Tony K

Just found this on another forum; So maybe Einstein was wrong afterall.

Tim, just as a matter of interest, what do you draw your plans on? Do you use draughting film?

BBC A symbolic improvement, I'd say.

PVA and pressure works for me. Surprised it's not been mentioned, but I think it is worth repeating, that you need to give the plywood a light sanding to remove the release agent before attempting to glue it.

Posted by WolstonFlyer on 13/10/2012 10:22:01: Actually it has no stars because the stars are grey and not filled in blue like other rated products on that website... but hey that is a minor point! Well, I am not sure which site you are looking at. I am looking at this one. Under "Bewertungen", Overall assessment - 4,9 from 5 Expectations fulfilled - 4,9 from 5 Price - performance relationship - 5 from 5 User friendliness - 5 from 5 Design - 4,6 from 5

Barrie, I use one of these. It has a five star customer rating on the German Conrad site.

Posted by Stephen Grigg on 13/10/2012 00:04:22: I thoroughly enjoyed the programme,The point made was that Black pilots were considered inferior but proved they were so good that the bomber crews they supported always requested them because they were best at the job of support and there identity was kept secret,but after a bomber crew crash landed an were brought to there base,the truth was out.They returned to the US and were treated as second citizens once more.Because a black person at that time could not give any ahite man an order,the whole squadren was black and highly successful. Stephen, good synopsis. My view is that the program is called "Heroes of the skies" and all pilots who flew in a war are heroes (whatever country they flew for) but shouldn't this program focus on those who were particularly heroic, whose deeds stood out from the standard heroism? It is my understanding that the featured fighter group used the tactic of staying with the bombers they were defending rather than chasing the enemy's fighters and that is the reason they were favoured by the bomber crews. Despite this, many bombers were lost to enemy fire. I may be wrong but I also understand that not one pilot of the 332 group was ever given the title of "ace". I am not denigrating any of that partcular group of heroes but the program should be about flying not racism.

Just watched it but I wished I hadn't bothered. What point were they trying to make?

Posted by John Privett on 27/09/2012 19:09:07: I'd say stay at McLaren. As an associate member of Myron's Pedant gang (see special issue feedback thread) I would like to congragulate you for the correct spelling of Bruce's surname. Edited By Tony K on 28/09/2012 12:28:46

Posted by Biggles' Elder Brother - Moderator on 27/09/2012 10:15:26: Posted by Martyn Johnston on 27/09/2012 09:06:53: Actually, I wasn't going to mention it - I just inwardly cringed! - but you bringing it up Martyn has given me renewed courage! Perhaps we should form the "Campaign for the Correct Use of Units" In which case, as you probably know, the units of torque are Newton metres (N.m) but N.cm would be more practical for our purposes. In fact I have a servo box in front of me (GWS IQ-140) on which the torque is quoted as 16Ncm. Nothing wrong with pedantry.

Posted by Erfolg on 20/09/2012 23:50:33: As I think we are all agreed, there does no(t) appear to be a fundamental issue with just statically balancing a prop. This sentence, along with the rest of Erfolg's post, just about sums it up.

Watched it this morning. I thought it was quite a well balanced overview considering the time available. So was it enemy fire which brought him down? Was the collisiion story just propaganda?

Posted by Biggles' Elder Brother - Moderator on 18/09/2012 22:31:22:   Static Balance - lieterally means "balanced whilst not moving" (hence the static bit!) This is what we do on a prop balancer.The problem is - as some have indicated - something can be statically in balance - but still not be in balance when you actually spin it up. Why?     That is the literal meaning of static balance however revolving parts can be, and often are, statically balanced by spinning them on a balancing machine. For example, a motor car crankshaft and flywheel. The crank would be dynamically balanced first, then the flywheel would be fitted and the assembly spun up on the same dynamic machine. The flywheel, however, is statically balanced. I'm with Graeme Evans and Shaunie on this. As for "inertia forces", surely they balance out? Edited By Tony K on 19/09/2012 06:32:49

I have spent some time reading and thinking about how a propeller works and I have arrived at the following conclusions. I would be pleased to hear any valid arguments against the following notes. A propeller or fan blade acts as an inclined plane. The actual shape of the blade and its aerodynamic properties, is a more advanced subject and is irrelevant here. By moving an inclined plane one can redirect a force so that the force out can act in a different direction to the force in. In the case of a fan, the power provider is forcing the blades to rotate in order to produce a force parallel to the axis of rotation. The inclined plane, the fan blade, is forced through the air and applies a force to the package of air immediately adjacent to it. As the blade has an area, and pressure is force/area, there is a pressure applied to the package of air which causes it to move. The back of the blade applies a push, the front does the same in reverse and applies a pull. As the air is caused to move it gains momentum and where it is not in contact with the thing making it move there is no more pressure acting on it. It does, however, have a dynamic pressure which will be presented later. The process is, of course, continuous. The fan does not throw out packages of air, but applies a constant force to the air which causes it to move, thereby endowing it with a momentum. Air can be considered as an incompressible fluid at mach numbers less than 0,5 so as the air moves away from the fan, new air must replace it. The rate of air coming in must be the same as the rate of air going out, that is, the mass flow entering the fan disc is equal to the mass flow exiting. Mass flow is air density times velocity times cross section area. So considering the flow in versus the flow out, we have no change in density and the cross section area of the front of the fan is the same as the back so the velocity must be the same. The speed of the air entering the fan must be the same as the speed of air exiting. There is no acceleration. Furthermore, we have no change in density so there is no change in the total pressure. Moving air has a dynamic pressure which is balanced by a reduction of static pressure. Dynamic pressure is calculated using density and velocity. If density and velocity are the same there can be no difference in dynamic pressure, therefore there can no difference in static pressure. The total pressure of the air entering the fan must be the same as the total pressure exiting. There is no pressure difference. Returning to momentum and thrust. Thrust is a force and force is the rate of change of momentum, that is, the rate of change of mass times velocity with respect to time. This can be arranged two ways. The first is change of velocity divided by change of time all multiplied by mass. Change of velocity divided by change of time is acceleration so force equals mass times acceleration. This is the commonly known equation F = m.a. Note that this is mass, not change of mass, times acceleration, not change of acceleration. The second arrangement is change of mass divided by change of time all multiplied by velocity. Change of mass divided by change of time is mass flow so force equals mass flow times velocity. The equation becomes F = mdot.v (mdot being the symbol for mass flow). Note again that this is not change of mass flow and is not change of velocity. From this it can be concluded that the thrust of a propeller/fan is purely the result of the weight of air it moves multiplied by the speed it moves that air. Now with the fan operating correctly, the speed of the air moving through it depends on the pitch and rotational speed of the blades (RPM). If the pitch is fixed, the velocity of the air therefore the angle of attack of the blade must depend entirely on the RPM. The airspeed of the aircraft makes no difference to the angle of attack of the propeller blade if it rotates at constant RPM. Following on, all of the theories of propellers which I have read have an illustration of the flow showing a truncated cone downstream of the blades. This suggests that the air accelerates after leaving the fan. One paper published by a university actually has a graph of velocity showing an acceleration. How can that be? If you take away the thing which makes something move, that something will slow down. The downstream air has left behind the propeller. It moves because of its momentum but this downstream flow has no constraints, it is not squeezed into a cone, so it must start to diffuse into the surrounding atmosphere and decelerate. There is no point in the downstream flow, as there is none in the upstream flow, where one can say that the velocity is such and such. In conclusion, this all suggests to me that the generally accepted theories of how propellers work are not quite accurate but, as mentioned, I would be pleased to discuss valid opposing arguments.

Posted by Garbo on 04/09/2012 21:59:58: Harry there are lots of suppliers in Germany who will supply you great products at great speed at a great price. Check out Lindinger they supply German versions (Planet Hobby) of the YT models at 70% of the price. But they wont send them here Probably not important but Lindinger is an Austrian company. In their magazine advertisement they state that, for orders over 90 euros, postage is free to all EU countries.

Posted by Andrew Garnell on 05/09/2012 12:06:36: Hi Harry Where did I get Tony from. Sorry. Regards Who knows? Anyway, I live near Cologne and use Derkum for most of my modelling stuff. Tony.

Posted by Daithi O Buitigh on 23/08/2012 11:51:06: Eric Carter, the last surviving member of Force Benedict, a secret mission to protect the northern Russian port of Murmansk, flew Spitfires during the Second World War. Mr. Carter might have flown a Spitfire but not in Russia. Force Benedict.

Posted by Peter Miller on 25/08/2012 19:35:27: Some should have said to that MP. "If it wasn't for those guns, you would be talking German now" More likely Russian, but thats another subject. To me this is a typical Daily Wail story which gets people exited for five minutes, even if they are not sure exactly what they should be exited about.

Posted by Swissflyer on 24/08/2012 11:21:06: You can find a much longer answer on the Yale University http://www.youtube.com/watch?v=lfXDJKKPGfY From 42m49s onwards the professor talks about energy addition in the context of Bernoulli. Hope that helps Kind regards Mark Mark, thanks for the link. I watched it all but it didn't really answer the question. I did, however, find something else which convinces me that thrust is entirely due to momentum or to be more precise, the rate of change of momentum of a fluid. It's at the bottom of page 3 on this link.

Posted by Swissflyer on 24/08/2012 10:11:39: Obviously there is much more that could be said but I hope to have given a basis to take the discussion further here. Happy flying Mark Excellent posts Mark. If I may make one comment, I think the 85% efficiency is a bit optomistic. Based on previous test figures, I think 75% would be more realistic for the installed system.

Posted by John Olsen 1 on 23/08/2012 22:12:46: ..., it can be found to a first approximation by divding the thrust by the area. John, do a units check and you will see that that is incorrect. But if you pick up the blocks of wood out of the water at the front of the boat, and then drop them back in at the back at the same place they would have been, you will not get any thrust. You may as well not have touched the block of wood. Same applies to the air with a plane. Unless you have acted on it, you have not acted on yourself. (Newton again!) How do the blocks get from the front of the boat to the back? Don't you have to carry them? Isn't that work being done? Bear in mind that although the fan is responsible for the acceleration of the air, that acceleration does not have to take place at the fan. With a parallel duct, as I have already pointed out somewhere earlier, much of that acceleration will take place ahead of the mouth of the duct. No argument with that. I made that point many months ago. I will ask this question once more; Given a fan/ propellor that has a front face and a rear face. The area of front and rear are the same. The air velocity at the front and the rear are the same - V2 = V3, continuity eqation. The density of the air doesn't change so the dynamic pressure is the same - (rho.v^2)/2. The kinetic energy of the moving air is the same - (A.rho.v^3)/2. Therefore the potential energy is the same - Bernoulli and conservation of energy. Therefore the static pressure is the same. Therefore the total pressure, static plus dynamic, is the same. So what or where is the pressure difference? Can somebody give me a mathematical scientific answer?

Posted by Erfolg on 23/08/2012 19:55:34: Tony Try using a "U" tube either side of the fan, then you will see that there is a pressure difference. To measure the dynamic pressure you would use a open ended tube pointing upstream before you start splitting hairs again. How ever you have your angels dancing on a pin, there is a pressure difference, you just have to speak to any H&V engineer. Well, I was hoping for some explanation of mechanical energy in a fluid but instead I get something about fictitious creatures. Not much point continuing, I'll get off here.

Posted by Erfolg on 23/08/2012 14:19:13: Are you seriously suggesting that the pressure in front of a propeller and directly behind it whilst rotating are the same. Yes, I am. The total pressure, static plus dynamic, is the same. I have not seen any mathematical or scientific argument that proves otherwise. If they were, what makes the propeller work? This what i am trying to find out. As dynamic pressure is a vector quantity perhaps it is momentum. There are a number of phenomena which are obvious by observation. Energy flows from hot to cold, pressure flows from high to low and so on. I think you mean a fluid flows from high pressure to low. Boyle was concerned with a closed system, when he derived the gas law. Although the principles can be seen over the weather system, massive scale. Our DF or propeller is not a massive system. There will be no heating due to pressure changes which are of practical value, as the values are very low (miniscule) and any increase quickly lost in the free air outside the duct. I have never suggested there was any significant temperature change. IF you cannot see that the velocity across a propeller (V2 &V3) is essentially constant, or is constant down a DF duct, Of course it is, this is the basis of my argument. In a constant section DF duct the velocity is constant along its length just as the velocity across a propeller disc is. if you cannot comprehend that the pressure immediately down stream of the fan is higher than the local ambient, and it is the fan doing work which accounts for pressure increases which leads to the flow, as the generated high pressure seeks ambient pressure, there is little more to be said. I do understand that the total pressure downstream of the fan is higher than local ambient, although I prefer to use the phrase free stream. It goes without saying that the fan is doing the work, what else could be? What I want to know is how the fan does the work. The phrase "pressure increase" suggests compression of the fluid. Are you suggesting that the air is being compressed by the fan, that its density is increased? Perhaps you mean dynamic pressure which is the product of density and velocity squared. If neither of those values changes then the result doesn't change. What I do not comprehend is why some commonly accepted theories can not be looked at again at a fundamental level, if only to understant them better. For many years it was thought that Bernoulli's theory was responsible for the lift generated by an aerofoil, now it seems Newton might be involved as well.

Posted by Erfolg on 23/08/2012 11:39:50: Tony Note the mention of 6". Where as V2 &V3 can have a difference approaching zero. What our good friend (Froude) realised, is that across the velocity would essentially be exactly the same, that is immediately in front and immediately behind. That does not say that the pressure either side would be identical. It is the pressure difference that causes the airflow and everything that flows from it. I personally would take the work of Froude, Hook, Newton, Euler, and so many more very seriously. Even if you know their are limitations to their work, the self same people often recognised/acknowledged the fact themselves, yet also knew that for most peoples understanding, and practical application, they worked well in our world. Erfolg, I will add Boyle to the list, the relationship between pressure and volume. The only way there can be a total pressure difference is if the density of the air changes. i think you will agree that doesn't happen. We are, of course, discussing V2 and V3 not some arbitrary distance outside of those boundaries. It is obvious that a particle of air will accelerate from free stream at V1 to V2 and will negatively accelerate back to free stream from V3 to V4. So, at V1 you have static pressure, at V2 you have static plus dynamic which is equal to static and dynamic at V3 (because V2 = V3 and the cross section area is the same), then at V4 you have static again. Where is the pressure difference?