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Firewall Forward max weight reasoning?

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SamP

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I’m trying to understand what drives the maximum firewall forward weight. Since the Engine is cantilevered, I am assuming that one component is simply the stress on the engine mount. If this is the case, then an airframe max g could be derated.

Another factor is elevator control authority. This has more to do with cg, and other components could be redistributed to keep it within the envelope.

are there other things to consider? I’ve been reading about engines that exceed FWF max weights. As always, it’s probably best to follow the aircraft manufacturer’s recommendation.
Thanks
 

mcrae0104

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I’m trying to understand what drives the maximum firewall forward weight. Since the Engine is cantilevered, I am assuming that one component is simply the stress on the engine mount. If this is the case, then an airframe max g could be derated.
Assuming you provide a strong enough mount, then you need to consider the mounting points, the longerons, the skins, etc., which all work together. You would need to need to be analyze quite a lot of the airframe structure to know how much to de-rate it, but in theory you could do it. But don't guess at the de-rating. It is not as simple as a decrease proportional to the amount of excess engine weight.

are there other things to consider?
  • Ability to rotate on takeoff
  • Ability to flare at landing
  • Weight on nose gear - is the structure sufficient?
  • May require adjustment to gear geometry to keep excess weight off nose gear
  • Effects on longitudinal stability - this might be overcome with a larger HS/elevator but at the cost of increased trim drag
  • May require a change in HS incidence
  • Increased polar moment of inertia - decreased yaw rate and degraded spin recovery
  • Decreased useful load
  • Decreased performance, particularly rate of climb and range
I'm sure there are others, but those are the first ones that come to mind. What kind of aircraft and engine do you have in mind?
 

Dana

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The maximum FWF weight is most likely primarily determined by the aircraft's CG limits. More engine weight and you may need tail weight to compensate and keep it within limits. But: The designer will have designed the engine mount around his chosen engine weight. Increase that, and the engine mount (or the firewall and forward fuselage structure) may not be strong enough. If you need tail weight then the tail structure has to be looked at. Yes, perhaps it could be derated, if there's sufficient margin. Others have mentioned moment of inertia, which particularly impacts spin recovery.
 

SamP

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Thanks very much. Definitely something you wouldn’t want to do without significant analyses and thought.

Looking at a Sonex B aeromomentum Am15 combination. Very cool engine, but mostly trying to understand aircraft design more. I also wondered about vibratory loads and whether that factors into FWF weight limits, or if those are so small 9n magnitude in comparison to the weight factors previously mentioned.
 

mcrae0104

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The AM15 is listed as 185lb dry weight. I don't know what you end up with for a full FWF weight, but it seems reasonable. Ask Mark (@aeromomentum). It's worth noting that the Corvair has made a successful pairing with the Sonex airframe (paging @Daleandee), and it is above the recommended FWF weight. I won't tell John Monnet if you don't.
 

SamP

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Thanks. I’ve had an opportunity to write Mark. He was kind enough to answer my questions, which I’m sure he’s been asked a million and one times before.
 

wsimpso1

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All these guys make it sound like there was high science involved. Maybe, maybe not. In some cases, I would bet that the designer intended X engine, built it with X engine, tested it with X engine, and then published that number and the CG range. Anything else is unknown territory for flying characteristics. Now some folks may go over that number and play with things like mount, cowling length, and even ballast to hold empty CG. I even knew one guy who filled his hollow crank end (no governor on his engine) with lead shot and resin - landed after a flight with most of the lead and most of the spinner gone, with the airplane really squirrelly on the way back to the airport. He would have been better served with a longer mount and cowling.

Seriously though, you do not know what parts came in really close to limits and which have a bunch of margin. And every design is likely different. Could be fuselage strength, spin and stall characteristics, ability to lift the nose wheel for takeoff, ability to flare in landing, the list goes on and on if you keep the engine CG in the same place but make it heavier. Even worse if you get it further forward and heavier.

Your challenge in going with a heavier engine is how much do you dare increase the bending moment into the airframe from the combined engine and mount? I will suggest zero is an excellent target. I recommend that a Free Body Diagram be done with the design recommended mount and engine, carefully compute FOS on the bolted joints at the firewall, and then see if you can get the same bending moments from your heavier engine by shortening the mount. For instance if you had a base engine at 185# with its CG 30" forward of the firewall, that is 5550 in-lb. The new engine is 200#, its CG should be about 27.75" forward of the firewall, and the FOS in the bolted joints and fuselage should still be on the safe side. They won't change much this way, but you better have a pretty good idea of what you did to your margins.

If you can maintain moments of the firewall forward stuff, you will only have a few less pounds of payload, but it will otherwise behave like the original. If you have a heavier engine that is longer too, now it can be tough to keep the CG in place without resorting to ballast. That can work, but you had better weigh the bird and make sure it is on for CG. I know of a guy who did all this, did a bunch of calculations but never weighed the airplane - it was tail heavy and the first flight ended in a fatal crash. Do not be that guy.

One other way to go with a water cooled engine that is a few pounds heavy is to go with a belly mounted radiator and maybe oil cooler too. See SDS's website and rv6ejguy's posts on this topic. This may keep the stuff forward of the firewall under control, put the extra few pounds somewhere nearer to the CG, and give you less stuff to make work under the cowl too.

Have fun,

Billski
 
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SamP

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Thanks very much. this is me mostly just trying to understand airframe design. I understand that controllability and structural integrity can be the limiting factor to FWF. It didn’t make much sense to me that FWF just be a weight limit in the case for structural limit. Rather, as you suggest, it’s a moment at the engine mount that is the desired parameter to match / not exceed.

If we say that there is a critical bending moment that should not be exceeded, then one of the derating calculation seems like the following. For example, we know a certain airframe is certified +6,-3, which translates to a certain moment at the mount. If we know the weight of the new engine, Then the calculation would be whatever G value is required to not exceed the limit. too simplistic?
 
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mcrae0104

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too simplistic?
Yes.

Edit for a bit more full answer:
Sure, if you keep the couple the same (i.e. same horizontal reactions at the firewall attach points), then that's the first hurdle. Next, have you considered/analyzed the increased vertical component of those reactions? Can the airframe take it--at what load factor? Next, you need to deal with all of the items listed above in post #3. That said, within reason, others have exceeded the FWF weight of a Sonex. Either they calculated their way to satisfaction, or they made themselves test pilots in the face of some unknowns. It's called experimental aviation, so you can suit yourself and your own level of risk tolerance.
 
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Dan Thomas

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A few years ago a Murphy builder demonstrated what can easily happen when you install a heavier engine than the airframe was designed for:


The excitement comes at around 3:30. Most of the preceding time shows a bunch of minimum-distance and altitude violations.

From the Youtube page:

The Murphy is very well built and this accident was not the fault of the design. The owner / builder wanted to put a larger engine in it. After consulting, Murphy said, NO. The builder put it in regardless, without additional engine-mount strengthening. The impact was to much for the additional weight of the larger engine and tore away from the fuselage. The aircraft is rebuilt with the same engine and now with additional strengthening of the engine mounts the pilot is back enjoying his hobby. I reiterate, the Murphy is a well designed and built aircraft, but only if you follow and abide by the design limits. Like any other aircraft, exceed the limits and face the consequences.

TC'd airplanes are designed for 9Gs vertically/forward on the engine mount structure. I don't think this impact was anywhere near that.
 

Bill-Higdon

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A few years ago a Murphy builder demonstrated what can easily happen when you install a heavier engine than the airframe was designed for:


The excitement comes at around 3:30. Most of the preceding time shows a bunch of minimum-distance and altitude violations.

From the Youtube page:

The Murphy is very well built and this accident was not the fault of the design. The owner / builder wanted to put a larger engine in it. After consulting, Murphy said, NO. The builder put it in regardless, without additional engine-mount strengthening. The impact was to much for the additional weight of the larger engine and tore away from the fuselage. The aircraft is rebuilt with the same engine and now with additional strengthening of the engine mounts the pilot is back enjoying his hobby. I reiterate, the Murphy is a well designed and built aircraft, but only if you follow and abide by the design limits. Like any other aircraft, exceed the limits and face the consequences.

TC'd airplanes are designed for 9Gs vertically/forward on the engine mount structure. I don't think this impact was anywhere near that.
Looks like "The 3rd time was the charm"
 

wsimpso1

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If we say that there is a critical bending moment that should not be exceeded, then one of the derating calculation seems like the following. For example, we know a certain airframe is certified +6,-3, which translates to a certain moment at the mount. If we know the weight of the new engine, Then the calculation would be whatever G value is required to not exceed the limit. too simplistic?
First off, there is shear load plus reactions from moments. They all count...

Second, Let's assume the airplane is well sorted and a good behaving bird with the stock engine. You make the engine heavier but leave the engine CG where it was, shifting the airplane CG forward. If you try to manage the bending moment and shear by reducing max g, several things happen. Sure, you are no longer allowed to do 6g, now the max is 5.4g, and that sounds doable to you... But what about everything else in the envelope?
  • In a tricycle geared airplane with the CG further forward, the nose gear now carries more load that it may not have been designed to handle and it sits more nose down for ground handling and takeoff run;
  • In a tricycle geared airplane with the CG further forward and the nose down, significantly higher speed on the runway is likely to be needed before the nose can be lifted, lengthening takeoff roll;
  • If a taildragger, tipping it over on its nose becomes more likely;
  • In flight the tail must make more load all of the time to balance the forward CG, which is added to the lift the wing must make, and which both accelerates fatigue of the tail and controls and increases aerodynamic drag over the entire flight;
  • Stall speeds are now higher because of greater downforce from the tail added to the rest of normal loads, and because the tail has less margin for control;
  • Approach speed and engine out speeds must be raised because stall speed is higher - it is still 1.3 times and 1.45 times whatever your new Vso is;
  • Increased polar moment of inertia in pitch will slow pitch response and may drive higher approach airspeed and/or less steep approaches so that you can rotate to arrest your sink before landing;
  • Increased polar moment of inertia will reduce ability to recover from spin entries and spins;
  • The sink rate at landing contact that the original design had resulted in so many g and so many pounds peak force in the gear legs. It now has a higher likelihood of occurring combined with more kinetic energy in the vertical direction to suck up, resulting in more peak pounds vertical force on the gear legs and more pounds spinup and drag loads on the gear driving higher likelihood of bent and/or broken landing gear;
  • The runway you have to consume to land increases because of raised Vso and raised weight;
  • Emergency landings now have more kinetic energy to dissipate and require a better job of flaring correctly to avoid hard contact, both decreasing survival odds;
  • Va and turbulent air penetration speeds must be reduced to prevent getting to max g's inadvertantly.
Preserve the CG while you add a few pounds, you get to choose between losing a little payload or losing a little performance.

Add some weight and slide the CG forward, and it is a different airplane with higher procedural speeds, lower Va, poorer handling and different procedures, higher likelihood of breaking stuff, and poorer survival odds in an emergency. Are any of these decrements deal breakers? Not really, but the combination of them all should be enough to convince anyone to preserve CG position if you are starting with a well worked out airplane.

There are numerous ways to preserve CG. Engine mount length can be changed, articles like batteries, avionics, etc can be installed elsewhere, lighter props and accessories can be installed, the number of things you can do is quite big.

Now if the bird you are starting is notorious for being somewhat aft CG'd , then yeah, you might benefit the beast with a heavier engine, but then you still might need a little longer engine mount and cowling to make it behave. I would ask if you could not find a better behaving bird instead of trying to fix one that has not finished being developed yet...

Billski
 
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