160 HP engine is under-powered for pontoons and amphibious

[dany93]

As several people noticed, the 160 hp-powered c172p is under-powered, which makes it "lazy" at full load. Floats and amphibious worsen everything (added drag). I tried to fiddle the propeller pitch angle, at no avail. The original one is not far from the best.

I have a test version with a 180 hp engine. On my c172-180hp branch https://github.com/dany93/c172p-detailed/tree/c172-180hp

Easy for the engine.

The propeller is much more questionable. It is the result of compromise between climbing rate, short field takeoff, and cruise or max velocity. For the propeller, I use the NACA 640 data http://naca.central.cranfield.ac.uk/reports/1938/naca-report-640.pdf. They worked rather well for DR400, DC3, Cap10B. But everything is never certain... And I refuse to cheat the values. I check their consistency, I (re-)calculate the efficiency when I extract them from the curves.

Positive:

• With default gear at full load (2400 lb), I have a 830 - 850 ft/mn climbing rate (80 KIAS).
• With amphibious at full load (2400 lb), 648 ft/mn (80 KIAS). In accordance with @tigert's data https://github.com/Juanvvc/c172p-detailed/issues/269#issuecomment-110268962

No so positive:

• The propeller (76" diameter) does not reach 2700 RPM, which means that the 180 hp are not reached during climbing. But if I decrease the pitch angle, the max air velocity will be lower than I expect. My attempt with a 78" diameter propeller makes me feel that it is not the solution (remains to be seen better).
• Levelled flight 2000 ft, 2500 RPM: with standard gears115 KIAS (hardly higher than with 160 hp). With amphibious 110 KIAS (I would have expected less, and do not know what to do).

My reasoning is that, even if this propeller is still questionable, the result is better than with the 160 hp engine. More pleasant.

[dany93]

Another question: If I set /controls/engines/engine[1]/magnetos to 1, 2, 3, does that mean I must set /controls/engines/engine[0]/magnetos to 0?

I think it is not compulsory, if the starter is selective for engine[0] or [1]. It is just more clean. To check, see the properties as written here https://github.com/Juanvvc/c172p-detailed/issues/273#issuecomment-113734466

[dany93]

I don't quite yet understand which code is reading /controls/engines/engine[1]/magnetos.

At first view, I would say: the FDM.

[onox]

I can't find the word "magnetos" in c172p.xml, other than in a comment.

[dany93]

Yes, but it has to check them to choose starting or not, or stop if we cut them. It contains a link to the Engines folder. Under <propulsion>

[onox]

In src/FDM/JSBSim/JSBSim.cxx I see this:

 631         FGPiston* eng = (FGPiston*)Propulsion->GetEngine(i);
 632         eng->SetMagnetos( globals->get_controls()->get_magnetos(i) );

It looks like JSBSim doesn't publish the magnetos values in its own properties subtree (/fdm/jsbsim). So we just have to read /controls/engines/ then.

[dany93]

Sorry, I have to quit. I think I can go on in one or two hours.

[dany93]

It looks like JSBSim doesn't publish the magnetos values in its own properties subtree (/fdm/jsbsim). So we just have to read /controls/engines/ then.

Confirmed from the Property tree under fdm/jsbsim/ But I see no drawback.

[onox]

I've got the magneto's and starter working here locally.

[dany93]

I saw this as the harder to do.

[onox]

I have already fixed the RPM and exhaust effect. Pushed.

[onox]

@dany93 I must say with the 180 HP it is much better to takeoff from the water :smile: With the 160 HP engine it was too difficult to gain altitude.

[dany93]

Notice that, As every engine is selected by default at start:

• Throttle for both engines is controled by the keyboard and a JS
• the same for mixture and magnetos (keyboard, likely for a JS too). Here, nothing to do, although deselecting the engine wich is not in use would be better. Currently, "s" key on the keyboard activates only starter for engine [0]

However, the bindings from the dashboard: contact key (for magnetos and starter), throttle, mixture lever are to be switched to the active engine. The animations: tachometer, propeller too.

What I would have seen at first thought (which does not mean that I know how to do it, easier to say than to do): Menu, Options to select one engine : flag 160 or 180 if the flag is 180:

• /sim/input/selected/engine[0]=false, engine[1]=true
• "s" on keyboard, magnetos-key and starter-key on dashboard: linked to controls/engines/engine[1]/magnetos and starter. Starter cancelled on keyboard and dashboard for engine[0]. In principle, this is enough to prevent engine[0] from starting. Not sure it will stop it if people do the engine change while flying (weird idea, but it will happen!).
• throttle and mixer dashboard levers: control engine 1
• float chamber priority: /fdm/jsbsim/propulsion/tank[2]/priority=0, tank[3]/priority=1 (not compulsory, can prevent unwanted start).
• Animations: tachommeter, propeller.
• Sound

But the more it is clean in the engine selection, the more it is long for the Repair list

[gilbertohasnofb]

Isn't that what I'm saying?

I don't know, is that a fact, is that the only reason they upgrade to a 180 HP engine?

If you give people the ability to fly default tires with 180 HP engine, then everybody will use the 180 HP engine :laughing:

I found some information about converted 172 that are not floats, see for instance: http://www.controller.com/list/list.aspx?manu=CESSNA&mdltxt=172%2F180+CONVERSION

That said, I am with onox on this one. I think we shouldn't give people too many options for every single thing possible. I guess 99.999% of all 172 model P have a 160 HP engine, and I guess that 99% of people will only fly the 180 HP version in the sim "because it's better". I would simply associate the 180 HP with the two float options and the 160 HP with the wheel options.

That's my 2 cents, but you guys are doing all the hard work here and I shouldn't be putting my finger here in the first place :wink: It's all up to you then!

[onox]

@dany93 I'm not using the /sim/input/selected/ right now. That is a generic way to select an engine and is to be used when your aircraft has actually multiple engines. You can set the current engine with the !, @, #, $ keys in that case.

I have fixed the magnetos, starter, RPM, exhaust. The throttle and mixture currently control both engines.

[onox]

Now going to fix float chamber and sound...

[dany93]

You do the code more quickly than I write...

[onox]

About the float chamber:

1. If using the 160 HP engine: fdm/jsbsim/propulsion/tank[2]/priority = 1 and fdm/jsbsim/propulsion/tank[3]/priority = 0?
2. If using the 180 HP engine: fdm/jsbsim/propulsion/tank[2]/priority = 0 and fdm/jsbsim/propulsion/tank[3]/priority = 1?

Is that correct?

[dany93]

About the float chamber:

1. If using the 160 HP engine: fdm/jsbsim/propulsion/tank[2]/priority = 1 and fdm/jsbsim/propulsion/tank[3]/priority = 0?
2. If using the 180 HP engine: fdm/jsbsim/propulsion/tank[2]/priority = 0 and fdm/jsbsim/propulsion/tank[3]/priority = 1? Is that correct?

Yes! And sufficient to prevent any unwanted start, or the other engine from running..

[onox]

Thanks! :+1:

[onox]

@dany93 I've a bit of trouble with the fuel system, I'm trying to do in Systems/fuel.xml:

 19     <channel name="Float Chamber Selection">
 20         <!-- Give tank[2] priority if 160 HP engine is used -->
 21         <switch name="propulsion/tank[2]/priority">
 22             <default value="0"/>
 23             <test logic="AND" value="1">
 24                 /controls/engines/active-engine EQ 0
 25             </test>
 26         </switch>
 27 
 28         <!-- Give tank[3] priority if 180 HP engine is used -->
 29         <switch name="propulsion/tank[3]/priority">
 30             <default value="0"/>
 31             <test logic="AND" value="1">
 32                 /controls/engines/active-engine EQ 1
 33             </test>
 34         </switch>
 35     </channel>

But JSBSim is just not setting values to the priority properties :confused:

[wkitty42]

FWIW: i've not found anything stating that the 180HP upgrade is limited only to floatcraft... indeed, it seems to be a standard upgrade for those that want a higher performance craft than the factory offers... i've looked at quite a few starting here -> https://www.google.com/search?q=cessna+c127+engine+180hp

[onox]

Fixed it, seems I need to use <output>.

[gilbertohasnofb]

Okay, so let's go for it. But does this mean we will leave the option of 160 HP for the floats? That may confuse some people as for why the plane doesn't climb.

[onox]

I would say automatic selection. Otherwise 99% are gonna cheat and use the 180 HP all the time.

[wkitty42]

how is it cheating?? IRL there are everyday many cessna c172 craft with the lycoming 180HP engine... c172r, c172d thru h, etc... it appears to be a normal and standard upgrade... just like changing the engine out in your car for a more powerful one... or your motorcycle or boat...

[wlbragg]

It's not cheating, it's a choice. I don't understand why we should care what users will do with their choice, it's not for us to say. If most of them use the bigger engine then that says something in and of itself.

[onox]

Alright, I'll make it a choice then.

[wlbragg]

:clap: :clap: :clap:

[wkitty42]

:+1: :+1: :+1:

[onox]

lol

[gilbertohasnofb]

:dog: :dog: :dog:

But what about the pontoons and amphibian? Are we allowing the 160 HP for them? And if not, how to elegantly handle the dialog? Can we force the 180 HP engine while on floats make the engine selection option "unclickable"?

[wlbragg]

If they still use the 160 engine on float planes in RL, then I say we leave that option. It is a delightful challenge to use it. If this engine is never used in RL on floats, then maybe not.

[gilbertohasnofb]

:+1:

I found some posts in aviation forums about this subject, and indeed there are people using 160 HP with floats. So I would suggest the following: when someone clicks on "pontoons" or "amphibian", the engine is automatically set to 180 HP in the same menu, but the person is allowed to change it back to 160 HP if he or she wants.

Here is a video of a 172 with 160 HP taking off. Looks as scary as it does in FG :wink: (see at around 0:50): https://www.youtube.com/watch?v=33E9_qQysoo

[onox]

haha, I was watching that video just a minute ago :smile:

[onox]

It's a regular float plane though I think, not amphibian.

[onox]

the engine is automatically set to 180 HP in the same menu, but the person is allowed to change it back to 160 HP if he or she wants.

I agree with that.

[wlbragg]

Great idea, that sound right to me. We should think "safety first" :smile:

[gilbertohasnofb]

Okay, so this is solved :) Should I open an issue for this or can you guys include the code handling these menus in the PR for this issue?

[onox]

No need for a new issue.

[onox]

I'm adding it to bug-273 branch.

[onox]

I have changed the milestone because without the 180 HP it will be very difficult for most people to takeoff from the water.

[wkitty42]

i found this, too... there are many 160HP c172 floatcraft but i don't know how many of those may be amphibious... how much additional weight is added for the additional amphib gearing?

https://www.google.com/search?q=cessna+c172+160hp+floats+site:www.cessna172club.com&num=100

[gilbertohasnofb]

i found this, too... there are many 160HP c172 floatcraft but i don't know how many of those may be amphibious... how much additional weight is added for the additional amphib gearing?

Let's just keep the solution I proposed above of forcing 180 HP to both pontoons and amphibian models, and let the user change it back if they want.

[wkitty42]

that's fine... i read these exchanges in my email and then come over here to reread them and respond... i just wanted to toss my two pesos in on the conversation about 160HP floaters :wink:

[wkitty42]

so how much extra weight is added to the floats by adding amphib landing gear? that is a serious (side) question :smile_cat:

[wlbragg]

Do we also want to force 160 on ! = float and allow user to switch back to180?

[wlbragg]

So in essence, the default behavior is what you originally wanted, but there is a choice to switch back.

[onox]

If !float then engine will be set back to 160 HP. If float then engine will be set back to 180 HP.

User can change engine afterwards.

[gilbertohasnofb]

@wlbragg good question. I guess that could work well.

[wlbragg]

I think that is a good compromise.