflyer

Well this certainly clears it up

BFoote

Welcome to Aeronautics. It gets far worse when you go super sonic. Then you have to add thermodynamics as well as Compressible fluid flow on top of normal aeronautics. Have the same symbol used in many different equations that means different things. Get the joy of making charts of symbology as the english/greek alphabet is not large enough.

rmh

what the prop actually affects -can be easily demoed-
I am a bit surprised more writers have apparently not used a small electric powered model to hand hold and actually feel the forces at work.
Better than thumbing thru some old textbook
Personally we resolved a ton of old questions-using this technique -also taking a small electric and trying different types of wing planforms.
CG as it relates to wing loading - (stability) was very informative.
power on vs power of stability -likewise informative.
As active modelers- this approach seems as direct as it can be
but then -I can't read very well.

MTK

All tractor propellers have an effect because the prop area (volume actually) that creates thrust has a longitudinal component and a transverse component, ie- the prop is twisted. If you look at the prop from the side, you will see area just the same as if you looked at the prop from the front. The area you see from the side view is analagous to adding this much area vertcally (and horizontally) tothe front of the airplane.

The current crop of models have adequate yaw stability for single prop drives up front....Interestingly, there has been an ongoing discussion in the Electric Pattern Forum regards to the destabilizing effect of a contra prop drive.

In a contra drive, the second propellerup font reduces the yaw stabilityof the current crop of models being fitted with this particular type of drive.The second prop adds longitudinal vertical area (horizontal too). Guys have resorted to adding vertical areato the stab and fin, thus effectively bringing the model's feel in yaw back to where it would have been with a single 2 blade prop driving up front.....

I know that driving the plane with a 3 bladed prop seems to reduce its yaw stability a bit but not enough to cause me to add contraptions to the back of my design.

I have never run anRCmodel with a single blade prop...Curious if that adds to the yaw stability since the effective area is smaller.

BFoote

MTK you have it reversed. Contra stabilizes along with increased efficiency. Big time. Saying otherwise is probably confusing CONTROL with STABILITY. They are not the same. Rudder authority will be greatly improved(dare I say "touchy"). Fin area can be decreased. Not to mention no pitching moment!

Your 3 blade performance was probably due to increased drag big time and LACK of efficiency compared to lift generated due to poor propeller/RPM/Power correlation.

MTK

Wrong on that one Bfoote. Rudder effectiveness becomes "twitchy" because yaw stability is decreased (not increased). That's fundamental aerodynamics....Same as when you add a dorsal or ventral strake at midships.. Yaw stability is decreased making rudder more effective. That's precisely the reason sharks have a dorsal at midbody....to be the top predator they are, they have to be able to turn on a dime. I think nature got that one right.....go figure

It's the same as keeping elevator area the same but reducing overall stab area. What do you think will happen in pitch control?

BFoote

Control and Stability are different.

Control is input needed to MOVE said object onto a different heading(dynamic). How much input is needed(twitchy if you NOW have too much which they do). Stability means that it will NATURALLY revert to its Neutral position. Contra rotating propellers will cause the plane to revert to its natural neutral stable position as it cancels yaw and pitch moments due to conservation of momentum. The end result is the need for smaller rudder throws. AKA CONTROL. Thus, when you read "yaw unstable" it is because their rudders are too big as they now don't need to compensate for a single out of balance moment caused by a single propeller.

After reading the thread for a while in electric pattern on contra rotating prop design by Brenner, I noticed that instead of doing the correct thing, they added drag to their rudders to get away from flutter. This is why some of the models had NO problems while others did.

The flutter is what they were calling "yaw" instability. The fact it was happening at neutral should have been a GIANT RED STOP SIGN. Their rudders are not balanced. Since there is NO preload on their rudders in the need to compensate for the momentum created by a SINGLE propeller anymore, their rudders are now fluttering at neutral creating a yaw "wobble". They fixed this by creating DRAG, blobs, triangles, on the rear of their rudder which dampens this flutter via drag though could be really bad at certain airspeeds. Or they could just do the smart thing: move the pivot point on the rudder itself along with internal weights to balance it. IT IS HOW ALL REAL PLANES rudders are balanced! It does create a harder plane to build and for this reason is not done in RC land.

Rudder Flutter around neutral = their so called "yaw instability"

If they took said contra and put it at a normal 1 degree offset like one does with a single blade propeller, they never would have even found this "problem" as this would preload the crude unbalanced rudders that nearly every RC plane has. They created their own problem by not following basic vibration dynamics.

Cheers.

EDIT: Might not be using a large enough spinner, creating large turbulence at low speed due to props hubs acting as a giant wind screen. One can gain quite a bit of extra thrust even on normal single blade planes by increasing spinner size due the drag/blocking going on.

EDIT: PS No one new jack squat about vibration dynamics before the 60's. They knew they existed, but only ever addressed a very small number of issues. So, showing pictures of planes with large fins and contra rotating props, after all it worked with larger props and horsepower before is their thinking... is equivalent of showing someone who is compensating for being stupid with something they know works. They just don't know why it works.

rmh

Well Brer Foote
A number of us DID know about vibration dynamics before the '60s-
My mentor then -in fact in the early 1950's was a wind tunnel model maker for a major mfgr
better yet he knew why things worked - without having to look up the answer.
I was designing fuselage construction and anti vibe motor mounts -in the mid 1980s -to resolve the problems of 4 cycle engines in light airframes .
be it a model or a full scale craft - most of the same isues crop up -
as for the contra props on pattern planes - Simply a difficult approach to a problem which really did not need resolving

BFoote

Yes, they knew vibration was a problem. They in fact had methods to help alleviate it(adding mass(balancing momentum) since the clock makers in the 1600's) No one had the mathematical model to actually describe the problem and how to resolve it. They had pretty good approximations along with common sense. Common sense goes a LONG ways. Give me someone with common sense over a PHD anyday. Testing and trial and error are quite often quicker and far more practical than any calculated computer model ever will be. Why the F-35 isn't finished yet. Too busy playing with computer models and the MUST NEVER FAIL mentality from the DoD where eggs and braid are trying to climb the ladder, when in fact it would be vastly cheaper/quicker to just make several different designs and start testing the damned things. Why in the 50s/60's/70's had dozens of new aircraft and thousands built when today we have one new design and maybe a couple hundred built. Of course good enough is good enough and no sense reinventing the wheel, so not a 100% true allegory.

rmh

We spend 100% of the money and effort, to develop products which are ultimately only 50% of the intended result.
Government efficiency at it's finest .
Of course -the real efficiency experts recommended we spend only half the projected costs and produce results which are 1/4 of the proposed goal.
( My real issue with the math approaches )
We need to THIMK SMARTER

otrcman

Alasdair,

Sorry to take so long to post an answer to your question. I'm finally home and caught up with my backlog of things to do.

First off, I should say that I agree with almost all of the prior posts. In short, there are many effects produced by a propellor, some favorable and some unfavorable. The destabilizing effect that you asked about is one of the "unfavorables".

The basic premise of a propellor is that it applies force to a mass of air, accelerating the mass rearward. In turn, the air mass applies an equal and opposite force on the propellor, pulling the airplane through the air. This all goes as intended until we misalign the airplane with the airstream direction (i.e. sideslip). In the sketch below I have attempted to help visualize the effect.

Referring to the sketch, you can readily imagine that the air mass passing through the propellor disc is accelerated rearward, providing thrust to move the airplane forward. But notice also that if the airplane is sideslipping at all, then the air mass is straightened slightly to flow along the long axis of the airplane. In effect, the propellor is slightly "turning" the air mass that passes through the prop disc. This "turning" requires exerting a side force to the air mass, which in return applies a side force to the propellor. Since the propellor is at the nose of the airplane, the result is a yawing moment. The yawing moment is zero if the sideslip is zero, but as sideslip increases, the side force also increases. The more the sideslip, the greater the side force to increase the sideslip. That's the very essence of an instability.

So what are the parameters controlling this unfavorable effect ? Well, they are just what you would expect. The larger the air mass to be deflected and the greater the deflection angle, the greater the side force. So a big propellor and a lot horsepower will maximize the destabilizing effect. Also, the greater the distance from the prop to the CG, the greater the yawing moment will be.

My first insight into this phenomenon came as the result of a fatal airplane accident involving a fellow engineer at my workplace. A very gifted machinist at the NASA Flight Research Center had created a neat little homebuilt, which he called the Mini Mustang. It was quite small, and had a lot of power for its size. The builder let a number of coworkers fly the plane, including some of our test pilots and engineers. A couple of people remarked that the airplane was very pitch sensitive and that the CG should be moved forward. Unfortunately, the forward CG was limited by landing gear position in that it would nose over if the CG were moved any further from it's present position. The inevitable soon happened, and one of the guest pilots pulled a wing off the plane while entering a loop at high power and high airspeed. Analysis of the wreckage showed that the wing had failed at about 14G's, indicating a pitch rotation far beyond what the pilot intended.

This all happened in 1964. I was a new hire at the time, but many of our senior people had come through the 1930's and 1940's and had participated in testing and development of the high high powered fighters of WWII. They went to work on analyzing the Mini Mustang accident and concluded that the destabilizing effect of the propellor was a major contributor to the accident. This was in pitch, mind you, but the effect is the same in either pitch or yaw. The machinist built a second Mini Mustang, incorporating fixes prescribed by our senior engineers. The second airplane was far more stable and flew for many years. That plane still exists, although I believe it was retired when the builder got too old to fly.

I was only a young observer of this event, but you can bet I was like a sponge, absorbing as much wisdom as I could from the folks who had "been there, done that" during the heyday of the ultimate piston engine fighters.

Edit: Ignore the first image. The first image was too light, so I darkened it up and re-posted. But now I can't seem to delete the first image.

Dick

rmh

The contra rotators - have two flywheels spinning up front
opposite directions but still- spinning flywheels
they also have precession DURING any deviation from a given flight path
Do you believe these forces cancel each other????????????
Nod once for yes - twice for no.

BFoote

Yes, Newton's first and second law of motion still apply.

MTK

The way they are set-up, they are not identical flywheels. They just about cancel but not quite. However, the difference is practically imperceptible.

I've flown a well set-up contra driven pattern plane exactly once. It flew very well. But so do my single prop models. Is it necessary to win? No....But Pattern competition at the highest level (actually at any level) is a very fickle endeavour. Once someone wins something important with ANYTHING different, everyone flocks to the difference. Many don't know how to think and follow blindly...

No nods, sorry = different

rmh

I agree - what is a supposed theoretical "perfect" arrangement - in practice is NOT.
Been at this pattern stuff a long time - seen the better ideas come n go.

In my book -you can't beat the most simple arrangement -fine tuned .
The FAI plane my son Guy is flying is pretty darn good - It is Jesky's latest design - with lots of power and is very light.
The result is constant speed - rapid deceleration OR acceleration on demand
No crackpot trim setups - very close to 0-0-0 as basic arrangement with some right thrust.
He does like about 3 % throttle down> down elev couple- very little with no surprise effects.
The good ol 4stroke setups -in my book- add too much complexity and vibes -(you can not eliminate em)
But it's what makes a horserace - believing you have a better setup and convincing others it is the best.
I love the 'theoretical approach to resolving a problemas long as it proves to work as intended in actual practice.
If you have not flown it to prove it - it is of no value - no matter how it looks on paper on in the bible.

MTK

In mine too....BUT I still won't fly big electric. Just because EVERyBODY in my area does, doesn't mean beans to me.

And now that the power of correctly sized 2 stroke gas has gotten big enough (rivals the YS big blocks) and friendly enough, with minimal vibes, it's about as simple and friendly as it gets. Still "need" the rubber iso mount tho....but even this gizmo has gotten simple and lightweight.....

I think I have an answer to the spiralling air stream conundrum of single props. Without further destabilizing the model in yaw

rmh

I did a few 40 cc pattern planes over 10 years ago - they worked just fine - but were unpopular. too different
Guy want's me to do another - - we had very little vibe - using our own rotary cradle mount. but the pipe setup demanded good air flow and header couplers had to be correct -etc.. The weight we found could easily stay within the rules .
One of the less expensive Hackers -instead of the high buck Plettenberg -is also a good performer The need to follow the leader still prevails -for many. I also think the YS is grossly overpriced for the results - my opinion.

MTK

I remember reading some of the 40cc stuff here, That's where I got my inspiration to make the change from glow to gas back in 2009.

I still have a ZDZ40F3A rear exhaust. Never was happy with its performance. Looked high and low for a really good set-up and I've found it withOS GT33. I hope OS offers it in rear exhaust. Much lighter than the ZDZ and Powerwise, at least equal to YS175. And some other makers are coming out with new stuff that promises to be better. We'll see!!

A hot pipe has not been much of an issue with Ed's new epoxy he uses. I have one 40G in service for 4 years now, some 600 runs, some150 hours. Didn't care much for available couplers so I created my own solution that's better than anything else I've seen or used before

But we digress from the original thread