What drives the 160HP limit?

I've seen several discussions on other message boards (Yahoo groups mainly) about using an 0-360 in the 9A, but I've never had a clear understanding on exactly what drives the 160 HP limit on the 9A.

Here are the facts as I see them:

1) 160 HP drives the plane no where near the yellow arc straight and level. (From what I've heard, 180 HP 0-360 doesn't either - I maybe wrong on this - but certainly not near red-line)
2) The 0-360 RV-7A mount is the exact same as an 0-320 RV-9A mount.
3) An 0-360 weighs about 10 lbs more than an 0-320. My 9A with a Catto 3-bladed prop could easily handle the added weight in the nose.

Does anyone know why Vans says to limit the HP to 160? Why not 180 HP for the 9A?

Just Saw that Somewhere ..

...I just read something from Van himself about that very topic. It may have been in the text pages in the front of the RV9 plan preview set. Without recalling specifics, I do recall his viewpoint being very clear, very logical, and well articulated. I'll check tonite the plans-preview set tonite and let you know...

All the pretty horses

Read Vans article in the RVator 2001 4th edition. "All the Pretty Horses". He answers your question in detail. It's an excellant article and convinced me that I made the right decision to not exceed 160hp.

Whew!

vanlle2000 said:

there was a good article -- maybe two -- in recent RVators about why more HP is not desirable. the key is that the never exceed speed is not an indicated airspeed number, but a true airspeed number. So at altitudes that the 9A can easily reach, bigger engines (actually more power, whether it's from a bigger engine or a supercharger or nitrous or a Prius batterypack...)can drive it over Vne, despite what that red line on the ASI says.

Applies to bigger engines in RV-10 and other RVs as well.

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Thank goodness I missed that article.

Just doesn't add up!

vanlle2000 said:

there was a good article -- maybe two -- in recent RVators about why more HP is not desirable. the key is that the never exceed speed is not an indicated airspeed number, but a true airspeed number.

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That statement is contrary to everything I have ever learned about Physics, aircraft operation and indicated/calibrated & true airspeed.
So, am I supposed to get my whiz wheel out and figure what my true airspeed is every time I think I'm flying fast?
The airplane fly?s to what the airspeed indicates. If you were to fly at, let?s say, 25,000 ft. your airspeed would indicate close to a stall, although you?re true airspeed would be much higher. All the airplane knows is that it is flying close to a stall. It has no idea what its true airspeed is.
Think about it...it just doesn?t add up.
I haven?t seen the article in the Aviator so I can only go with what you have written, but to me it just doesn?t add up.

Roger

Kahuna said:

Thank goodness I missed that article.

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ROTFLMAO!! Gotta love the Super 8!

Well, I missed that article, too, so if someone can post the particulars, I'd appreciate it. I'll have to take your word that Vne are true airspeeds, but if Van says it, I suppose it is.

I guess I understand that Van has to have that position to keep him in business, but if higher altitidues is the only issue, then a TAS indicator and a head on your shoulders should keep you out of trouble.

Indicated Air speed

Vans article and explanation uses IAS, not true airspeed. He explains the maximum aerodynamic load or lift factors and the effect on the wings. His diagrams show G load that a given airframe will experience when it encounters a sharp vertical gust. You can probably get a 180hp RV9A closer to Maximum structural cruise speed (Ve). And in smooth air, probably no problem. But it appears that you would be flying right on the edge of the envelope and if sufficient gusts were encountered, it's possible to exceed the design limits of the airframe. As airspeed increases G loads increase linearly.

The above is a brief summary of the 9A article (All the pretty horses). There is a lot more info and should be recommended reading for anyone considering installing a 180hp in their 9(A). In the end you can put whatever you want FWF. If you want more speed (the 9 is not slow) why not build the 7 or 8 with more horses. My $.02

Build9A said:

Vans article and explanation uses IAS, not true airspeed. He explains the maximum aerodynamic load or lift factors and the effect on the wings. His diagrams show G load that a given airframe will experience when it encounters a sharp vertical gust. You can probably get a 180hp RV9A closer to Maximum structural cruise speed (Ve).

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Actually thats Vno

And in smooth air, probably no problem. But it appears that you would be flying right on the edge of the envelope and if sufficient gusts were encountered, it's possible to exceed the design limits of the airframe. As airspeed increases G loads increase linearly.

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See this is precisely what I don't understand about Van's logic. This is completely normal, even for certfied aircraft. That is why there is manuvering speed (Va). At that speed, a wing will stall before the load limits are reached.

The above is a brief summary of the 9A article (All the pretty horses). There is a lot more info and should be recommended reading for anyone considering installing a 180hp in their 9(A). In the end you can put whatever you want FWF. If you want more speed (the 9 is not slow) why not build the 7 or 8 with more horses. My $.02

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That was the deciding factor for me . S7

R.P.Ping said:

All the airplane knows is that it is flying close to a stall. It has no idea what its true airspeed is

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Sure it does. The little air molecules pass over the plane's surface at true airspeed, not indicated. The fact that there are fewer of them per cubic foot of air doesn't alter this fact. Flutter is the issue here.

Steve Zicree

The wise man set the limit

OneTwoSierra said:

I've never had a clear understanding on exactly what drives the 160 HP limit on the 9A.

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The 160hp limit is set by a guy named Richard Vangrunsven. I hate to be glib like Tom Cruise would say, but....

Engineering an aircraft is a million trade offs. It may sound trite but build it to plans. This is the common words of wisdom people much smarter than you and I have spoken. You can't go wrong. If you want to go faster in a side by side get a RV-6/7. That is my opinion. Since in a previous life I was an aerospace engineer and have built two RV's I have some perspective, and this is the short answer. However it's a free country, the FAA has given builders a blank check to modify or build a one-off custom. You want a twin engine / Bi-plane / RV-9a, than go for it.

However usually changing one thing brings on an avalanche of other changes or cause & affects, many of which are not as desirable as one might hope for.

I would suggest building clean, light , simple 160hp RV-9a and concentrate on reducing drag and weight thru things like a Sam James style type cowl (I say "type" because you can make your own cowl and plenum from a stock cowl. Check out the Sam James Yahoo group). The net effect of adding a SJ type cowl and plenum will add about 6-10 mph or more. In cruise 6 mph is like adding 10-15 hp. Using Van's spec for the RV-7 going from 160 to 180 hp is worth about 8 mph top speed, 300 fpm greater climb, 25 to 75 feet shorter T/O and about 60-75 sm less range.

Also people rationalize they will get 160hp fuel burn with their 180hp engine by throttling back. It does not usually work that way, and even if you did you will still have overall higher fuel consumption. There is no free lunch.

The RV-9a departed from the traditional 23013.5 NACA wing airfoil other RV's use.

"The RV-9/9A uses a new Roncz airfoil design with a slightly longer wing span, shorter chord and slotted flaps for better low speed performance."

To get low speed performance there are trade offs to top end. If the plane was designed to cruise 6-8 mph faster, Van would have used a different airfoil and yes specified a bigger and heavier engine? Adding more HP may mean a little more speed but with less efficiently.

Keeping it light will also make the plane more fun to fly and feel better on controls, not to mention preserve its low speed performance. The thought of a O-360 adding only 10 lbs is very optimistic, even if that is what the specs say. (not to mention clone O-360's weigh more than stock Lycomings.) The installed weight will be at least 21 lbs. Not to mention one add-on begets another. A 180hp engine is begging for a constant speed prop, with another 35lb or more additional weight over a fixed prop.

I did a survey of RV weights and all boiled down to O-320 powered RV's (all models) are 21 lbs to 51 lbs lighter than a similar O-360 powered RV's. Needless to say you will add some weight, at least 21 lbs and likely more. Now add a fat panel, upholstery, paint job and extra do-dad's and you will have a heavy RV. Trust me light is the way to go. From my survey one RV-9a weighed about 100lbs more than Van's max empty weight of 1075 lbs. If you flew that one next the one built to the lower 1028 lb empty weight limit (147 lbs lighter), you would know what I mean.

Forget the engineering/aero reasons. This has everything to do with feel. Also the benefit of more power does not always produce the performance you might predict because of the extra weight.

Since the 160Hp engine is already at high compression, I would suggest you could make small changes such as a blue printed engine with electronic ignition and get a fat +160 hp. I am not a fan of real high compression pistons in a daily flyer. 8.50:1 is plenty.

My opinion is 160 hp is PLENTY. If you have not flown a RV, trust me 160 hp is lots of power. Heck 150hp is plenty in a RV.

I can get into structural and aerodynamic considerations of 20 extra hp and +20 extra lbs on the nose, but I'll just leave the bottom line as: If "The wise Man", Van, would have wanted 180 hp in a RV-9a he would have designed it that way. There is a balance to get the "total performance" the RV-9 was designed to meet. Next you all will want to aerobatics in the RV-9? (there was another thread on that already). Build a RV-7 already.

Good Luck, George RV-4, RV-7

PS no one mentioned money. It seems people have unlimited money. A used 150/160hp is cheaper and easier to find than a used 180hp Lycoming.

vanlle2000 said:

Also in the RVator, in the 6th issue of 2004, Ken Krueger wrote the lead article FLYING HIGH AND FAST, which gives an even better explanation. There are several criteria that go into establishing a Vne, one of which is flutter. And as Ken points out, flutter is excited by the velocity of airstream. Dynamic pressure, which is what the pitot tube sees, is something different.

In most normally aspirated airplanes it's not an issue. there isn't enough power available at altitude to make the airplane go too fast. But with more power, whether it's from gravity, turbo or more cubes....

This was all news to me, as an average private pilot. I don't recall anything about it my training. But once it was explained, it made sense.

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Well...it's news to me too! I would think the only problem would be flutter at high speeds.
So far I have had my -9 up to redline (indicated a/s) at 10,000 ft. using gravity on the way down. The equivalent true a/s was a little over 230 mph.

I guess I'm safe up to that but I don't think I'll be pushing it any further.

Doug is there any chance you can print the article by Ken?

Roger Ping

I appreciate the idea that unless one understands the consequenses, one should stick with the recommendations of the engineers. I am, however, unsatisfied with the notion that engineers say so so I should stop thinking about it - or that I should stop trying to understand my plane. And this is the purpose of this thread - to understand the driving factors - not to suggest that anyone blindly go beyond limits without care.

It sounds like an article was publshed on the topic, and it sounds like two issues were described- (1) too high a TAS creating the possibility of flutter, and (2) overloading the G load limits in turbulent air.

I have questions about both:

(1) Is it true that a dangerous, flutter-inducing TAS exists at an IAS below redline? Did the article indicate what this TAS is?

(2) I think I've read somewhere that Van designed the plane for +/- 50 vertical ft/s turbulence factor, while 'normal' category planes are typically designed for +/- 30 ft/s. Is this true or am I dreaming? Whatever the case, did the article indicate a speed where this design limit no longer holds?


Thanks,
Brian
RV-9A, flying, (1030lbs empty)

P.S., My motives for asking these questions is not dissatisaction as some may suggest. I wouldn't trade my 9A for any 6, 7 or 8. I built the right plane for me. YMMV

email

Brian: I will get the article to you. Vans mentions the 50 ft. per sec. gust factor in the article. I'm no engineer and I don't play one on TV, but Vans rationale was convincing. Keep asking questions, it's good for all of us. Jack

(2) I think I've read somewhere that Van designed the plane for +/- 50 vertical ft/s turbulence factor, while 'normal' category planes are typically designed for +/- 30 ft/s. Is this true or am I dreaming?

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I could be wrong (and often am) but I think this was referring to Van using the current FAR 23 standards which specify the 50 fps gust loading, rather than the older CAR 3 standards which specified 30 fps.

Most "legacy" designs in the fleet were originally certified under CAR part 3. Examples would include Cubs, bonanzas, the Cessna line, etc.

IIRC Cessna recertified the 172 under part 23 when they restarted single engine production.

I am going from very distant memory here, but the first single certified under the (then new) FAR part 23 standards might have been the Rockwell Commander series, which made much of the fact in its advertising.

Brian,

It sounds like an article was publshed on the topic, and it sounds like two issues were described- (1) too high a TAS creating the possibility of flutter, and (2) overloading the G load limits in turbulent air.

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Ken's article in the sixth 2004 RVator is definitely worth digging out of your back issues.

Is it true that a dangerous, flutter-inducing TAS exists at an IAS below redline? Did the article indicate what this TAS is?

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It could be true, depending on density altitude and Van's design flutter margins. The article says only that their design margins are 'conservative', and operating with a normally-aspirated engine of specified power should keep you away from the margin.

Ken's examples showed that an airplane capable of hitting 168 mph TAS at 20,000 feet would have a negative flutter margin, assuming that margin is zero at 230 mph TAS. Is that the real limit? I don't know.

(2) I think I've read somewhere that Van designed the plane for +/- 50 vertical ft/s turbulence factor, while 'normal' category planes are typically designed for +/- 30 ft/s. Is this true or am I dreaming? Whatever the case, did the article indicate a speed where this design limit no longer holds?

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The +/-50 fps gust is a FAR 23 certification design criteria for all but commuter category airplanes and is applied out to Vc, the design cruising speed. Van's claims to use FAR 23 in their designs, but only they know how it was applied here, and what they used for Vc. Ken's article doesn't say.

Dave

Bigger engine

Fun with numbers:

I don't understand why there is such a desire to go faster by a few knots by installing a bigger engine.

My airplane has travelled 35 miles in the last 3.5 years... that's 10 miles per year or 0.0011 miles per hour. As you guessed, It hasn't flown yet.

Why does one spend years building an airplane, then worry about an extra couple of knots? To save five minutes per tank of fuel? After 525,600 tanks of fuel, you'd gain back 5 years of building time. Of course, you'd probably have to refuel more often.... and then you'd probably never catch up.

I think that climb rate is probably a better reason to choose a more efficient airframe/prop/engine. Getting to cruise altitude faster will save more time, is safer, and allows you to take advantage of the winds.

If anyone out there knows of some good modifications (other than HP or a CS prop) that would increase climb rate, then THAT would very interesting. Oh, and I bet an RV-9A will outclimb an RV-7A with the same engine/prop. That would make for some interesting bets on a hamburger run.



Vern Little

Flutter

OneTwoSierra said:

(1) Is it true that a dangerous, flutter-inducing TAS exists at an IAS below redline? Did the article indicate what this TAS is?

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Flutter is a frequency phenomenon, which is why TAS matters rather than IAS. There was a discussion about it a few weeks ago when discussing control balancing on another thread. I've never seen the article, but I doubt a TAS was given because flutter isn't 'induced' at any particular speed, it is just one of the many factors that contribute to it's possibility.

Dennis Glaeser

Fun with numbers

vlittle said:

I don't understand why there is such a desire to go faster by a few knots by installing a bigger engine.........................Oh, and I bet an RV-9A will out-climb an RV-7A with the same engine/prop. That would make for some interesting bets on a hamburger run. Vern Little

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Well lets go to the numbers (160hp):

Notice the effect weight has?

RV-7............RV-9
202.............197 (mph) top speed solo wt. (1400lb/1350lb)
201.............196 (mph) top speed gross wt. (1800lb/1750lb)

51...............44 (mph) stall speed solo wt. (1400lb/1350lb)
58...............50 (mph) stall speed gross wt. (1800lb/1750lb)

1,900...........2,000 (ft/min) ROC solo wt. (1400lb/1350lb)
1,400...........1,400 (ft/min) ROC gross wt. (1800lb/1750lb)

21,000.........24,500 (ft) ceiling solo wt. (1400lb/1350lb)
18,500.........19,000 (ft) ceiling gross wt. (1800lb/1750lb)

What can you tell from above? Not much but the RV-9 has slight climb advantage and a big ceiling advantage at solo wt., but this advantage* is reduced with higher gross weights (hummmm). The RV-7 as a speed advantage, but the RV-9 has a low speed advantage. Again notice effect of wt. (hummmm). This is really no surprise, the RV-9 has more wing area and a higher aspect ratio. The trade off is more cruise drag, and with the longer wing, max load factor is less (so its not "aerobatic rated").

"I don't understand why there is such a desire to go faster by a few knots"

I understand it totally, but there are better (more efficient) ways go faster than adding a bigger (heaver) engine with more HP. George

*Van's numbers say the RV-9 performance numbers for *Empty weight and performance measured with fixed pitch propeller*. So Vern it would be interesting to see what at the real climb difference would be between two similar prop'ed and engine-ed RV-7/9's.

Whats a few knots?

vlittle said:

Fun with numbers:

I don't understand why there is such a desire to go faster by a few knots by installing a bigger engine.

Vern Little

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Yeah me either. I just dont get it. Trying to push the envelop that the design engineers frown upon. Its all fun and games till someone gets their eyes poked out. Slow down! Enjoy the scenery. Whats a couple knots amongst friends?

Aircraft designers have always pushed the speed envelope by adding more power. Isn't that how we got all these neat Military fighters?

I decided on the 9A when I compared performance specs at 160HP like GM did. I knew then that I would eventually use the best aircraft motor (Mazda Renesis) which has similiar weight characteristics as the Lyc 320/360 but delivers more vibration-free HP by (safely) increasing engine RPM. I assume that the 9A speed, takeoff, and climb performance will be very close, or better, to the 7A specs listed at 180-200 HP. Most of my flying will be in the mountains, so extra power should be handy as long as top speeds are watched (I would like to see a near 200 mph cruise)

I still have a lot of questions about the best Prop to use though. The engine is capable of running at 8500 rpm all day long if desired, which equates to between 3000-3900 prop rpm depending on redrive gearing. Max torque @ 5500 rpm (prop 2000-2600). The best prop choices seem to be either MT (expensive) c/s or Ivo's (very affordable and light) in-flight composite adjustable prop (not recommended for Lycs) at this time.

I wish I understood flutter better. The fast WWII fighters didn't seem to be bothered by it at high speeds, and they were not particularly clean designs. All Ive gathered so far is that it seems to be some kind of harmonic vibration triggered at some unpublished speed threshold by air movement over the wing/elevator (?) surface. Makes me wonder if it can be dampened or tuned to a higher frequency somehow.

The first customer-built -9

I do know for a fact that THE first -9 (taildragger) built by anyone other than Van's, was built by a fellow named Tim Donham in North Texas (note: plane has recently been sold).

Tim went with the 0-360 in that plane, and said he loved the results. I spoke to him at length at the Texas RV Fly-In earlier in the year. Not sure the specific reasons he went with the 0-360 versus the suggested 0-320, but I just wanted to post that the first one ever built by an outside individual did go that route.

...why more power is not always a good thing

OneTwoSierra (and all),

Van's web site has a article on this topic now online which adresses this topic in detail. Short answer: because it will go too fast. It's a good read and worth your time - I learned stuff.

http://www.vansaircraft.com/pdf/hp_limts.pdf

Best,
Doug

OneTwoSierra said:

snip..., but I've never had a clear understanding on exactly what drives the 160 HP limit on the 9A. ....snip

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I just like the fact that these birds are experimentals, which gives us the option to do things like this. Bigger engines are always the first thing on some people minds, as evidenced by people that put angle valves in RV-4's, 540's in 7's , and 360's in -9's.

Boys will be boys, 'maam.

Well kind of

cobra said:

Aircraft designers have always pushed the speed envelope by adding more power. Isn't that how we got all these neat Military fighters?

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True breakthroughs in engine technology (like Jets) have always driven airframe design, but that is not the case here, nice try Cobra G

osxuser said:

I just like the fact that these birds are experimental's, which gives us the option to do things like this. Bigger engines are always the first thing on some people minds, as evidenced by people that put angle valves in RV-4's, 540's in 7's , and 360's in -9's.

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Yep and the results are not always good. My 150HP RV-4, kicked 160HP and a few 180HP RV's bee-hinds. Many angle valve 200hp RV-4's get their "A's" handed to them by a clean 180HP RV . Just slapping a 540 in a RV-7 may not be great unless you "clip" the wings (to reduce drag for the intended new top speed) and stretch the fuselage (maintain w&b and stability & control). Clearly the Harmon Rocket II is NOT a 540 powered RV-4 (kind of). But to put a 540 in a stock RV-4 would be a.......? (pig) My point is there's no free lunch (Either one of Newton's or Murphy's laws). G

The thing I got from the Vans articles and charts (could easily be wrong...) is that the (suggested) Vne is driven more by aircraft strength (G-loadings and 50fps downdraft safety margin) than by observed flutter avoidance.

I remember seeing the charts in the RVatior but not anywhere in the build plans. Are similiar data available somewhere for other planes and/or solo weight?

I'm curious about the 160hp limit too. I want a commuter and so the 9 fits my needs more than a 7 but I'd like to consider putting a Toyota D4D alum. block diesel for added frugality. I'm only just beginning to reserch this so it may not be feasible [I just joined this forum today and I did already post to a diesel thread regarding this engine]. It weights a little more [probably 400# totally decked out] but has 180 hp without chipping it or anything. It has great potential and would burn jet-a or diesel.

If a pilot would cruise at a lower % of power and keep the speed down [and assuming the extra weight gained by using the engine is offset by the reduced weight of less fuel needed to be carried] why would it matter if it were a particular hp? it seems speed is the critical element. Simply keep it within a safe range?

I'm a newbie so I could be all wrong. Probably am.

John

Good questions

Deuskid said:

I'm curious about the 160hp limit too. I want a commuter and so the 9 fits my needs more than a 7 but I'd like to consider putting a Toyota D4D alum............

If a pilot would cruise at a lower % of power and keep the speed down [and assuming the extra weight gained by using the engine is offset by the reduced weight of less fuel needed to be carried] why would it matter if it were a particular hp? it seems speed is the critical element. Simply keep it within a safe range?

I'm a newbie so I could be all wrong. Probably am. John

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Let me give you the best advice on the web:

Build it per plans, build it light and keep it simple.

The advice is good because it was given to me from wiser folks than I.

Look, you will never totally account or compensate for dead empty weigh, especially on the nose. It is weight that is there all the time. RV's fly nice, especially at light weights. Every extra pound takes just a little away from the light, quick handling and feel. Using a (I)O320 you will build the lightest plane possible.

Power: 150hp or 160 hp is PLENTY!!!! of hp for a RV. You don't need more, and adding more HP for more pounds is really diminishing returns. Can't you deviate from the plans and go with a 180hp? Sure, but see advice above, and repeat it like a mantra:
keep it to plans, light, simple......
per plans, light, simple........
per plans, light, simple........

WHY not do aerobatics in the RV-9 also, and while we are at it, up the gross weight to 2100lbs. Are you getting my point. There are many factors and the RV-9 was designed with a mission in mind. Changing from plans will change the characteristics. Yes you might pick up some speed but at expense of stall speed and landing distance. Why not just get a RV-7?

You won't regret sticking to plans. You will finish faster and have known performance, maintenance, reliability and flying qualities.

Alternative engines: THE BEST ALTERNATIVE ENGINE CONVERSION IS TO CONVERT $19,000 INTO A NEW LYCOMING (CLONE). I know that might ruffle a few feathers, but if you want a commuter that is simple and reliable you will not get any simpler, lighter than a Lycoming. The alternative guys can be quite infectious with their enthusiasm, but if you are a newbie and want to finish the plane before you retire go per plans. You will finish faster, have a lighter plane with the best performance. I don't know your back ground or abilities but going with a Toyota engine that no one has used before is asking for a tremendous about of extra work and unknown results. Aircraft engines, air-cooled, direct drive engines are made for aircraft. They may be "old fashion" but they are purpose built for the application. Also unless you are an expert at fabrication and have the resources, an alternative engine may end up costing more than a Lycoming.


Based on your goal to finish a RV for commuting to work I would recommend you stick with a 160HP Lyc or Lyc clone. 180hp will get you a little more climb and a few mph, but the weight will be there all the time. You don't need it.

Don't take my word for it, but do your research and make your mind up. I can see you are not drinking the cool aid and just asking good questions.

George

John: Huge mistake. Don't even think about it. New guys (and God bless 'em, we need 'em) are always asking why they can't drop in a large block V8, a marine diesel, or a steam boiler and go flying. Well, if it was feasible someone would already have done it, and if it worked the skies would be full of them. But it isn't and there aren't any, the reason being that it is a gargantuan engineering problem. IF you are smart enough, IF you are rich enough, IF you have the engineering experience, AND you have the time to do this, you might get off the ground someday in the far distant future. Your airplane will be heavier, slower, less reliable, harder to repair, harder to insure, and have less resale value than a Lycoming powered version. If you think this is what you want, well, the rules allow such experimentation, but chances are you will not have an airplane you can use for commuting. That is the history of amateur powerplant experimentation. What you don't yet realize is that Van has engineered marvelous compromises into his designs. Most are invisible to a new guy. Things in an RV, built as designed, work together harmoniously in the proper proportions. Stick to the plans and you will get a great airplane. The kits themselves are wonderous to behold. Van recommends using a Lycoming. The best engine conversion, to paraphrase Van, is to take your money and convert it into a Lycoming. Steve