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Aircraft too fast, aircraft too slow

Vitus

Vitus

Resource contributor
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newzealand
I am at a loss with this...
The Vega has really interesting flight characteristics:
Cruise speed: ~165 mph
Max speed: ~185 mph
Landing speed: 60-65 mph (no flaps installed!)
Climb rate : ~1,300 fpm

An interesting piece of observation is that she has "gliding capabilities like a boulder", which points towards a high drag factor (looking at you, landing gear!).

I'd say that those are some really impressive numbers but I'm having trouble to put it all together in the sim. My main issue is that when tuning for these values the aircraft seems to be waaaaaay overpowered. It takes off within a few seconds but I know that the take-off run of an empty Vega takes around 15-20 seconds, she seems to be very sluggish on the acceleration.

Could you please give me directions which variables I should have a closer look at to achieve both: a sluggish acceleration as well as the correct cruise speed?
 
I'll leave the details to the experts (especially when it comes to prop characteristics), but slow acceleration means low engine power (or prop efficiency) and high cruise speed means low drag scalars.

While you can get away with tweaking things in the aircraft.cfg to get into ballpark regions, the correct approach would be adapting the AIR file based on aerodynamic reference data for a Vega 5. But that's an entire different field and requires an excursion into the SDK documentation and the resources section here for Yves' documentation on flight dynamics in MSFS.
 
I did spent a considerable time with the aerodynamics already and read the documents you mentioned. The overall aerodynamic model is pretty good as it is right now and it's based on the sparse real-world data I have. I updated the air file with the variables for thee clark-y and adjusted the weight and balance.

So right now I am more looking at some coarse methods to get the performance into the envelope. I agree with your logic - low power + low drag. However that doesn't really square with the observation of a shitty gliding experience and that's a bit puzzling...
 
Glide performance is strictly Lift/Drag ratio controlled. Drag at that airspeed is different than drag at cruise speed. You have parasite and induced drag... consider which one is more important at a given condition of flight. Just a suggestion.
 
Vitus,
To add anything sensible to what you have already received, I would need more detail. Specifically which version are you modeling. I see that the early versions had 225 HP and later ones had 450 HP. That makes a lot of difference. I see that empty weight is 2565 lbs and loaded weight is 4500 lbs. Which weight are you using for testing.
There is a lot of science in modeling flight characteristics and what you do is try to match data points. When you do not have these you have to see what other similar airplanes might have a manual. You will not find vague statements like "gliding capabilities like a boulder" in a manual. Was the poor take-off acceleration associated with a model that might have had a fixed pitch prop? That was typical of a lot of "high speed" airplanes of its time. To get the high speed the prop would have a coarse pitch which was very inefficient on take-off.
I'd like to offer some help, but without more detail I'd be purely guessing
Roy
 
Don't forget to minimize any prop wash effects. I figure that the high wing doesn't get much, if any wash.
 
A piston prop plane has negative prop thrust when the throttle is closed as it might be on an approach. That acts like an increase in drag and could give a steep approach path. Seems to me your issues are prop related.
Roy
 
Hey guys,
thank you all for helping me out! Much appreciated!

I am still to come across proper historical documents in regards to the Vega, I think I mentioned somewhere else that I wasn't able to find the old operating manual or detailed performance charts. Not even the Smithsonian seem to have those documents. So all I can go by is vague pilot reports and general specs that I could find.

And Roy, just to clarify:
I am currently looking at the Vega 5C. Equipped with the P&W 1340 of about 450 HP. Prop is fixed pitched, however I do not have the data for the prop that was used for this aircraft. I know it's a metal Hamilton with about 9ft diameter but that's all the information I could find. :confused:
The Vega in the video I mentioned is one built by the "Detroit Aircraft Corporation" - a metal aircraft, while the Vega I represent is the original wooden one. I don't know if that makes a lot of difference in weight, but I doubt it. If anything the metal Vega might even be a wee bit lighter. Also the Vega in the video is using a constant speed prop, yet it's much more sluggish than my representation.

I am looking at both, the empty aircraft as well as the fully loaded one and I have no idea if the performance data I have is for a the empty plane, the fully loaded one or something in between. As you can see, there are a lot of unknowns in this equation. The good news is that if I wasn't able to find detailed data, neither will the users. And so ultimately the performance data I will come up with will be the only one available :rotfl:
It does give me a lot of creative freedom, but also poses a challenge. If it was as easy as punching the right numbers into the flight model and everything is fine, I wouldn't be starting this thread.

Thank you for the remark on the propeller. It didn't even occur to me that the drag of the propeller, combined with the inefficient itch during takeoff, could be part of the solution. I'll investigate that.

All of your input has been great so far! Keep it coming! :wizard:
 
I think I'm getting somewhere with this. I adjusted the power section of the air file and I can see the effect immediately. It does need a lot more tweaking but I am getting results with this now. So again, thank you for your input guys!

I have a tiny follow up: does the propeller pitch angle actually play any part in the flight model or is it just a reference to the associated power curves? And also I'd like your opinion as to what type of propeller would be installed in an high-performance Vega? What pitch am I probably looking here? 20 degrees? 30?
 
And here's another follow-up question. I put a "performance tuner" in my debug gauge to observe the engine's variables.
oAmU7Rk.jpg

One thing I noticed is that the brake power can exceed the maximum power of my engine. How can that be a thing? The engine is rated 450 HP, but during take-off I get a brake power of 655 hp. One thing I might need to note is that I am using a gear-reduction of 1:2, which is why the prop rpm is lower than the engine rpm.
 
In the simplest of terms, and pointing out that I grabbed text from a ground vehicle discussion... this is why it's different:
Brake horsepower (bhp) is the measure of an engine's horsepower without the loss in power caused by the gearbox, generator, differential, water pump and other auxiliaries. The actual horsepower delivered to the driving wheels is less.
Click to expand...
 
This doesn't make much sense to me. Brake power is whatever your engine delivers onto the shaft. My understanding is that the only way to measure an engine's power output is to observe its torque and rpm. From that I thought that the max HP given by any engine is actually the maximum brake power this engine can produce.

Here comes the weird part: The BHP indicated is HIGHER when I put a gear reduction of 2:1 in my prop section. If I change it to 1, I get about 2/3 of the BHP indicated before. This makes no sense to me. The BHP is a function of torque and rpm. When I have a gear reduction in place, the rpm goes down, the torque goes up. The Horsepower should stay the same. Where am I wrong in my thinking?
 
HP is measured using a torque converter... BHP is measured using brake band. HP is measured at the output (rear axle as example), BHP is measured at the flywheel (main engine shaft). 450HP rating is not the BHP value. As far as I'm aware, HP is the value referenced when discussing engines and BHP is rarely a spec that is mentioned or actually known.
 
Ah! I think the lights are coming on now :idea:

The reason why all of this comes up is this: in many manuals I find power limitations of the engine. I try to get a representation of these variables in my debug gauge. When the manual lists maximum take-off power or maximum continuous power, how can I see my engine's current power, since BHP doesn't seem to get me anywhere?
 
I see you got it working the way you want. I remember someone, I think Roy Holmes, found a way to add drag to pavement motion, like tire drag when on the ground, which helps slow the plane down a bit on take-offs when it seems its just accelerating too fast.
 
Maximum take-off power, its the maximum power at maximum RPM, in a certified airplane engine, the engine should be capable to handle that power for 5 minutes before blow up the pistons.
So, you need to match the maximum power at maximum RPM.
I don't remember if PW R1340 has a gear reduction box, i think the engine RPM should be the same as propeller RPM.
 
Yes, this issue is pretty much resolved by now. I still tinker a little bit, but overall I got what I want, thanks to the help here in the forum I have a much better understanding of the performance model used by the sim and thanks to some amazing people over at sim-outhouse I have a very good understanding of the engine I am modelling. So I can help with the memory by saying: it depends. I'd say that the R-1340 is not a designation for an engine, but rather for an engine family. It was developed over a loooooong period of time and some things changed fundamentally from one version of the engine to the other. If you dig into http://www.enginehistory.org/ you'll find most answers to those questions. Some of the R-1340s did have a gear-reduction and the civilian version of the engine got a designator "G" to indicate that it's a geared engine. Turns out the R1340s for the Vega were mainly C1s and SC1s, so no reduction gear. :teacher:
 
Vitus said:
Yes, this issue is pretty much resolved by now. I still tinker a little bit, but overall I got what I want, thanks to the help here in the forum I have a much better understanding of the performance model used by the sim and thanks to some amazing people over at sim-outhouse I have a very good understanding of the engine I am modelling. So I can help with the memory by saying: it depends. I'd say that the R-1340 is not a designation for an engine, but rather for an engine family. It was developed over a loooooong period of time and some things changed fundamentally from one version of the engine to the other. If you dig into http://www.enginehistory.org/ you'll find most answers to those questions. Some of the R-1340s did have a gear-reduction and the civilian version of the engine got a designator "G" to indicate that it's a geared engine. Turns out the R1340s for the Vega were mainly C1s and SC1s, so no reduction gear. :teacher:
Click to expand...
Thanks for this, this site http://www.enginehistory.org/ is pure gold!
 
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