Fokker D.VIII (FC2 vs. RoF)

[ZachariasX]

2 hours ago, Holtzauge said:

 

I saw you made an edit @ZachariasX and I found some new info as  well:

 

I just found a couple of French tests for captured Albatros D.Va:

 

Test 1: SL top speed 172 Km/h with 2.75 m dia 1.97 m pitch propeller at 1500 rpm.

 

Test 2: SL top speed 171 Km/h with 2.8 m dia 1.795 m pitch propeller at 1555 rpm.

 

I think this also fits the momentum theory idea well: Both these aircraft are almost identical in speed: 171 versus 172 Km/h but one attains the speed by using a larger diameter propeller with lower pitch and the other one a smaller diameter propeller with a larger pitch.

 

However, if we use the rpm/pitch method to predict speed:

 

1500/60*1.97*3.6=177 Km/h

 

1555/60*1.795*3.6=167 Km/h

 

So in one case positive and in the other negative “slip”……..

 

If we now instead put on a theoretical prop in between those two diameters and pitches then we would probably be right on the money so for some diameter and pitch combinations the method seems to work well and for others it doesn't. I think this is why you see such good correlations in some cases, e.g. for the Dr.1 and why it does not work out so well in others like for the Morse and the D.VIII.

 

You know, I do this "rule of the thumb" mostly due to sanity checking data as well as to get more of an idea of what kind of arrangement one is looking at. Professionally I do make audits of the work of smart people, often smarter than me. But everyone makes mistakes, especially myself. (And yes, I can be awful to people, but you knew that.) The trick is now to find a way to put numbers in relation and if the relations do not fit anymore, then you know that "something is going on".

 

The"slip" is easily documented for aircraft and can readily be found (and calculated) from the airspeed tables in the PN.

 

The slip increases when:

• wrong prop
• low revs / excessive revs
• coarse prop
• draggy airframe
• heavy aircraft
• thin air

Maybe in that order. Or not, but you get the idea.

 

Now lets look at the ONLY case I'm looking at, this is steady state speed at given rpm setting. We can hopefully agree that identical propellers perform (for practical purposes) identacally on different aircraft.

 

Steady state speed at given rpm are defined exclusively by generated thrust and aircraft drag. Drag having a component of induced drag, but in low altitude high speed flight I'll ignore that factor, knowing that I'm making a slight error. Then I just have airframe drag that can vary.

 

Now the point is, if I have conceptually identical aircraft that feature the same engine/prop, one with lots of slip and the other with little slip, the ONLY reason for having more slip is larger drag. And we can make our life easy by looking at sea level speeds, where the engine has power and the propeller has bite.

 

So if I have two aircraft, the Camel and the Fokker D.VIII, both having a similar propeller, D2.5, P2.6 and the Camel only has 5% slip, but the cool, new, monoplane D.VIII has 10% slip, this would mean that for this 5% speed penalty, the Fokker D.VIII is 10% draggier than the Camel! Can this be? If you say the D.VIII had a 2.6 m pitch prop, you're saying just that, draggy D.VIII! Actually coming back to the error I made previously, it is worse as I would guess the the D.VIII has considerably less induced drag than the Camel!

 

See, when people state flight speeds, they at times publish far more info than that. If you look at what a specific datum means, you can judge a test under a whole new light.

 

Hence, I am not convinced that the D.VIII in the test has the propeller as stated. If there is another publication stating that, they might have well copied the error. As long as I do not see the factory sheet of that type of AXIAL propeller, the most likely possibility is the guy typing goofed up there.

 

Btw. I have yet to see any flight  test of someone revving the Camel to 1380 rpm in level flight! You get 1200ish, and that's that. The idea that the D.VIII has so much power and it can crank the 2.6 m propeller to 1380 rpm, then it would actually have almost 20% slip! And the aircraft would still be as slow as the Camel. It's just ridiculous. If the Camel did 1380 revs, it would go almost 130 mph. I mean, just no.

 

 

 

[jollyjack]

Now after a few days of this Fokker D8's release, i still feel like it's sort of a flying orange crate ... must be personal: i like the Nieuport a lot more LoL.

Must try the F8 in RoF ...

[Holtzauge]

3 hours ago, ZachariasX said:

 

You know, I do this "rule of the thumb" mostly due to sanity checking data as well as to get more of an idea of what kind of arrangement one is looking at. Professionally I do make audits of the work of smart people, often smarter than me. But everyone makes mistakes, especially myself. (And yes, I can be awful to people, but you knew that.) The trick is now to find a way to put numbers in relation and if the relations do not fit anymore, then you know that "something is going on".

 

The"slip" is easily documented for aircraft and can readily be found (and calculated) from the airspeed tables in the PN.

 

The slip increases when:

• wrong prop
• low revs / excessive revs
• coarse prop
• draggy airframe
• heavy aircraft
• thin air

Maybe in that order. Or not, but you get the idea.

 

Now lets look at the ONLY case I'm looking at, this is steady state speed at given rpm setting. We can hopefully agree that identical propellers perform (for practical purposes) identacally on different aircraft.

 

Steady state speed at given rpm are defined exclusively by generated thrust and aircraft drag. Drag having a component of induced drag, but in low altitude high speed flight I'll ignore that factor, knowing that I'm making a slight error. Then I just have airframe drag that can vary.

 

Now the point is, if I have conceptually identical aircraft that feature the same engine/prop, one with lots of slip and the other with little slip, the ONLY reason for having more slip is larger drag. And we can make our life easy by looking at sea level speeds, where the engine has power and the propeller has bite.

 

So if I have two aircraft, the Camel and the Fokker D.VIII, both having a similar propeller, D2.5, P2.6 and the Camel only has 5% slip, but the cool, new, monoplane D.VIII has 10% slip, this would mean that for this 5% speed penalty, the Fokker D.VIII is 10% draggier than the Camel! Can this be? If you say the D.VIII had a 2.6 m pitch prop, you're saying just that, draggy D.VIII! Actually coming back to the error I made previously, it is worse as I would guess the the D.VIII has considerably less induced drag than the Camel!

 

See, when people state flight speeds, they at times publish far more info than that. If you look at what a specific datum means, you can judge a test under a whole new light.

 

Hence, I am not convinced that the D.VIII in the test has the propeller as stated. If there is another publication stating that, they might have well copied the error. As long as I do not see the factory sheet of that type of AXIAL propeller, the most likely possibility is the guy typing goofed up there.

 

Btw. I have yet to see any flight  test of someone revving the Camel to 1380 rpm in level flight! You get 1200ish, and that's that. The idea that the D.VIII has so much power and it can crank the 2.6 m propeller to 1380 rpm, then it would actually have almost 20% slip! And the aircraft would still be as slow as the Camel. It's just ridiculous. If the Camel did 1380 revs, it would go almost 130 mph. I mean, just no.

 

 

 

 

OK @ZachariasX, I still remain sceptical but I know you are fond of this method so I won't torture you with any more exceptions and leave it at that.

[ZachariasX]

8 hours ago, Holtzauge said:

 

OK @ZachariasX, I still remain sceptical but I know you are fond of this method so I won't torture you with any more exceptions and leave it at that.

Oh, please do. The „method“ is just one way of making a „checksum“ of data. Sometimes it is off and sometimes you know why and sometimes you just don‘t. In this case it looks weird and it makes me want to find a test of an aircraft that flew at a similar rating, yet all the numbers Iˋve seen with the British/French aircraft are done on a lower rating with 100 rpm or so less.

 

Why would‘t they run a Camel with 1380 rpm when such could be conveniently done? Or did they and did they produce hard numbers?

[ZachariasX]

Maybe I have to put it in another way, reduce the issue to the laymans perspective.

 

Thommy and Tony are buiding and selling aircraft. Both try to sell you their aircraft. Both aircraft have the same traction unit.

Oneself is shopping for the fastest available, because speed matters a lot to one, but also endurance to some extent.

Thommys aircraft makes 115 mph while having the engine rev at 1280 rpm.

Tonys aircraft makes 115 mph while revving the engine to 1380 rpm.

Do you buy at Thommys' or at Tonys'?

 

 

[BMA_Hellbender]

30 minutes ago, ZachariasX said:

Thommys aircraft makes 115 mph while having the engine rev at 1280 rpm.

Tonys aircraft makes 115 mph while revving the engine to 1380 rpm.

Do you buy at Thommys' or at Tonys'?

 

I choose the third option: I buy from the guy who figured out how to make the wings withstand >10g using only wood, wires and fabric.

[Holtzauge]

And I choose a fourth option: Buying a plane from Fritz who builds planes that work according to Newtonian physics.

[ZachariasX]

16 minutes ago, Holtzauge said:

And I choose a fourth option: Buying a plane from Fritz who builds planes that work according to Newtonian physics.

Always the good idea. But you are saying that does not give me anything on relative performance, according to Newton?

[Holtzauge]

Apparently it's not only the US testers that tested the Fokker D.VIII at McCook that cannot document propellers properly:

 

In this test of a captured Fokker D.VII the French claim that the Fokker they tested had a Wolf serial number 12004 with a diameter of 2.8 m and a pitch of 1.81 m at 0.8 m and 1.89 at 1 m.

 

In the test they got 188,2 Km/h top speed at 1565 rpms at SL:

 

Let’s enter that into pitch/rpm formula:

 

1565/60*1.81*3.6= 170 Km/h

 

OK, let’s be generous and use 1.89:

 

1565/60*1.89*3.6= 177 Km/h

 

Still nowhere close to 188.2 Km/h and the "slip" now has the opposite sign as well?

 

Jacques who documented the propeller data is now at Verdun with a Lebel rifle wishing he was not such a sloppy reader.........

 

Edited June 4, 2021 by Holtzauge

[ZachariasX]

That's interesting thank you!

 

2 hours ago, Holtzauge said:

Still nowhere close to 188.2 Km/h and the "slip" now has the opposite sign as well?

Actually, I have yet to find a single article in literature where it has. Or just one diagram. Or even just the mentioning that a propeller could in theory advance his aerodynamic/hydrodynamic pitch. You would know of one? One single textbook diagram? I am actually looking for it, but to no avail.

 

I just know this:

The plane is never ahead of the propeller. Never. (In this case though it could, as aerodynamic pitch of cambered propeller profiles actually does precede the geometric pitch, but but to a very limited degree compared to what is expected for lagging.)

 

Until then, Jaques might have had a keen eye, but accurate speed measurements are not trivial. And I indeed have less problems believing the US report than Jaques testimony.

 

Let me ask you this: If jaques measured 198 km/h, would you go with it? 208? 218? Would there be a cutoff value where you'd be sceptical of the data? Or, is there a range in which you would say "If it is like that, then it is bad."?

 

The example form Tony and Thommy I took from the actual airctaft. One is the test I linked above, the other is this:

All I did is round the Clerget Camel to 115 mph (which I guessed as sutable sea level speed) from the "recorded" 113 at 10'000 ft.

 

Now if you campare the the two at given power settings, the Camel and the D.VIII, which one has more drag? I'm asking. I don't want to frustrate anyone here, I'm honestly curious. I kept it in the thread as it still relates to the D.VIII, but if there is something general, unrelated about all that, I'll leave it here like that.

 

In the end, I'm just happy that they did match the in game D.VIII with the US report mentioned. So they were definietly doing more than copying a RoF aircraft to FC. And I think that shows promise.

[Monostripezebra]

All I can say is it looks pretty amazing from up close:

 

[Diggun]

2 minutes ago, Monostripezebra said:

All I can say is it looks pretty amazing from up close:

 

You're rapidly carving out a niche for yourself as the zany video guy, and I love it ?

[Holtzauge]

1 hour ago, ZachariasX said:

That's interesting thank you!

 

Actually, I have yet to find a single article in literature where it has. Or just one diagram. Or even just the mentioning that a propeller could in theory advance his aerodynamic/hydrodynamic pitch. You would know of one? One single textbook diagram? I am actually looking for it, but to no avail.

 

I just know this:

The plane is never ahead of the propeller. Never. (In this case though it could, as aerodynamic pitch of cambered propeller profiles actually does precede the geometric pitch, but but to a very limited degree compared to what is expected for lagging.)

 

Until then, Jaques might have had a keen eye, but accurate speed measurements are not trivial. And I indeed have less problems believing the US report than Jaques testimony.

 

Let me ask you this: If jaques measured 198 km/h, would you go with it? 208? 218? Would there be a cutoff value where you'd be sceptical of the data? Or, is there a range in which you would say "If it is like that, then it is bad."?

 

The example form Tony and Thommy I took from the actual airctaft. One is the test I linked above, the other is this:

All I did is round the Clerget Camel to 115 mph (which I guessed as sutable sea level speed) from the "recorded" 113 at 10'000 ft.

 

Now if you campare the the two at given power settings, the Camel and the D.VIII, which one has more drag? I'm asking. I don't want to frustrate anyone here, I'm honestly curious. I kept it in the thread as it still relates to the D.VIII, but if there is something general, unrelated about all that, I'll leave it here like that.

 

In the end, I'm just happy that they did match the in game D.VIII with the US report mentioned. So they were definietly doing more than copying a RoF aircraft to FC. And I think that shows promise.

 

First of all, I think the reason you are not finding any diagrams with a shall we call it negative slip is that those diagrams you are finding on-line are made with the good intentions to explain how propellers work in general sense without going into momentum theory.

 

Second point I think you already answered yourself: Propellers have a “wing” type profile so they will be producing thrust even if the local angle of attack is at zero like at the geometric pitch speed. Also, if you look at which are the cases where there is a negative slip? Note that they are all for larger diameter propellers, e.g. 2.8 instead of 2.6 m which again is exactly what would expect from momentum theory.

 

Another good exercise could work out the advance ratio J=v/(n*D) and the pitch ratio d/D and use those and look in NACA report 141. For example use the French Fokker D.VII test with 188 Km/h top speed: If I worked it out correctly this gives gives a J=0.71 and pitch ratio of 0.65 to 0.68. You will find a number of different propellers configurations that still have quite decent efficiencies under those conditions.

 

Finally ask yourself: Why are there so many cases now from IRL flight measurements that do not fit the pitch/rpm model? I mean, maybe one transcriber can’t read what’s written on the propeller or it gets “lost in translation” but when you start seeing three or four sources pointing in the same direction you have to ask yourself what is the most plausible explanation? Limitations in the pitch/rpm model or a long string of mismeasured speeds and propellers?…..

 

In some sense this is an interesting discussion but I feel we are going in circles. In my simulations I get perfectly valid results for all the IRL aircraft measurements we have discussed above so I don't need to assume there is anything wrong with them to fit my model. So summing up my take on this is this: The pitch/rpm model works better for some diameter and pitch combinations than others but it has its limitations and this will have to be the point at which I call it a day and move on!

[Monostripezebra]

7 hours ago, =IRFC=Hellbender said:

 

I choose the third option: I buy from the guy who figured out how to make the wings withstand >10g using only wood, wires and fabric.

 

You mean Alfred Vogt, right?

 

 

1 hour ago, Diggun said:

You're rapidly carving out a niche for yourself as the zany video guy, and I love it ?

 

I have always been doing that.. though. Torough close up testing.